Appendix A

Solutions of Linear Differential Equations

A.l Linear Differential Equations with Constant Coefficients

Linear diflFerential equations with constant coefficients are usually writ­ ten as 2/("> + ai2/("-i) + ... + a„_i2/(i) + anV = g, (A.l) where a^, fc = 1,..., n, are numbers, y^^^ = ^ , and g = g{t) is a known of t. We shall denote hy D = ^ the derivative operator^ so that the differential equation now becomes

p{D)y = (D^ + aiD^-i + ... + a^_iD + an)y = g. (A.2)

If g(t) = 0, the equation is said to be homogeneous. If g{t) ^ 0, then the homogeneous or reduced equation is obtained from (A.2) by replacing g byO. If y and y* are two different solutions of (A.2), then it is easy to show that y — y* solves the reduced equation of (A.2). Hence, if y is any solution to (A.2), it can be written as

y = y*+y\ (A.3) where y* is any other particular solution to (A.2) and y^ is a suitable solution to the homogeneous equation. Therefore, solving (A.2) involves (a) finding all the solutions to the homogeneous equation, caUed the gen­ eral solution, and (b) finding a particular solution to the given equation. 364 A. Solutions of Linear Differential Equations

The rest of these notes indicate how to solve these two problems. Given (A.l) the auxiliary equation is p{m) = mP + aim^'^ + ... + an-im + an = 0, (A.4) In other words, p{m) is obtained from p{D) by replacing D by m. The auxiliary equation is an ordinary of nth degree and has n real or complex roots, counting multiple roots according to their multiplicity. We will see that, given these roots, we can write the general solution forms of homogeneous Unear differential equations.

A.2 Homogeneous Equations of Order One

Here the equation is (D - a)y = y'-ay = 0, which has y = Ce^^ as its general solution form.

A.3 Homogeneous Equations of Order Two

Here the differential equation can be factored (using the quadratic for­ mula) as (D-mi)(Z)-m2)2/-0, where m\ and m^ can be real or complex. Examples are given in Table A.l and the solution forms are given in Table A.2.

Differential Equation General Solution Form

1. y"-Ay' + Ay = Q y{t) = e2*(Ci + Cit)

2. y" - %' + 3y = 0 y{t) = e2*(Di sinh t) + D2 sinh t)

3. y" - 4y' + 5y = 0 y{t) = e^\Disva. t + Di cos t)

Table A.l: Examples of Homogeneous Equations of Order Two A.4. Homogeneous Equations of Order n 365

Root General Solution Form

^1 7^ ^2? real y{t) = Cie^i* + C2e^2t

= e^*(Cie^^ + C2e-^^)

or

mi = a + 6, 1712 = a — b y(t) = e"*(Ci sinh 6t + C2 cosh bt)

mi = 7722 = ^^ y{t) = iCi+C2t)e^'

^1 7^ ^2 J complex y(t) = Cie^i* + C2e^2t

- e"^(Cie^^* + C2e-'^^)

or

mi = a + bi^ m2 = a — bi y(^) 3.: e"^(Di sin 6t + D2 cos bt)

or

y(^) = e^'^lEi sm{bt + ^2)]

or

y{t) = e''^[Ficos{bt + F2)] |

Table A.2: General Solution Forms for Second-Order Linear Homogeneous Equations, Constant Coefficients

A.4 Homogeneous Equations of Order n

When (A.2) is of order n, the auxiliary equation p(m) = 0 has n roots, when multiple roots are coimted according to their multiplicity. Also, complex roots occur in conjugate pairs. The general solutions of the homogeneous equations is the sum of the solutions associated with each multiple root. They can be foimd in Table A.4 for each root and should be added together to form the general solution. First, we give some examples in Table A.3. 366 A. Solutions of Linear Differential Equations

Differential Equation General Solution Form

1. D2(2)2 _4D + 4)2/-0 y(t) = Ci + C2i + e2*(C3 + C^t)

2. (D-3)2(D + 5)3(D2-4D + 5)2y=0 2/(t)-e3*(Ci+C2t)

+e-5t(C3 + C4t + C5t2)

+e2*[(C6+C7)sin t

^(Cs + CgOoos t]

3. (D2 -2D-\- 2)3(D2 - 2D -- 3)^2/ = 0 2/(i) = e*[(Ci -f C2* + C3t2) sin t

+(C4-f-C5t-f C6t2)cos i]

+(C7 + Cst)e^^ + (Cg + Ciot)e-^

Table A.3: Examples of Homogeneous Equations of Order n

Root Multiplicity General Solution Form

r,=l yj{t) = Ce'^J^

rrij, real rj >1 yj{t) = (Ci 4- C2* + ... + Crjf-J -i)e"^i*

Complex Conjugate r,=l e'^J*(Ci sin 6jt + C2 cos 6jt)

aj lb bj2 0>1 e^i*[Ci + C2t + ... + Cr^-Ti -^)sin 6jt]

+(Cr^- + l + Cr^+2t + .. . + C2r,-r:'' -^) COS bjt]

Table A.4: General Solution Forms for Multiple Roots of Auxiliary Equation

A.5 Particular Solutions of Linear D.E. with Constant Coefficients

The particular solution to the inhomogeneous equation (A.2) can usually be found by guessing the form of the answer and then verifying the guess by substitution. Table A.5 shows the correct forms for guessing for various kinds of forcing fimctions g(t). Note that the form of the guess depends on whether certain nimibers are roots of the auxiliary equation. Table A.6 gives examples of differential equations along with their particular integrals. A,5. Particular Solutions of Linear D,E, — Constant Coefficients 367

Forcing Function, g{t) K Particular Integral, y{t)

(i)c 0 A

(2) h{t) 0 X

(3) csin qt or ccos qt 9 A sin qt — B cos qt

(4) ce"* 9 Ae

(5) ce^* sin qt or ce^* cos qt p + iq AeP* sin ^t - Be^* cos ^i

(6) /i(i)e«* Q Xe«*

(7) /i(t) sin ^t or /i(f) cos qt iq X sin gi + y cos qfi

(8) /i(t)eP*sin gt or /i(i)eP*cos gt p + iq XeP*sin ^i + FeP^cos qt

Notation.

(a) In the forcing function column, p, q, and c are given constants and h{t) is a given polynomial of degree s. (b) In the p>articular integral column, A and B are coefficients to be determined and

X = ^0 + Alt + ... + Ast\ Y = Bo-\-Bit+ ... + Bst^ are s degree whose coefficients are to be determined.

Rules.

(a) If the number in the K column is not a root of the auxiliary equation p(m) = 0, then the proper guess for the particular integral is as shown. (b) If the number in the K colimin is a root of the auxiliary equation of degree r, then multiply the guess in the last column by t^.

Table A.5: Particular Solution Forms for Various Forcing Functions

If the forcing function g{t) is the sum of several functions, 9^=91 + g2 + *"+9ky each having one of the forms in the table, then solve for each Qi separately and add the results together to get the complete solution. In the next table, we wiU apply the formulas and the rules in Table A.6 to obtain particular integrals in specific examples. 368 A. Solutions of Linear Differential Equations

Differential Equation Particular Integral

1.2/'"-32/" = 5 At^

2. y'" - 3y" = l + 3t + 5t^ t^Ao + Ait + A2t'^)

3. 2/"-V + 42/ = 3-i2 Ao + Ait + A2t^

4. y" -4y' + 4y = 2sm t Asm t + Bcos t

5. 2/"-43/'+ 42/= 5sin 2t Asm 2t + Bcos 2t

6. 2/" - 42/' + 4y = lOe^* Ae'^

7. 2/" - 4y' + 42/ = lOe^* i2(Ae2«)

8. 2/" - 42/' + 52/ = e^* cos t t{Ae^^ sin t + Be^* cos t)

9. 2/"-42/' + 52/ = t^sin f X^r_o(^r^'^ sin i + B^V cos i) 10. y" - 42/' + 5y = i^e^* cos t * Er=o(^r*''e2* sin « + B^fe^* cos *)J

Table A.6: Particular Integrals in Specific Examples

A.6

Consider the first-order linear equation

2/' + ay = /(i). (A.5)

If we multiply both sides of the equation by the integrating factor e"*, we get d ^(ye«')=j/'e"*+aye«* = e"V(i). (A.6)

Integrating from 0 to t we have

y{t) = y{0)e--' + T e'^(--^)/(T)rfr, (A.7) Jo which is the complete solution (homogeneous solution plus particular solution) to the equation. A, 7. Reduction of Higher-Order to First-Order Linear Equations 369

A.7 Reduction of Higher-Order Linear Equations to Systems of First-Order Linear Equations

Another way of solving equation (A.l) is to convert it into a system of first-order linear equations. We use the transformations

zi = y, Z2 = y^^\...,zn = y^'' ^\ (A.8) so that (A.l) can be written as

0 1 0 .. 0 zi 0

4 0 0 1 .. 0 ^2 0 +

0 0 0 .. 1 Zn-1 0

Z^ —dn -an- -1 -CLn- -2 • ai Zn 9 J (A.9) In vector form this equation reads

z' = Az + b (A.10) with the obvious definitions obtained by comparing (A.9) and (A. 10). We will present two ways of solving the first-order system (A. 10). The first method involves the matrix exponential function e*"* defined by the power series

/2 42 (A.11) E A;!

It can be shown that this series converges (component by component) for all values of t. Also it is differentiable (component by component) for all values of t and satisfies

^(e*^) = Ae'^ = {e'^)A. (A.12)

By analogy with Section A.6, we try e^* as the integrating factor for (A. 10) to obtain 370 A. Solutions of Linear Differential Equations

(Note that the order of matrix multiphcation here is important.) Using the product rule for matrix multiphcation of fimctions, which can be shown to be vahd, the above equation becomes

dV ' Integrating from 0 to i gives

TA b{r)dT. Jo Evaluating and solving, we have

z{t) = e'^z{0) + e'^ r .-TA b{T)dr, (A.13) Jo The analogy between this equation and (A.6) is clear. Although (A.13) represents a formal expression for the solution of (A. 10), it does not provide a computationally convenient way of getting explicit solutions. In order to demonstrate such a method we assimie that the matrix A is diagonalizable, i.e., that there exists a nonsingtilar square matrix P such that

P-^AP = A. (A.14)

Here A is the diagonal matrix

Ai 0 ••• 0

0 A2 ••• 0 A = (A.15)

0 0 An where the diagonal elements, Ai,..., A^i, are eigenvalues of A, The ith colunm of P is the column eigenvector associated with the eigenvalue A^ (to see this multiply both sides of (A.14) by P on the left). By looking at (A.ll) it is easy to see that p-l^tAp^^tA^ (A.16)

Suppose we make the following definitions: z = Pw, z(0) = Pw{Q), z' = Pw\ (A.17) A. 7. Reduction of Higher-Order to First-Order Linear Equations 371

These in turn imply

w = P-^Z, w{0) = P-h{0), w' = p-^z\ (A.18)

Substituting (A. 17) into (A. 10) gives

Pw' = APw + b, vJ = p-^APw + p-^b, which by using (A. 14) gives

w' = Aw + p-^b. (A.19)

Since A is a diagonal matrix, it is easy to solve the homogeneous part of (A.19), which is w' = Aw. (A.20)

The solution is

Wi = Wi{0)e~^^^ for i = 1,..., n.

We solve (A.19) completely by multiplying through by the integrating factor e~*^:

^(e-'^w) = e'^w' - e-^^Aw = e'^^p-^b. at

Integrating this equation from 0 to ^ gives

w{t) = e^^w(0) + e^^ f e-^^p-^b{T)dT. (A.21) Jo Using the substitutions (A.18) yields

z{t) = {Pe^^p-^)z{0) + Pe^^ f e-^^p-^b{r)dT, (A.22) Jo which is the formal solution to (A. 10). Since well-known algorithms are available for finding eigenvalues and eigenvectors of a matrix, the solution to (A.22) can be found in a straightforward manner. 372 A. Solutions of Linear Differential Equations

A.8 Solution of Linear Two-Point Boundary Value Problems

In linear-quadratic control problems with linear salvage values (e.g., the production-inventory problem in Section 6.1) we require the solution of linear two-point boundary value problems of the form

X 11 An X hi + (A.23) 21 A22 A 62 J with boundary conditions

a;(0) = XQ and A(T) = Ay. (A.24)

The solution of this system will be of the form (A.22), which can be restated as

x{t) Quit) Quit) rr(O) Rx{t) + (A.25) m Q2l{t) Q22{t) . ^(°^ . Mt) where the A(0) is a vector of unknowns. They can be determined by setting AT = Q2i{T)x{0) + Q22(T)A(0) + R^iT), (A.26) which is a system of linear equations for the variables A(0).

A.9 Homogeneous Partial Differential Equations

A homogeneous partial differential equation is an equation containing one or more partial derivatives of an unknown function with respect to its independent variables. If the highest appearing ex­ plicitly in the equation has order n, then the partial differential equation is said to be of order n. As we saw in the previous sections, the general solutions of ordinary differential equations involve expressions containing arbitrary constants. Similarly, the solutions of partial differential equations are expressions containing arbitrary (differentiable) functions. Conversely, when arbi­ trary fimctions can be eliminated algebraically from a given expression, A.9, Homogeneous Partial Differential Equations 373

after suitable partial derivatives have been taken, then the result is a partial differentiable equation.

Example A,l Eliminate the arbitrary function / from the expression u = f{ax — by), where a and b are non-zero constants.

Solution. Taking partial derivatives, we have Ux = af and Uy = —bf so that fru^ + auy = abf - abf = 0. (A.27) Here u = f{ax — by) is a, solution for the equation bux + aUy = 0. To show that any solution u = g{x, y) can be written in this form, we set s = ax — by, and define \s + by G{s,y) = g .y = 9{^^y)'

Then, gx = Gs% = aGs and gy = G,^ + Gy = -bGs + Gy. Since we assume g solves the equation bux + aUy = 0, we have Q = bgx- agy = abGg - abGs + aGy = 0, (A.28) but this implies Gy = 0 so that G is a function oi s = ax — by only, and hence g{x, y) = G{s) = G{ax — by) is of the required form. We conclude that u = f(ax — by) is a general solution form for bux +auy = 0.

Example A.2 Eliminate the arbitrary fimctions /i and /2 from the expression u = fi{x)f2{y).

Solution. Taking partial derivatives, we have

"^x = /i/2, Uy = /1/2, and Uxy = /I/2 so that uuxy - UxUy = /i/2/{/2 - /i/2/1/2 = 0- As in Example A.l we conclude that u = h{^)f2{y) is the general solu­ tion form of the equation uUxy — UxUy = 0. The subject of partial differential equations is too vast to even sur­ vey here. However, Table A.7 gives general solution forms for all the homogeneous partial differential equations we will consider in this book, as well as others. 374 A. Solutions of Linear Differential Equations

Partial Differential General Solution Form

Ekjuation

(1) bux + auy = 0 u = f(ax — by)

(2) XUx + yUy =0, X y/^0 u = f{y/x)

(3) xux — yUy == 0 u = fi^y)

(4) Ux -\-Uy =^ au u = h{x- y)e^ + f2{x - y)e''y

(5) Uz-\-Uy = au^, k ^ 1 u^[{k-\){h{x-y)-ax)Y/^^-'^ ^[k-l)h{x-y)-ay)Y^^^-')

(6) ^xx ^ yy — ^ u=:fi{y + ax) + hiy - ax)

(7) Uxx + CL^Uyy = 0 u = fi{y + iax) -\- f2(y- iax), i = y/^

(8) Uxy =0 u = fi(x) - f2(y)

(9) UUxy —UxUy =0 u = fi(x)f2(y)

(10) UUxy +UxUy=0 u = Mx)/f2{y)

(11) bcux + acuy + abuz = 0 u =: fi{ax - by) ^ f2(by - cz) + fsicz - ax)

(12) xUx + yuy 4- zuz = 0 u = fi(x/y)-\-f2{y/z)

(13) xUx — yUy -1- xuz = 0 u = fi(xy)-\-f2{yz)

(15) U^Uxyz — UxUyUz = 0 u = Mx)f2(y)f3(z)

(16) ^txy^ = 0 u=-fi{x)-\-f2iy)-\-f3{z)

(17) Uxx — 0^{Uyy 4- Uzz) = 0 u = fi(y + ax) + f2{y - ax)-\- h{z + ax) +f4{z-ax)

(18) Ux -\- Uy + Uz = au u = h{x- y)e^ + h{y- ^)e"^ + h{z - ^)e"' [

Note. The function fi are arbitrary differentiable functions of a single variable; a, 6, c,... stand for arbitrary (non-zero) constants.

Table A.7: General Solution Forms for Some Homogeneous Partial Differential Equations

A. 10 Inhomogeneous Partial Differential Equations

As in the ordinary case, an inhomogeneous partial differential equation is obtained from a homogeneous one by adding one or more forcing A.iJ. Solutions of Finite Difference Equations 375 functions. The general case of this problem is too difficult to treat here. We consider only the case in which the forcing functions are separable, i.e., can be written as a sum of functions each involving only one of the independent variables. In solving such problems we can make use of the solutions to ordinary differential equations considered earlier.

Example A.3 Solve the partial differential equation

Ux + Uy = 3x + e^.

Solution. We know from the previous section that the general solution to the homogeneous equation is of the form f{x—y). To get the particular solutions we solve separately the ordinary equations

Ux = Sx^ and Uy = e^^ obtaining solutions x^ and e^. Therefore, the general solution to the original equation is u = f{x-y)+x^ + e^. Generally speaking, the above philosophy of finding particular solu­ tions to separable partial differential equations (when it works) follows the same method of "dividing and conquering." Other methods involve the use of series. We will not go further here for lack of space.

A. 11 Solutions of Finite Difference Equations

In this book we will have uses for finite difference equations only in Chapters 8 and 9. For that reason we will give only a brief introduction to solution techniques for them. Readers who wish more details can consult one of several texts on difference equations; see, e.g., Goldberg (1986) or Spiegel (1971). If f{k) is a real function of time, then the difference operator applied to / is defined as

A/(fc) = /(fc + l)-/(fc). (A.29) The factorial power of k is defined as

A;(") = k{k - l)ik -2)...{k-{n- 1)). (A.30) It is easy to show that Aifc("> = nfc("-i). (A.31) 376 A. Solutions of Linear Differential Equations

Because this formula is similar to the corresponding formula for the derivative d{k'^)/dk^ the factorial powers of k play an analogous role for finite differences that the ordinary powers of k play for differential calculus. K /(fc) is a real function of time, then the anti-difference operator A~-^ applied to / is defined as another fimction g — A~^f{k) with the property that

A^ = f(k). (A.32) One can easily show that

A-ifcW - (l/(n + l))fc(^+i) + c, (A.33) where c is an arbitrary constant. Equation (A.33) corresponds to the integration formula for powers of k in calculus. Note that formulas (A.31) and (A.33) are similar to, respectively, differentiation and integration of the power function k"^ in calculus. By analogy with calculus, therefore, we can solve difference equations in­ volving polynomials in ordinary powers of k by first rewriting them as polynomials involving factorial powers of k so that (A.31) and (A.33) can be used. We show next how to do this.

A. 11.1 Changing Polynomials in Powers of k into Facto­ rial Powers of k We first give an abbreviated list of formulas that show how to change powers of k into factorial powers of k: fcO = fc(o) = 1 (by definition), k'=k('\

fc4 = fc(i)+7/c(2)+6fc(3)+fcW, k^ = fc(i) + 15fc(2) + 25fc(^) + lOfcW + fc(^).

The coefficients of the factorial powers on the right-hand sides of these equations are called Stirling numbers of the second kind, after the person who first derived them. This list can be extended by using a A,ll. Solutions of Finite Difference Equations 377

more complete table of these nimibers, which can be found in books on difference equations cited earlier.

Example A.4 Express A;^ — 3fc + 4 in terms of factorial powers.

Solution. Using the equations above we have

k^ = fed) + 7fc(2) + 6fc(3) + fcW, -3k = -3fc(i), 4 = 4,

SO that

Example A.5 Solve the following difference equation in Example 8.7:

AA^ = -k + 6,X^ = 0.

Solution. We first change the right-hand side into factorial powers so that it becomes AA^ = -fc(^) + 5. Applying (A.33), we obtain

A^ = -(l/2)fc(2) + 5fcW + c, where c is a constant. Applying the condition A^ = 0, we find that c= —15, so that the solution is

A^ = -(l/2)fc(2) + 5fc(^) - 15. (A.34) However, we would like the answer to be in ordinary powers of fc. The way to do that is discussed in the next section.

A. 11.2 Changing Factorial Powers of k into Ordinary Powers of k In order to change factorial powers of k into ordinary powers of fc, we make use of the following formulas: fc(i) = k, ki'^) = ^k + k\ fc(3) = 2k- 3A;2 + k^, k^^) = -6k + llfc2 _ 6^3 ^ ^4^ 378 A. Solutions of Linear Difkrential Equations

fc(5) = 24k - 50A;2 + 35k^ - lOJt^ + k^.

The coefficients of the factorial powers on the right-hand sides of these equations are called Stirling numbers of the first kind. This list can also be extended by using a more complete table of these nimibers, which can be found in books on difference equations.

Solution of Example A.5 Continued. By substituting the first two of the above formulas into (A.34), we see that the desired answer is

A'^ = -(1/2)A;2 + (n/2)k - 15, (A.35) which is the solution needed for Example 8.7.

EXERCISES FOR APPENDIX A

3 2 5 0 1 1 A.l If ^ = show that A andF = 2 3 0 2 1 -1

Use (A.22) to solve (A. 10) for this data, given that ^(O) =

3 3 6 0 1 3 A.2 If A , show that A = andP = 2 4 0 1 1 -2

0 Use (A.22) to solve (A. 10) for this data, given that z{0) = 5

A.3 After you have read Section 6.1, re-solve the production-inventory example stated in equations (6.1) and (6.2), (ignoring the control constraint (P > 0) by the method of Section A.8. The linear two-point boundary value problem is stated in equations (6.6) and (6.7). Appendix B Calculus of Variations and Optimal Control Theory

Here we introduce the subject of the calculus oi variations by analogy with the classical topic of maximization and miniiir:zation in calculus; see Gelfand and Fomin (1963), Young (1969), and Leitmann (1981) for rigorous treatments of the subject. The problem of the calculus of varia­ tions is that of determining a function that maximizes a given functional, the objective fimction. An analogous problem in calculus is that of de­ termining a point at which a specific function, the objective function, is maximum. This, of course, is done by taking the first derivative of the function and equating it to zero. This is what is called the first-order condition for a maximum. A similar procedure will be employed to de­ rive the first-order condition for the variational problem. The analogy with classical optimization extends also to the second-order maximiza.- tion condition of calculus. Finally, we will show the relationship between the maximum principle of optimal control theory and the necessary con­ ditions of the calculus of variations. It is noted that this relationship is similar to the one between the Kuhn-Tucker conditions in mathematical programming and the first-order conditions in classical optimization. We start with the "simplest" variational problem in the next section.

B.l The Simplest Variational Problem

Assume a function x : C-^[0^t] —> E^, where C^[O^T] is a class of func­ tions defined over the interval [0,T] with continuous first derivatives. (For simplicity in exposition, assimie x to be a scalar fimction. The 380 B. Calculus of Variations and Optimal Control Theory extension to a vector function is straightforward.) Assume further that a function in this class is termed admissible if it satisfies the terminal conditions x{0) = xo and x{T) = XT- (B.l) We are thus dealing with a fixed-end-point problem. Examples of admis­ sible functions for the problem are shown in Figure B.l; see Section 6 and Chapters 2 and 3 of Gelfand and Fomin (1963) for problems other than the simplest problem, i.e., the problems with other kinds of conditions for the end points.

0 T

Figure B.l: Examples of Admissible Functions for the Problem

The problem under consideration is to obtain the admissible function X* for which the fimctional fT J{x) = / g{x^x^t)dt (B.2) Jo has a relative maximum. We will assume that all first and second partial derivatives of the function g : E^ x E^ x E^ —^ E^ are continuous.

B.2 The Euler Equation

The necessary first-order conditions in classical optimization were ob­ tained by considering small changes about the solution point. For the B.2, The Euler Equation 381

variational problem, we consider small variations about the solution func­ tion. Let x{t) be the solution and let y{t)=x{t) + sri{t), where T]{t) : C^[0,T] —» E^ is an arbitrary continuously satisfying r){0) = r,{T) = 0, (B.3) and £ > 0 is a small number. A sketch of these functions is shown in Figure B.2.

Figure B.2: Variation about the Solution Function

The value of the objective functional associated with y{t) can be considered a fimction of s, i.e., fT V{e) = J{y) = g{x + erj, x + ef],, t)dt Jo However, x(t) is a solution and therefore V{e) must have a maximum at £ = 0. This means A dV\ -0, de e=Q where 8J is known as the variation 8J in J. Differentiating V{e) with respect to e and setting e — Q yields

8J = = / {9xV + 9xf])dt = 0, de =0 ^0 382 B. Calculus of Variations and Optimal Control Theory which after integrating the second term by parts provides

«5^ = -^1 = j^ 9xVdt + {9iv)\o - I Jt^9i)vdt = 0. (B.4)

Because of the end conditions on 77, the expression simplifies to

= [9x- •:^9x]vdt = 0. de zzo Jo dt We now use the fundamental lemma of the calculus of variations which states that if /i is a continuous fimction and /Q h(t)rj{t)dt = 0 for every r]{t)^ then h{t) — 0 for all t G [0, T]. The reason that this lemma holds, without going into details of a rigorous proof which is available in Gelfand and Fomin (1963), is as follows. Suppose that h(t) ^ 0 for some t G [0,T]. Since h{t) is continuous, there is, therefore, an interval (^1, ^2) C [0, T] over which h is nonzero and has the same sign. Now selecting r/(t) such

>0, te{ti,t2) r]{t) is < 0, otherwise, it is possible to make the integral /Q h(t)r){t)dt 7^ 0. Thus, by contrar- diction, h{t) must be identically zero over the entire interval [0,T]. By using the fimdamental lemma, we have the necessary condition

9x - -Tjgx = 0 (B.5) known as the Euler equation, which must be satisfied by a maximal solution X*, We note that the Euler equation is a second-order ordinary differen­ tial equation. This can be seen by taking the total time derivative of QX and collecting terms:

^9xx + i^9xx + {9tx - 9x) = 0.

The boimdary conditions for this equation are obviously the end-point conditions x{0) = XQ and x{T) = x^- B.3. The Shortest Distance Between Two Points on the Plane 383

Special Case (i) When g does not depend explicitly on x.

In this case, the Enler equation (B.5) reduces to

which is nothing but the first-order condition of classical optimization. In this case, the dynamic problem is a succession of static classical optimization problems.

Special Case (ii) When g does not depend explicitly on x.

The Euler equation reduces to

j^9^ = 0, (B.6) which we can integrate as

gx = constant. (B.7)

Special Case (iii) When g does not depend explicitly on t,

Finally, we have the important special case in which g is explicitly independent of t. In this case, we write the Euler equation (B.5) as

j^{9-i9x)-9t = 0. (B.8)

But gt = 0 and therefore we can solve the above equation as

g-xgx = C, (B.9) where C is the constant of the integration.

B.3 The Shortest Distance Between Two Points on the Plane

The problem is to show that the straight line passing through two points on a plane is the shortest distance between the two points. The problem 384 B. Calculus of Variations and Optimal Control Theory can be stated as follows:

imn Jo subject to

x(0) = xo and x{T) = XT-

Here t refers to distance rather than time. Since g = — vT+^ does not depend explicitly on x^ we are in the second special case and the first integral (B.7) of the Euler equation is

This implies that i is a constant, which results in the solution

x{t) = Cit + C2, where Ci and C2 are constants. These can be evaluated by imposing boimdary conditions which give Ci — {XT — xo)/T and C2 = XQ. Thus,

XT-XQ x{t) = t + Xo, T which is the straight line passing through XQ and XT-

B.4 The Brachistochrone Problem

The problem arises from the search for the shape of a wire along which a bead will slide in the least time from a given point to another, imder the influence of gravity; see Figure 1.1. The Brachistochrone problem has a long history. It was first studied (incorrectly) by Galileo in 1630. The problem was correctly posed by Johann Bernoulli in 1696 and later solved by Johann Bernoulli, Jacob Bernoulli, Newton, and L'Hospital. Note that Euler deduced the Euler's equation in 1744, and we will solve the Brachistochrone problem using Euler's equation. But first we must formulate the problem. Assimie the bead slides with no friction. Let m denote the mass of the bead, s denote the arc length, t denote the horizontal axis, x denote the vertical axis (measured vertically down), and r denote the time. Assimie ^0 = 0, rr(io) = 0, T-l, x{T) = l. B.4. The Brachistochrone Problem 385

We wish to minimize ^^ ds Jo Jo V where v represents velocity, and ST is the final displacement measured on the curve. We can write

ds = \/l + x'^dt

and, from elementary physics, it is known that if v{to = 0) — 0 and a denotes the gravitational acceleration constant, then

v = V2^ax, Therefore, the variational problem can be stated as

, 1 f'/* ! h 1+x^ min^TT- / \l—- dt \2aJo V X where x = dx/dt (note that t does not denote time), and x(0) = 0 and x{l) = 1. Since a is a constant, we can rewrite the problem as

mm J{x) = / g(x^x,t)dt= / \ dt> .

Since g does not depend explicitly on f, the problem belongs to the third special case. Using the first integral (B.9) of the Euler equation for this case, we have

1/2 i;2[x(l+i2)]-i/2 1+x^ = Ci (a constant). X

We can reduce this to dx dt y xCf To solve this equation, we separate the variables as

•^xdx dt ^l ""x 386 B. Calculus of Variations and Optimal Control Theory and substitute

X = (sin^ e)/Cl = (1 - cos 26)/Cl (B.IO)

The resulting expression can be integrated to yield

t=[e- (1/2) sm2e]/Cf + C2. (B.ll)

The condition ^ = 0 at ^ = 0 implies C2 = 0 providing Cf > 0. The value of Cl can be obtained in terms of the value of 6 at t = 1; let this be 9i. Then, since a; = 1 at ^ = 1, we have Ci = sin 6, where Oi satisfies

26>i --I = sin20i-cos26>i.

This equation must be solved nimierically. An iterative numerical pro­ cedure yields 0i = 1.206 and therefore Cf = 0.873. Defining 0 = 26, we can write (B.IO) and (B.ll) as

X = 0.573(1-cos 20), t = 0.573(0-sin0), which are equations of a cycloid in the parametric form. The shape of the curve is shown in Figure 1.1 in Chapter 1.

B.5 The Weierstrass-Erdmann Corner Conditions

So far we have only considered functionals defined for smooth curves. This is, however, a restricted class of curves which qualify as solutions, since it is easy to give examples of variational problems which have no solution in this class. Consider, for example, the objective functional

min h{x) = I x'^il - xfdt\ , x(-l) = 0, x{l) = 1.

The greatest lower bound for J(x) for smooth x = x{t) satisfying the boimdary conditions is obviously zero. Yet there is no x € C-^[—1,1] with x(—l) = 0 and x{l) = 1, which achieves this value of J(x). In fact, the minimum is achieved for the curve

0, -1 < < < 0, x(t) t, 0

which has a comer (i.e., a discontinuous first derivative) at t = 0, Such a piecewise smooth extremal with corners is called a broken extremal We now enlarge the class of admissible functions by relaxing the requirement that they be smooth everywhere. The larger class is the class of piecewise continuous functions which are continuously differentiable almost everywhere in [0,r], i.e., except at some points in [0, T]. Let x{t) be an extremal with a corner at r G [0, T]. Let us decompose J{x) as

rT PT PT J{x) = / g{x,x^t)dt= / g{x,x,t)dt+ / g{x^x,t)dt Jo Jo JT = Ji{x)+J2{x), It is clear that on each of the intervals [0, r) and {r^T], the Euler equation must hold. To compute variations SJi and SJ2, we must recognize that the two 'pieces' oi x are not fixed-end-point problems. We must require that the two pieces of x join continuously at ^ = r; the point t = T can, however, move freely as shown in Figure B.3.

Figure B.3: A Broken Extremal with Corner at r

This will require a slightly modified version of formula (B.4) for writ­ ing out the variations; see pp. 55-56 in Gelfand and Fomin (1963). Equat­ ing the sum of variations SJ = SJi +SJ2 = 0 388 B. Calculus of Variations and Optimal Control Theory for x{t) to be an extremal and using the fact that x{t) must be continuous at t = r imphes g^l^- = g^l^^ , (B.12) [9 - igxlr- = [9- i9x]T+' (B.13) These conditions are called Weierstrass-Erdmann corner conditions, which must hold at the point r where the extremal has a corner. In each of the interval [0, r) and (r, t], the extremal x must satisfy the Euler equation (B.5). Solving these two equations will provide us with four constants of integration since the Euler equations are second-order differential equations. These constants can be foiind from the end-point conditions (B.l) and Weierstrass-Erdmann conditions (B.12) and (B.13).

B.6 Legendre's Conditions: The Second Variation

The Euler equation is a necessary conditions analogous to the first-order condition for a maximum (or minimimi) in the classical optimization problems of calculus. The condition analogous to the second-order nec­ essary condition for a maximum is the Legendre condition

9xx < 0. (B.14)

To obtain this condition, we use the second-order condition of classical optimization on function V(e) to be a maximum at e = 0, i.e.,

d^V{e) PT / {9xxrj^ + '^gxxm + 9xxf]^)dt < 0. (B.15) de'^ e=0 JO Integrating the middle term by parts and using (B.3), we can transform (B.15) into a more convenient form

/ {Qri^ + Pfi^)dt

While it is possible to rigorously obtain (B.14) from (B.16), we will only provide a qualitative argument for this. If we consider the quadratic functional (B.16) for functions r]{t) satisfying 77(0) = 0, then r]{t) will be B.7, Necessary Condition for a Strong Maximum 389

small in [0, T] if f]{t) is small in [0, T], The converse is not true, however, since it is easy to construct r]{t) which is small but has a large derivative 7]{t) in [0,r]. Thus, Pfj^ plays the dominant role in (B.16); i.e., Pif can be much larger than Qrf but it cannot be much smaller (provided P 7^ 0). Therefore, it might be expected that the sign of the fimctional in (B.6) is determined by the sign of the coefficient P(t), i.e., (B.16) implies (B.14). For a rigorous proof, see Gelfand and Fomin (1963). We note that the strengthened Legendre condition (i.e., with a strict inequahty in (B.14)), the Euler equation, and one other condition called strengthened Jacobi condition are sufficient for a maximum. The reader can consult Chapter 5 of Gelfand and Fomin (1963) for details.

B.7 Necessary Condition for a Strong Maximum

So far we have discussed necessary conditions for a weak maximum. By weak maximum we mean that the candidate extremals are smooth or piecewise smooth functions. The concept of a strong m>axim.um, on the other hand requires that the candidate extremal need only be continuous functions. Without going into details, which are available in Gelfand and Fomin (1963), we state a necessary condition for a strong maximum. This is called the Weierstrass necessary condition. The condition is analogous to the one in the static case that the objective function be concave. It states that if the fimctional (B.2) has a strong maximum for the extremal 7 satisfying (B.l), then E{x,x,t,v) <0 (B.17) along 7 for every finite v, where E is the Weierstrass Excess Function defined as E{x^ i, t, v) = g{x^ v^ t) — g{x^ i, t) — gxi^i ^, t){'^ — x), (B.18) Note that this condition is always met if g{x^x^t) is concave in x. The proof of (B.17) is by contradiction. Suppose there exists a r G [0, T] and a vector q such that E{T,x{T),x{T),q) >0, where x = x(t) is the equation of the extremal 7. It is then possible to suitably modify 7 to /? which is close to 7 in C^[0,T] such that

^J — I g{x^x^t)dt— / g{x^x^t)dt > 0, JB J-i 390 B. Calculus of Variations and Optimal Control Theory contradicting the hypothesis that J{x) has a strong maximum for 7.

B.8 Relation to the Optimal Control Theory

It is possible to derive the necessary conditions of the calculus of varia­ tions from the maximum principle. This is strongly reminiscent of the relationship between the first-order conditions of classical optimization and the Kuhn-Tucker conditions of mathematical progranmiing. First, we note that the calculus of variations problem can be stated as an optimal control problem as follows:

max J — j g{x,u^t)dt >

subject to

X = n, x(0) = xo, x(T) = XT^

The Hamiltonian is

H(x^ u^ A, t) = g(x^ u, t) + Xu (B.19) with the adjoint variable A satisfying

A = —Hx = —Qx- (B.20)

Maximizing the Hamiltonian with respect to u yields

Hu = 9x + ^=> ^ = -Qx- (B.21)

Differentiating with respect to time, we have

d A dt 9x' This equation with (B.20) implies

9x - ^9x = 0, which is the Euler equation of the calculus of variations. B.8, Relation to the Optimal Control Theory 391

The second-order condition for the maximization of the Hamiltonian, i.e., Huu < 0 => pi;i < 0, which is the Legendre condition. Again, by the maximum principle, if u is an optimal control, then

H{x,t,X,t) >H{x,v,\t), where v is any other control. By the definition of the Hamiltonian (B.19) and equation (B.21), we have

g{x, i, t) - QxX > g(x, v, t) - g±v, which by transposition of terms yields the Weierstrass necessary condi­ tion E(x^ ir, i, v) = g{x^ v^ t) — g(x^ i, t) — gx{v — i) < 0. We have just proved the equivalence of the maximum principle and the Weierstrass necessary condition in the case where Q is open. In cases when O is closed and when the optimal control is on the boundary of f2, the Weierstrass necessary condition is no longer valid, in general. The maximum principle still applies, however. Finally, according to the maximum principle, both A and H are con­ tinuous functions of time. However,

A == -gx and H = g - g±x, which means that the right-hand sides must be continuous with respect to time, i.e., even across corners. These are precisely Weierstrass-Erdmann corner conditions. Appendix C An Alternative Derivation of the Maximum Principle

Recall that in the derivation of the maximum principle in Chapter 2, we assumed the twice differentiability of the return function V, Looking at (2.32), we can observe that the smoothness assumptions on the return function do not arise in the statement of the maximum principle. Also since it is not an exogenously given fimction, there is no a priori reason to assume the twice differentiabihty. In many important cases as a matter of fact, V has no derivatives at individual points, e.g., at points on switching manifolds. In what foUows, we wiU give an alternate derivation. This proof fol­ lows the course pointed out by Pontryagin et al. (1962) but with certain simplifications. It appears in Fel'dbaum (1965) and, in our opinion, it is one of the simplest proofs for the maximiim principle which is not related to dynamic programming and thus permits the elimination of assumptions about the differentiability of the return fimction V(t, x), We select the Mayer form of the problem (2.5) for deriving the max­ imum principle in this section. It will be convenient to reproduce (2.5) here as (C.l):

max {J = cx(T)} u{t)en{t) subject to (C.l) X = f{x,U,t), x{0) =Xo, 394 C. An Alternative Derivation of the Maximum Principle

C.l Needle-Shaped Variation

Let u*{t) be an optimal control with corresponding state trajectory x*{t). We sketch u*{t) in Figure C.l and x*{t) in Figure C.2 in a scalar case. Note that the kink in x*{t) aX t = 9 corresponds to the discontinuity in u*{t) 8itt = e.

•-> t 6 T-8 T T

Figure C.l: Needle-Shaped Variation

Let r denote any time in the open interval (0,r). We select a suffi­ ciently small e to insure that r — e > 0 and concentrate our attention on this small interval (r — e^r]. We vary the control on this interval while keeping the control on the remaining intervals [0, r — s] and (r, T] fixed. Specifically, the modified control is

^-> t 0 T-s T r

Figure C.2: Trajectories x*{t) and x(t) in a One-Dimensional Case. C.l. Needle-Shaped Variation 395

V eft, t e (r — s,r], u{t) = I (C.2) u*{t), otherwise. This is called a needle-shaped YSuYiaXion as shown in Figure C.l. It is a jump function and is different from variations in the calculus of variations (see Appendix B). Also the difference v —u* is finite and need not be small. However, since the variation is on a small time interval, its influence on the subsequent state trajectory can be proved to be 'small'. This is done in the following. Let the subsequent motion be denoted by x{t) ^ x*{t) for t > r — s, In Figure 0.2, we have sketched x{t) corresponding to u{t). Let 6x{t) = x{t) - a;*(i), t>T-e, denote the change in the state variables. Obviously 6X{T — S) = 0. Clearly, Sx{T)^e[x{s)-x\s)], (C.3) where s denotes some intermediate time in the interval (r — e^r]. In particular, we can write (C.3) as

6x{r) = 4H^) - ^*('^)] + ^(^) = ^[/(x(r),^,r) -/(x*(r),u*(r),r] + o(s). (C.4)

But Sx{r) is small since / is assumed to be boimded. Furthermore, since / is continuous and the difference 6X{T) = X(T) — X*{T) is small, we can rewrite (C.4) as

6x{t) « e[f{x*{T),v,T)-f{x*{r),u*iT),T)]. (C.5)

Since the initial difference SX(T) is small and since U*{T) does not change from t > T on, we may conclude that Sx{t) will be small for aU t > r. Being small, the law of variation of Sx{t) can be foimd from linear equa­ tions for small changes in the state variables. These are called variational equations. From the state equation in (C.l), we have

^^El^M. = f(a:* + Sx,u*,t) (C.6) or, ^ + ^^fix\u*,t)+fjx (C.7) 396 C. An Alternative Derivation of the Maximum Principle or using (C.l),

-r{Sx) ^ U{x\ u*, t)6x, for t > r, (C.8) tit/ with the initial condition 6X{T) given by (C.5). The basic idea in deriving the maximum principle is that equations (C.8) are linear variational equations and result in an extraordinary sim­ plification. We next obtain the adjoint equations.

C.2 Derivation of the Adjoint Equation and the Maximum Principle

For this derivation, we employ two methods. The direct method, similar to that of Hartberger (1973), is the consequence of directly integrating (C.8). The indirect method avoids this integration by a trick which is instructive.

Direct method. Integrating (C.8) we get

6x{T) = 6x{r)+ f f^[x\t),u\t),t\8x{t)dt, (C.9) where the initial condition 6X{T) is given in (C.5). Since 8x{T) is the change in the terminal state from the optimal state x*(r), the change in the objective function 6J must be negative. Thus,

8J = c8x{T) = C8X{T) + f cfx[x%t),u*{t),t\8x{t)dt < 0. (CIO)

Furthermore, since (C.8) is a linear homogeneous differential equation, we can write its general solution as

8x{t) = ^t,T)8x{T), (C.ll) where the fundamental solution matrix or the transition matrix $(t, r) G

^$(^,r) = Ux%t),u*m^t,r), ^T,T) ^ I, (C.12) where / is an n x n identity matrix; see Appendix A. C.2. Derivation of Adjoint Equation and the Maximum Principle 397

Substituting for 6x{t) from (C.ll) into (C.IO), we have

6J = C6X{T) + / cfa,[x*{t), u\t), t]^{t, T)8x{T)dt < 0. (C.13)

This induces the definition

X*(t) = J cf4x'{t),u*{t),t]^t,r)dt + c, (C.14)

which when substituted into (C.13), yields

SJ=^X*{T)SX{T)<0, (C.15)

But Sx{r) is supphed in (C.5). Noting that £ > 0, we can rewrite (C.15) as X*{T)f[x*(T),v,T]-X*(r)f[x*iT),u*iT),T]<0. (C.16) Defining the Hamiltonian for the Mayer form as

H[x, u, A, t] = A/(x, u, t), (C.17) we can rewrite (C.16) as

if[:r*(r),u*(r),A(r),r] >ff[ar*(r),t;,A(r),r]. (C.18)

Since this can be done for almost every r, we have the required Hamil­ tonian maximizing condition. The differential equation form of the adjoint equation (C.14) can be obtained by taking its derivative with respect to r. Thus,

-C/X[X'(T),M'(T),T]. (0.19)

It is also known that the transition matrix has the property:

^^^ = -$(i,r)/,[x*(r),«*(r),T], which can be used in (C.19) to obtain

rfA(r) rT ^^ -j cU[x*{t),u*{t),t]^{t,T)U[x*{Tlu*{r),T]dt -cU[x*{T)X{r),T]. (C.20) 398 C. An Alternative Derivation of the Maximum Principle

Using the definition (C.14) of A(r) in (C.20), we have

^ = -A(r)/,[x*(r),«*(r),r] with A(T) = c, or using (C.17) and noting that r is arbitrary, we have

A= -A/^[x*,n*,t] = -H^[x*,u\\t), X(T) = c. (C.21)

This completes the derivation of the maximum principle along with the adjoint equation using the direct method.

Indirect method. The indirect method employs a trick which simplifies considerably the derivation. Instead of integrating (C.8) explicitly, we now assume that the result of this integration yields cSx{T) as the change in the state at the terminal time. As in (C.IO), we have

6J = c6x{T) < 0. (C.22)

First, we define A(r) = c, (C.23) which makes it possible to write (C.22) as

SJ - c6x{T) = X{T)6x(T) < 0. (C.24)

Note parenthetically that if the objective fimction J = S{x{T)), we must define A(r) = dS[x{T)]/dx{T) giving us

*^ = ^i^^^(^) = A(r)6^(T).

Now, X(T)Sx{T) is the change in the objective fimction due to a change Sx{T) at the terminal time T. That is, A(T) is the marginal return or the marginal change in the objective function per unit change in the state at time T. But Sx{T) cannot be known without integrating (C.8). We do know, however, the value of the change SX{T) at time r which caused the terminal change Sx(T) via (C.8). We would therefore like to pose the problem of obtaining the change 6J in the objective function in terms of the known value SX{T); see FeFdbaimi (1965). Simply stated, we would like to obtain the marginal return A(r) per unit change in state at time r. Thus,

X{T)SX{T) = 6J = X{T)SX{T) < 0. (C.25) C,2. Derivation of Adjoint Equation and the Maximum Principle 399

Obviously, knowing A(r) will make it possible to make an inference about SJ which is directly related to the needle-shaped variation applied in the small interval (r — e^r], However, since r is arbitrary, our problem of finding A(r) can be translated to one of finding A(t), t G [0,r], such that

X{t)6x{t) = \{T)6x{T), t e [0,T], (C.26)

or in other words,

X(t)Sx{t) = constant, A(r) = c. (C.27)

It turns out that the differential equation which X{t) must satisfy can be easily foimd. From (C.27),

^^[X(t)Sx{t)] = A^ + XSx = 0, (C.28) which after substituting for dSx/dt from (C.8) becomes

Xfjx + X6x = (A/^ + X)Sx = 0. (C.29)

Since (C.29) is true for arbitrary 6x, we have

A = -XU = -Ha: (C.30) using the definition (C.17) for the Hamiltonian. The Hamiltonian maximizing condition can be obtained by substi­ tuting for 6x{r) from (C.5) into (C.25). This is the same as what we did in (C.15) through (C.18). The purpose of the alternative proof was to demonstrate the valid­ ity of the maximum principle for a simple problem without knowledge of any return function. For more complex problems, one needs compli­ cated mathematical analysis to rigorously prove the maximum principle without making use of return fimctions. A part of mathematical rigor is in proving the existence of an optimal solution without which necessary conditions are meaningless; see Young (1969). Appendix D

Special Topics in Optimal Control

In this appendix we will discuss three specialized topics. These are linear- quadratic problems, second-order variations, and singular control. These topics are referred to but not discussed in the main body of the text because of their advanced nature. While we shall not be able to go into a great detail, we will provide an adequate description of these topics and list relevant references.

D.l Linear-Quadratic Problems

An important problem in systems theory, especially engineering sciences, is to synthesize feedback controllers. These controllers provide optimal control as a function of the state of the system. A usual method of ob­ taining these controllers is to solve the Hamilton-Jacobi-Bellman partial differential equation (2.19). This equation is nonlinear in general, which makes it very difficult to solve in closed form. Thus, it is not possible in most cases to obtain optimal feedback control schemes explicitly. It is, however, feasible in many cases to obtain perturbation feedback control, which refers to control in the vicinity of an optimal path; see Bryson and Ho (1969). These perturbation schemes reqiiire the approx­ imation of the problem by a linear-quadratic problem in the vicinity of an optimal path (see Section D.2), and feedback control for the approx­ imating problem is easy to obtain. A linear-quadratic control problem is a problem with Unear dynamics 402 D, Special Topics in Optimal Control and quadratic objective function. More specifically, it is:

max I J = -x'^Gx + - I {x^Cx + u^Du)dt I (D.l) u subject to x = Ax + Bu, x{Q) =- XQ. (D.2) The matrices G, C, D, A, and B are in general time dependent. Fur­ thermore, the matrices G, (7, and D are assumed to be negative definite and the superscript ^ denotes the transpose operation. Note that this problem is a special case of Row (c) of Table 3.3. To solve this problem for the explicit feedback controller, we write the Hamilton-Jacobi-Bellman equation (2.19) as

0 = max[if + Vt] = max I--(a:^Cx + u^Du) + V^\Ax + Bu\ -\-v\ (D.3) with the terminal boundary condition

V{x,T) = \x^Gx. (D.4)

The maximization of the maximand in (D.3) can be carried out by taking its derivative with respect to u and setting it to zero. Thus,

^M^ = ^ = (^Duf + V,B = 0=^u' = -V,B{D^)-\ (D.5) au ou Note that (D.5) is the same as the Hamiltonian maximizing condition. Substituting (D.5) in (D.3) and simplifying, we obtain

0 = ^x^Cx + V^Ax - ^V^BD-^B^V^. (D.6)

This is a nonlinear partial differential equation of first order and it has a solution of the form

V{x,t) = lx'^S{t)x. (D.7)

Substitution of (D.7) into (D.6) yields

0 = \x^\S + 5A + ^S - SBD-^B^S + C\x. (D.8) D.l, Linear-Quadratic Problems 403

Since (D.8) must hold for all x, it implies the following matrix differential equation S + SA + A^S - SBD'^B^S + C = 0, (D.9) called a matrix Riccati equation, with the boundary condition

S(T) = G. (D.IO)

A solution procedure for Riccati equations appears in Bryson and Ho (1969). Once we have the solution S(t) of (D.9) and (D.IO), the optimal feedback control can be written as

u\t) = D{t)-^B' {t)S{t)x{t), (D.ll)

A generaUzation of (D.l), which would be useful in the next section on the second variation, is to set

1 T C N X J = -x^ Gx + jf'^^"^) dt. (D.12) N^ D u

The state equation is given by (D.2). It is possible to derive the optimal control for this problem as

u*{t) = Dity^iN'^it) + B'^{t)S{t)]x{t), (D.13) where

S+SA+A^S-{SB+N)D-^(B^S+N^)+C = 0, S{T) = G. (D.14)

For other variations and extensions of the linear-quadratic problem (D.l) and (D.2), for which explicit feedback controllers can be developed, the reader is referred to Bryson and Ho (1969).

D.1.1 Certainty Equivalence or Separation Principle Suppose equation (D.2) is changed by the presence of a Gaussian white noise w{t) and becomes

X = Ax + Bu + w, where E[w{t)] = 0, E[w{t)w{Tf] = Q{t)6{t - r), 404 D. Special Topics in Optimal Control and x{0) is a normal random variable with

E[x{0)] = 0, E[x{0)x{Of] = Po.

Because of the presence of uncertainty in the system equation, we must modify the objective fimction in (D.12) as follows:

( \ C N ' X max < J = E ^x'^Gx + ^{x^,u^) dt\ iV^ D ^ [u] Assume further that x cannot be directly measured and the measure­ ment process is given by (13.21), i.e.,

y{t) = H{t)x{t)+v{t), where v{t) is a white noise as defined in (12.72). The optimal control u* (t) for this linear-quadratic stochastic optimal control problem can be shown to be given by (D.13) with x{t) replaced by its estimate x{t); see Bryson and Ho (1969). Thus,

n*(t) - D{t)-^[N^{t) + B^it)S{t)]x{t), where S is given in (D.14) and x is given by the Kalman-Bucy filter.

'x = Ax + Bu*+w + K[y-Hx], :r(0) = 0, K - PH^R-\ P = AP + PA^ -KHP + Q, P{0)^Po,

The above procedure has received two different names in the liter­ ature. In economics it is called the certainty equivalence principle; see Simon (1956). In engineering and literature it is called the separation principle, Joseph and Tou (1961). When we call it the cer­ tainty equivalence principle, we are emphasizing the fact that x(t) can be used for the purposes of optimal feedback control as if it were the certain value of the state variable x{t). Whereas the term separation principle emphasizes the fact that the process of determining the optimal control can be broken down into two steps: first, estimate x by using the optimal filter; second, use that estimate in the optimal feedback control formula for the deterministic problem. D,2. Second-Order Variations 405

D.2 Second-Order Variations

Second-order variations in optimal control theory are analogous to the second-order conditions in the classical optimization problem of calculus. To discuss the second-order variational condition is difficult when the control variable u is constrained to be in the control set fl. So we make the simplifying assimiption that Q = R^, and thus the control u is unconstrained. As a result, we are now dealing with the problem:

max \j=f F{x, u, t)dt + ^x{T)] \ (D.15)

subject to X = f{x^ u, t), x{0) = xo> (D.16) From Chapter 2, we know that the first-order necessary conditions for this problem are given by

A = -Ha:, A(T) - 0, (D.17)

Hu = 0, (D.18) where the Hamiltonian H is given by

H = F + Xf, (D.19)

Since u is unconstrained, these conditions may be easily derived by the method of calculus of variations. To see this, we write the augmented objective fimctional as

J = ^x(T)] + I [Hix, u, A, t) - Xx]dt (D.20) Jo Consider small perturbation from the extremal path given by (D.16) - (D.19) as a result of small perturbations 6x{0) in the initial state. Define the resulting perturbations in state, adjoint, and control variables by Sx{t), <5A(i), and 6u(t), respectively. These, of course, will be obtained by linearizing (D.16 - D.18) around the external path:

dux —rr = fx^x + fu^'^i 6x{0) specified, (D.21) do ^ = -{HxJxf - SXf - (H^uSu), (D.22) 406 D. Special Topics in Optimal Control

SHu = {HuxSxf + SX{HuXf + (HuuSuf = {HuuSxf + 6Xfu + {HuuSuf = 0. (D.23)

Alternatively, we may consider an expansion of the objective function and the state equation to second order since the first-order terms vanish about a trajectory which satisfies (D.15 - D.18), Prom Bryson and Ho (1969), this may be accomplished by expanding (D.20) to second order and all the constraints to first order. Thus, we have

^x dt ^ 2 Jo 8u (D.24) subject to d8x IT = fxSx + fuSu^ 6x{0) specified. (D.25) Since we are interested in a neighboring extremal path, we must deter­ mine 6u{t) so as to maximize 6^J subject to (D.25). This problem is a linear-quadratic problem discussed in the previous section. For this problem, the optimal control 6u*{t) is given by the formula (D.14), pro­ vided Huu{i) is nonsingular for 0 < t < T. The case when Huuify is singular for a finite time interval is treated in Section D.3. Thus, rec­ ognizing that G = ^xx, C = Hxx, N = H^u, D = Huu, A = fx, and B = fu, we have

6u%t) = H-^[Hux + f^S{t)]8x{t), (D.26) where

S + Sfx + f^S-{Sfu+Hxu)H-^{f^S + Hux) + Hxx = 0, S{T) = $^^. (D.27) While a ntmiber of second-order conditions can be obtained by pro­ ceeding further from this manner, we shall be interested only in the concavity condition (or strengthened Legendre-Clebsch condition). It is possible to show that neighboring stationary paths exist (in a weak sense; i.e., Sx and Su are small) if

Huu{t) < 0 for 0

represent sufficient conditions for a trajectory to be a local maximum. We are not being specific here because in this book we would be relying mostly on the sufficiency conditions developed in Chapters 2-4, which are based on certain concavity requirements. We are stating (D.28) because of its similarity to the second-order condition for a local maximum in the classical maximization problem. We must note that

Hu = 0 and Huu < 0 (D.29)

form necessary conditions for a trajectory to be a local maximimi.

D.3 Singular Control

In some optimization problems including some problems treated in this text, extremal arcs satisfying Hu = 0 occur on which the matrix Huu is singular. Such arcs are called singular arcs. Note that these arcs sat­ isfy (D.29) but not the strengthened condition (D.28). While no general sufficiency conditions are available for singular arcs, some additional nec­ essary conditions known as the generalized Legendre-Clebsch conditions have been developed. A good reference on singular control is Bell and Jacobson (1975). We shall only discuss the case in which the Hamiltonian is linear in one or more of the control variables. For these systems, Hu = 0 implies that the coefficient of the linear control term in the Hamiltonian vanishes identically along a singular arc. Thus, the control is not determined in terms of x and A by the Hamiltonian maximizing condition Hu = 0. Instead, the control is determined by the requirement that the coefficient of these linear terms remain zero on the singular arc. That is, the time derivatives of Hu must be zero. Having obtained the control by setting dHu/dt = 0 (or by setting higher time derivatives to equal zero) along the singular arc, we must check additional necessary conditions analogous to the second-order condition (D.28). For a maximization problem with a single control variable, these conditions turn out to be

(P^Hu (-')'!; <0, it = 0,1,2,.... (D.30) dfi^

The conditions (D.30) are called the generalized Legendre-Clebsch conditions. 408 D. Special Topics in Optimal Control

Example D.l We present an example treated by Johnson and Gibson (1963): max \j=-{—ir^- I ?x\dt \ (D.31) subject to

xi = X2+u^ xi{Q) = a^ (D.32) i2 = -u, a:(0) = fe, (D.33)

xi{T) - X2{T) = 0. (D.34)

Solution. We form the Hamiltonian

H = --x\ + Xi{x2 + u)+ Mi-u), (D.35) where the adjoint equations are Ai=xi, A2 = -Ai. (D.36)

The optimal control is bang-bang plus singular. Singular arcs must sat­ isfy if = Ai - A2 = 0 (D.37) for a finite time interval. The optimal control can, therefore, be obtained by ^ = Ai-A2 = :ci+Ai = 0. (D.38)

Differentiating once more with respect to time t, we obtain

2 = xi + \i = X2 + u + xi = Q, which implies u = -{xi+X2) (D.39) along the singular arc. We now verify for the example, the generalized Legendre-Clebsch condition (D.30) for fc = 1: Appendix E

Answers to Selected Exercises

Completely worked solutions to all exercises in this book are contained in a forthcoming Teachers' Manual, which wiU be made available to instructors by the publisher when it is ready.

Chapter 1

1.1 (a) Feasible. J = -333,333. 1.2 J = 36. 1.3 (a) C = $157,861/year. (b) J = 103.41 utils. (c) $15,000/year.

1.4 (b) W{20) = 985,648; J = 104.34.

1.12 imp(Gi,G2;i) = (Gi - G2)e-^*.

Chapter 2

2.2 The optimal control is

2 if 0

u* (t) = { undefined if i = 2 - In 2.5,

0 ift>2-ln2.5. 410 E. Answers to Selected Exercises

2.8 u* = bang(0,1; Ai - A2), where X{t) = (8e-2(*-i8), 4e-2(*-i8)).

2.10 (a) u* = bang[0,1; (giKi + 52^2)(Ai - A2)].

(c) i = T- (1/52) Hio^bi - 9ih2)/{g2 - giM- 2.12 (a) a;(100) = 30 - 20e-i° « 30. (b) w* = 3fori€ [0,100].

3 for is [0, 100-101n2], (c) u*{t) = 0 otherwise.

2.14 (a) C*{t) = pWoe^'-P^yil - e-P^). (b) C*it) = Kir-p). 2.17 (a) X = x + 3Aa;2, A(l) = 0, and x =-x^ + A, x{0) = 1. 2.18 X = f{x) + b{x)u, x{0) = xo, x{T) = 0.

u = [b{xfg'{x) - 2<^u{b(x)f'{x) - V{x)f{x)y\l\l(?h{x)\.

Chapter 3

3.1 X — Ml > 0, «1 — M2 > 0, Ml > 0, 1 + M2 > 0- 3.2 X = [-1, 5]. 3.7 L = F(x, v) + A/(x, M, i) + iig{x, u, t),

A = -(a/a)X - 1^, /x > 0, /x^ = 0.

3.11 A(i) = t - 1.

3.12 (a) A(i) = 10 I _g0.1(t-100)

0 if ii: - 300,

-10 2 _ gO.iCic/s-ioo) if ii: < 300, M*(t) = bang[0,3;A + /Lt].

The problem is infeasible for K > 300. E. Answers to Selected Exercises 411

(b) r* = niin[0, 100-K/3], 0 for t < t**, u*{t) = I 3 iort>t**. V 3.18 11.87 minutes. 3.19 u* = -1, T* = 5. 3.20 tx* = -2, T* = 5/2. 3.29 (a) {I, P,X} = {h - p{S - Pi), S, 2(5 - Pi)}. (b) I = h. Chapter 4 4.1 u*{t) = -l, iJ,i = -X = 1/2-t, H2 = 7] = 0. 4.2 One solution appears in Figure 3.1. Another solution is u(t) = 1/2 for t G [0,2]. There are many others. 4.4 (a) u* = 0.

1, 0

(e) J = -(1/8 + 1/8K). (f) J =-1/8. Chapter 5

5, t< l + 61n0.99«0.94, 5.1 (a) u*{t) 0, t> 0.094.

-5, 0 < ^ < 0.28,

0, 0.28 < t < 0.4, (b) A2(t)/Ai(i) = e3(*'-«+i)/i2,^t*(f) = J 5, 0.4 < i < 0.93,

0, 0.93 < t < 1.0. 412 E. Answers to Selected Exercises

5.5 u* = v* = 0 for all t.

5.7 u* = 0, V* = 4/5 for t € [0,49],

u* = 0, ^;* = 0 for ^€[49,60],

J* = 34,420.

5.10 (b) /(**) = t* - 101n(l - 0.3e°-"*).

(c) t* = 1.969327, J{t*) = 19.037.

Chapter 6

6.5 Q(t) = t^ - 160*3 ^ 1740^2 _ 7350^ + 9639.

6.7 V* = sat[-F2,14; (A2 - Aip)2/?Ai]. 6.9 J* = 10.56653. 6.10 y*{t) « 3e-3*, y*(t) w 1 - Se'^*.

0, 0 < i < 7/3,

2, 7/3

0, 13/3

-fi+|, t€[0,l], 6.13 /xi = < 0, tG{0,3].

0, ie [0,1.8), f^2= \ -ii + f, ie [1.8,3].

0, tG[0,l)U(1.8,3], 77= < -fi+|, «€ [1,1.8). E. Answers to Selected Exercises 413

-1 for tG [0,1.8), 6.14 (a) v*{t) = { 1 for ie (1.8,3].

(b) t;*(t) = 1 for iG [0,10].

-1 for tG [0,1/2],

0 for f G (1/2, ti], where ti = 23/12, 6.15 V* = < +1 foriG(ii,ii + l/2],

0 for tG (ii + 1/2,4].

0, for0

where ti=T- ^2BC/h.

Chapter 7

7.1 p* = 102.5 + 0.2G. 7.7 {u)/{pS) = {Sp)/{rj{p + S)). 7.10 G + SG = bang[0, oo; A + 1],

-X + (p + S)X = 7r'{G).

7.12 The equations corresponding to (6.28) and (6.29) can be obtained

by replacing p by p+r/r. The form of (6.30) remains imchanged.

7.17 (b) 1 1 Xo _, 1 . X-X^ h rQ + 6:ln— x^ , t2 = rQ„ + S. In •X — XT 7.18

T > ^ In ^<^(1 ~ ^o) ~ ^^0 + i In fi ~ rQ + 6 rQ(l — x^) — 6x^ 6 XT ' 414 E. Answers to Selected Exercises

7.19 The reachable set is [xoe'^^, {xo - x)e-(*+'''5)^ + x],

where x = rQ/{W + rQ).

7.25 1-A imp{A^B;t) = - r 1-B 7.26 (b) J = 0.6325. Chapter 8

8.1 (a) y = l^z = 3. (b) y - 2, z - 10. 8.2 (a) (1,3) is a relative maximum. (b) (2,10) is a relative maximum. 8.3 x = 50;x = 80. 8.6 (a) a: = 4 is a local maximum. (b) a: = 8 is a local maximimn and a: = 20 is a local and a global maximum. 8.7 (a) (0, 0) is the nearest point. (b) (1/2,1/2) is the nearest point. 8.8 (1/V^, 2/\/5) is the closest point. 8.9 (a) (2^,0). (b) (0,2). (c) (0,2). 8.10 Xj = dF/dxJ for i = 1,2,..., n; A^+i = 1. Note that here T

denotes the terminal time, and not the transpose operation.

+1 ifA'=+i6>l,

k* 8.14 u -1 ifA'^+iJx-l ,whereA*= = (7 + A)^-'=A^

0 if iX^+^b] < 1. E. Answers to Selected Exercises 415

Chapter 9 9.1 t' = 5.25, T = 11. 9.3 T = i* = 2.47. 9.4 t^ = 0, T = 30.

l2 9.5 u*{t) = sat[0,1;u^{t)], where u^{t) = \2 - e005(t-34.8)|'/(i + i),

ti^S;t2-T = 34.8. Chapter 10 10.3 X = 0.734. 10.4 (a)

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Abad, P.L., 417, 474 Axsater, S., 418 adjoint equation, 31, 32, 36, 230 adjoint function, 322 Baar, T., 425 adjoint variables, 10, 32 backlogging of demand, 5, 348 adjoint vector, 30, 33 Bagchi, A., 419 admissible control, 24 Balachandran, B., 445 advertising model, 5, 6 Balakrishman, A.V., 455 affine function, 19 bang function, 16 Agnew, C.E., 417 bang-bang, 42, 78, 84, 85, 87, Alam, M., 249, 265, 417, 465 122, 132, 135, 190, 194, AUen, K.R., 286, 417 232, 244, 247, 257, 258, Amit, R., 154, 279, 418 313, 322, 328, 329, 333, Amoroso-Robinson relation, 187 334, 408 Anderson, R.M., 418 bankruptcy, 358 anti-difference operator, 376 Bamea, A., 452 Aoki, M., 345, 418 Basar, T., 308, 419, 420, 430, applications to biomedicine, 295 446 applications to finance, 119 Bass, F.M., 419 applications to marketing, 185 Bayes theorem, 342 Arnold, L., 344, 347, 348, 418 Bean, J.C, 182, 419 Aronson, J.E., 418, 471 begiiming game, 321 Arora, S.R., 243, 418 Bell, D.J., 42, 407, 419 Arrow, K.J., 10, 46, 58, 81, 82, Belhnan, R.E., 9, 27, 419 186, 188, 289, 290, 360, Bennett, R.J., 474 418, 456, 459 Bensoussan, A., 10, 58, 88, 154, Arthur, W.B., 304, 418 173, 182, 191, 315, 322, Arugaslan, O., viii 324, 360, 419, 420, 444, Arutyunov, A.V., 103, 418 461, 470 Aseev, S.M., 103, 418 bequest function, 7, 355 Aubin, J.-R, 418, 466 Berkovitz, L.D., 10, 27, 308, 420 autocorrelation function, 343 Bernoulli, Jacob, 8, 9, 384 autonomous, 52, 81 Bernoulli, Johann, 8, 9, 384 484 Index

Bertsekas, D.P., 236, 346, 420, Bulirsch, R., 9, 108, 422, 461, 430 474 Bes, C, 173, 182, 420 Bullinger, H.J., 479 Beyer, D., viii, 421 Bultez, A.V., 315, 419, 422 Bharucha-Reid, A.T., 480 Burdet, C.A., 236, 239, 422 Bhaskaran, S., 182, 421 Burmeister, E., 290, 291, 422 bionomic eqmlibrium, 269, 314 Buskens, C., 455 Black, F., 355, 421 Butkowskiy, A.G., 317, 422 Blaquiere, A., 325, 326, 331, 421, Bylka, S., 182, 254, 423 464, 473 BUss point, 306 Gaines, P., 423 BUss, G.A., 9 calculus of variations, 8, 379 Boiteux problem, 241 Canon, M.D., 235, 423 Boiteux, M., 241, 421 canonical adjoints, 33 Boltyanskii, V.G., 9, 27, 421, canonical system of equations, 462 33 Bolza, 9 capital accumulation model, 290 Bolza form, 25, 26, 229, 235, 239 Caratheodory, C., 9 Bookbinder, J.H., vi, 10, 304, Carlson, D.A., 10, 82, 423-425 421 Carraro, C, 315, 425 boundary conditions, 61, 350 Carrillo, J., 154, 425 boundary interval, 105 Case, J.H., 308, 325, 425 Bourguignon, F., 421 Cass, D., 425 Brachistochrone problem, 8, 9, Cassandras, C.G., 236, 461 384 cattle ranching problem, 318 Breakwell, J.V., 190, 421 Caulkins, J.P., 477 Brekke, K.A., 360, 422 CeUina, A., 418 Brito, D.L., 304, 422 certainty equivalence, 403, 404 Brock, W.A., 360, 454 Cesari, L., 26, 425 Brockhoff, K., 420 chain of forests model, 276-278 broken extremal, 387 chain of machines, 254 Brotherton, T., 423 Chand, S., 182, 254, 425, 469 Brown, R.G., 164, 422 Chandra, T., 445 Brownian Motion, 356 Chang, S., 360, 418 Bryant, G.F., 27, 422 Charnes, A., 304, 425 Bryson, Jr., A.E., 33, 87, 105, Chatterjee, R., 463 108, 341, 401, 403, 404, Chen, S.F., 425 406, 422 Cheng, F., viii Buchanan, L.F., 345, 422 Chiarella, C, 426 Bucy, R., 345, 447 Chichilinsky, G., 426 Index 485

Chikan, A., 440 Cottle, R.W., 430 Chintagunta, P.K., 315, 426 Cowling, K., 444 Chow, G.C., 345, 426 critical points, 218 Clark, C.W., 10, 241, 267, 268, Crouhy, M., 173, 182, 419 273, 286, 312, 314, 426 Cruz, J.B., Jr., 472 Clarke, F.H., 27, 106, 167, 427 CuUum, CD., 235, 423 Clemhout, S., 427 current-value adjoint variables, closed-loop control, 346 70 closed-loop Nash solution, 311 current-value formulation, 58, Coddington, E.A., 193, 427 65, 239 Cohen, K.J., 36, 187, 427 current-value functions, 67 common-property fishery re­ current-value Hamiltonian, 66 sources, 312 current-value Lagrange multipli­ comparison lemma, 198 ers, 67 complimentary slackness condi­ current-value Lagrangian, 66 tions, 60 current-value maximum princi­ computational methods, vii, ple, 68, 111 108, 236 cycloid, 9 concave function, 18, 64 Cyert, R.M., 36, 187, 427 Connors, M.M., 10, 427 Conrad, K., 427 D'Autimie, A., 428 Constantinides, G.M., 427 Dantzig, G.B., 418, 427 constraint of rth order, 105 Darrat, A.F., 456 constraint qualification, 60, 105, Darrough, M.N., 427 226 Dasgupta, P., 279, 428, 474, 476 constraints, 24 Davis, B.E., 119, 129, 152, 360, consumption model, 7, 8 428 consumption-investment prob­ Davis, M.H.A., 346, 428 lem, 355 Davis, R.E., 456 contact time, 105 Dawid, H., 428 continuous wheat trading model, Day, G., 463 164 DDT, 299, 300, 303 control of pest infestations, 295 Deal, K.R., 311, 315, 337, 428 control trajectory, 2, 24 decision horizon, 173, 175, 177, control variable, 2, 24 179, 180 convex combination, 17 Deger, S., 428 , 18, 64 Deissenberg, C, 428, 429 convex huU, 17 Deistler, M., 476 convex set, 17 derivative operator, 363 CorelDRAW, vii derived Hamiltonian, 45 486 Index

Derzko, N.A., 45, 130, 279, 315, dynamic efficiency condition, 317, 360, 420, 429, 466, 291 469 dynamic programming, 27, 345, DeSarbo, W., 304, 445 393 Dhrymes, P.J., 213, 429 difference equation, 229, 341 economic applications, 289, 360 diflference operator, 375 economic interpretation, 34, 69, differential games, 308 138, 291, 322 differentiation with scalars, 12 educational policy, 21 differentiation with vectors, 12, eigenvalues, 370, 371 13 eigenvectors, 370, 371 diffusion process, 344 El-Hodiri, M., 431 , 323, 344 EUashberg, J., 304, 431, 445, 475 direct adjoining method, 100 Elton, E., 119, 431 direct contribution, 35 Elzinga, D.J., 119, 152, 428 discount factor, 6 ending correction, 158 discount rate, 6 ending game, 321 entry time, 105 discrete maximum principle, environmental management, 315 217, 228, 229 EOQ, 153 discrete-time optimal control epidemic control, 295 problem, 228, 229 equilibrimn relation, 36 distributed parameter maximimi Erickson, G.M., 431 principle, 317 Erickson, L.E., 10, 154, 315, 443 distributed parameter systems, Euler, 8, 384 315 Euler equation, 380, 382, 387, Dixit, A.K., 429 388 Dobell, A.R., 290, 291, 422 EXCEL, vii, 48-50 Dockner, E.J., 10, 289, 304, 308, exhaustible resource model, 279 315, 429, 430, 432, 446 exit time, 105 Dohrmann, C.R., 236, 430 Dolan, R.J., 430, 445 factorial power, 375 Dorfman, R., 430 Fan, L.T., 154, 304, 431, 443 Dornoff, R.J., 474 Farley, J.U., 475 Drews, W., 430 Feichtinger, G., vi, viii, 10, 33, Dreyfus, S.E., 420 58, 69, 81, 99, 106, 108, dual variables, 36 113, 154, 167, 185, 194, Dubovitskii, A.Y., 430 211, 289, 304, 322, 427- Dunn, J.C, 236, 430 429, 431-434, 438-440, Durrett, R., 347, 431 442, 446, 451, 453, 455, Index 487

457^59, 463-465, 470, Furst, E., 476 473, 475^78, 480 Feinberg, F.M., 434 Gaandolfo, G., 436 Fel'dbaum, A.A., 31, 393, 398, Gaimon, C, 154, 249, 304, 322, 434 331, 425, 435, 436 Ferreira, M.M.A., 103, 434 Galileo, 384 Ferreyra, G., 434 Gamkrelidze, R.V., 9, 436, 462 Filar, J., 315, 425 Gandolfo, G., 432 Filipiak, J., 434 Gaskins, Jr., D.W., 436 filtering, 339, 341 Gaugusch, J., 437 finite diflference equations, 375 Gaussian, 341-343 first-order pure state con­ Geismar, N., viii straints, 106, 108 Gelfand, I.M., 379, 380, 382, Fischer, T., 434 387, 389, 437 Fisher, A.C., 461 general discrete maximum prin­ fishery management, 315 ciple, 234 fishery model, 268, 312 general inequality constraints, fishing mortality function, 314 97 fixed-end-point problem, 70 generalized bang-bang, 87, 132 Fleming, W.H., 346, 360, 434 generalized derivative, 344 Fletcher, R., 434 generalized Legendre-Clebsch Fomin, S.V., 379, 380, 382, 387, condition, 152, 407, 408 389, 437 forecast horizons, 173 Geoffrion, A.M., 453 forest fertilization model, 287 Gerchak, Y., 437 forest thinning model, 273, 276 Gfrerer, H., 437 forgetting coefficient, 5 Gibson, J.E., 408, 445 Forster, B.A., 435 Gihman, LI., 344, 353, 437 Fourgeaud, C, 435 Gillessen, W., 455 Francis, P.J., 295, 435 Girsanov, I.V., 437 Frankena, J.F., 435 Glad, S.T., 437 free-end-point problem, 70 goal level, 319 Friedman, A., 308, 435 GOAL SEEK, 48, 49 Fromovitz, S., 228, 455 Goh, B.-S., 108, 267, 437 Fruchter, G.E., 315, 435 Goh, C.J., 476 fuU-rank condition, 20, 60, 105, Goldberg, S., 375, 437 106 Golden Path, 82 Fuller, D., 280, 435 Golden Rule, 82, 93 fundamental lemma, 382 Goldstine, H.H., 437 Funke, U.H., 435 goodwill, 5, 186 488 Index goodwill elasticity of demand, 304, 317, 322, 428, 432, 188 433, 438-442, 453, 473 Gopalsamy, K., 437 Harvey, A.C., 442 Gordon's formula, 145 Haunschmied, J., 304, 433 Gordon, H.S., 268, 269, 437 Haurie, A., 10, 82, 173, 308, 315, Gordon, M.J., 145, 437 317, 322, 420, 424, 425, Gould, J.P., 191, 210, 214, 437 438, 442 Green's theorem, 185, 196, 198, Haussmann, U.G., 442 205, 211, 214, 270, 296, Heal, G.M., 279, 428, 443, 476 297, 305, 314 Heaps, T., 241, 443 Grimm, W., 438, 459 Heckman, J., 443 Gross, M., 438 Heineke, J.M., 427 Gruber, M., 119, 431 Hestenes, M.R., 10, 58, 62, 443 Gruver, W.A., 448 higher-order constraints, 105 HJB equation, 30, 31, 354 Hadley, G., 10, 58, 289, 438 HMMS model, 153 Hahn, M., 438 Ho, Y.-C., 33, 87, 105, 308, 311, Halkin, H., 27, 235, 438 341, 401, 403, 404, 406, Hamalainen, R.P., 315, 438 422, 443, 459, 473 Hamilton, 9 Hochman, E., 360, 464 Hamilton-Jacobi equation, 349 Hoffmann, K.H., 443 Hamilton-Jacobi-BeUman equa­ Hohn, F., 174, 457 tion, 27, 30, 345, 349 Holly, S., 443 Hamiltonian, 30, 35, 60, 230, Holt, C.C., 153, 443 291, 397 Holtzman, J.M., 235, 443 Hamiltonian maximizing condi­ homogeneous equation, 363 tion, 30, 34, 397 homogeneous equations of order Han, M., 438 n, 365 Hanson, M., 457 homogeneous equations of order Hanssens, D.M., 438 one, 364 Harris, F.W., 153, 438 homogeneous equations of order Harris, H., 303, 439 two, 364 Harrison, J.M., 357, 439 homogeneous function of degree Hartberger, R.J., 27, 396, 430, one, 19 439 homogeneous partial differential Hartl, R.F., viii, 10, 26, 27, 33, equations, 374 58, 62, 69, 81, 89, 99, Horsky, D., 443 100, 105, 106, 108, 113, Hotelling, H., 279, 443 115, 154, 167, 174, 185, Hung, N.M., 442 194, 211, 239, 243, 303, Hurst, Jr., E.G., 10, 58, 154, 419 Index 489

Hwang, C.L., 154, 443 Jazwinski, A.H., 444 Hyun, J.S., 438 Jedidi, K., 304, 445 Jennings, L.S., 445 Ijiri, Y., 153, 165, 444 Jeuland, A.P., 430, 445 Ilan, Y., 154, 418 Jiang, J., 445 imp, 17 Johnson, CD., 408, 445 impulse control, 16, 125, 202, Jones, P., 445 204, 322, 324, 325 J0rgensen, S., viii, 10, 154, 267, impulse control model, 88 289, 304, 308, 315, 429, impulse Hamiltonian, 326 430, 433, 440, 445, 446 impulse maximum principle, Joseph, P.D., 341, 404, 446 326, 332 jirnip conditions, 103, 108 impulse stochastic control, 360 jump Markov processes, 360 imputed value, 219 junction times, 105 indirect adjoining method, 98, 100, 104,111 Kaitala, V.T., 315, 433, 438, 446 indirect contribution, 35 Kalaba, R.E., 419 infinite horizon, 6, 80 Kalish, S., 304, 435, 446, 447 inhomogeneous partial differen­ KaU, P., 439, 458, 470 tial equation, 374 Kahnan filter, 339, 340, 342 instantaneous profit rate, 25 Kahnan, R.E., 341, 345, 447 interior interval, 105 Kaknan-Bucy filter, 339, 345 Intriligator, M.D., 290, 444, 456 Kamien, M.I., 10, 248, 253, 289, inventory control problem, 117 303, 447 loffe, A.D., 444 Kamien-Schwartz model, 249 Isaacs, R., 9, 133, 308, 444 Kaplan, W., 447 isoperimetric profit constraint, Karatzas, I., 340, 344, 347, 359, 214 360, 447 Ito stochastic differential equa­ Karreman, H.F., 421 tion, 344, 345 Keeler, E., 448 Ivanilov, Y.P., 304, 480 Keller, H.B., 299, 448 Kemp, M.C., 10, 58, 289, 426, Jabrane, A., 424 438, 448 Jacobi, 9 Kendrick, D.A., 448 Jacobson, D.H., 42, 108, 407, Keon, J.W., 476 419, 444, 451 Khmelnitsky, E., 10, 448, 449, Jacquemin, A.P., 188, 191, 444 454 Jagpal, S., 444 Kilkki, P., 273, 274, 448 Jain, D., 315, 426 Kirakossian, G.T., 304, 440 Jamshidi, M., 444 Kirby, B.J., 448 490 Index

Kirk, D.E., 31, 448 L'Hospital, 384 Klein, C.F., 448 Lagrange, 8 Kleindorfer, G.B., 234, 449 Lagrange form, 25 Kleindorfer, P.R., 174, 182, 234, Lagrange multipliers, 57, 218 448, 449, 470 Lagrangian, 57, 60 Kleinschmidt, P., 440 Lagtmov, V.N., 450 Knobloch, H.W., 449 Lakhani, C, 465 Knowles, G., 449 Lansdowne, Z.F., 206, 450 Kogan, K., 10, 448, 449, 454 Lasdon, L.S., 450 Kopel, M., 428 Leban, R., 10, 289, 450, 451 Kort, P.M., 10, 154, 267, 289, Leclair, S.R., 445 303, 304, 433, 440, 441, Lee, E.B., 129, 450 446, 478 Lee, S.C, 213, 469 Kortanek, K., 304, 425 Lee, W.Y., 450 Kotowitz, Y., 449 left and right limits, 15 Kozlowski, J., 480 Legendre, 8 Krabs, W., 443 Legendre's conditions, 388 Kraft, D., 108, 422 Legey, L., 304, 450 Krarup, J., 430 Lehoczky, J.P., 130, 359, 447, Krauth, J., 304, 441 450, 469 Kreindler, E., 449 Leibniz, 8 KreUe, W., 420, 449 Leitmann, G., 27, 267, 308, 379, Krichagina, E., 449 425, 437, 438, 442, 451, Kriebel, C.H., 234, 449 457, 473 Kriendler, E., 108 Leizarowitz, A., 424, 425 Krouse, CO., 129, 449, 450 Leland, H.E., 451 Krutilla, J.V., 447 Lele, M.M., 108, 444, 451 Kugelmann, B., 450 Lele, P.T., 243, 418 Kuhn, H.W., 472 Lenclud, B., 435 Kuhn-Tucker conditions, 218, Leonard, D., 10, 289, 451 220, 228 Leondes, C.T., 422, 473, 479 Kuhn-Tucker constraint qualifi­ Lesourne, J., 10, 289, 360, 419, cation, 226, 227 420, 450, 451 Kumar, S., viii Lev, B., 436 Kurawarwala, A.A., 450 Levine, J., 451 Kurcyusz, S., 103, 450 Levinson, N.L., 193, 427 Kurz, M., 10, 46, 58, 81, 82, 188, Lewis, T.R., 451, 452 289, 290, 418 Li, G., 154, 452 Kushner, H.J., 450 Lieber, Z., 174, 182, 438, 449, Kydland, F.E., 450 452 Index 491

LigneU, J., 452 maintenance and replacement LiUen, G.L., 304, 446 model, 241, 242, 248, line integral, 197 331 linear differential equations, 363 Majumdar, M., 454 linear independence, 20 Malinowski, K., 115, 454, 472 linear Mayer form, 25, 26, 239 Malliaris, A.G., 360, 454 linear programming, 87, 132 Mangasarian, O.L., 46, 64, 226- linear-quadratic case, 85 228, 455 linear-quadratic problems, 401 Manh-Hung, N., 279, 455 linearly independent, 20 Mantrala, M.K., 345, 458 Lintner, J., 452 MAPI (Machinery and Applied Lions, J.L., 88, 317, 324, 360, Products Institute), 241 420, 444, 452, 470 MAPLE, 150 Little, J.D.C., 452 marginal cost, 36, 187 little-o notation, 15 marginal cost equals marginal Liu, P.-T., 418, 421, 428, 451, revenue, 36 452 marginal return, 398 Loewen, P.D., 106, 427 marginal revenue, 36 logarithmic Brownian Motion, Markus, L., 450 356 martingale problems, 360 Long, N.V., 10, 289, 308, 315, Martirena-Mantel, A.M., 455 426, 430, 448, 451, 452 Marzano, P., 432 long-rim stationary equilibrium, Masse, P., 241, 455 82 Mate, K., 443 Loon, P.J.J.M., van, 453 Mathematica, 150 Lou, S., 449, 453 mathematical requirements, 1 Lucas, Jr., R.E., 304, 453 Mathewson, P., 449 Luenberger, D.G., 228, 453 matrix Riccati equation, 345, Luhmer, A., 433, 453 403 limiped parameter systems, 315 Matsuo, H., 450 Lundin, R.A., 174, 182, 453 Maurer, H., viii, 108, 115, 118, Luptacik, M., 303, 441, 453, 463 455 Luus, R., 454 maximized Hamiltonian, 114 Lynn, J.W., 249, 417 maximum, 388 maximum likelihood estimate, Macki, J., 454 342 Magat, W.A., 454 maximum principle, 23, 33, 34, Magill, M.J.P., 454 57, 58, 67, 104,113, 217, Mahajan, V., 430, 447, 452, 454 393 Maimon, O., 10, 448, 449, 454 May, R.M., 418 492 Index

Mayer form, 25, 397 model type (b), 85-87 Mayne, D.Q., 27, 108, 236, 422, model type (c), 85 456, 460, 462 model type (d), 85 McCabe, J.L., 451 model type (e), 85, 86 McCann, J.M., 454 model type (f), 85, 86, 251 McEneaney, W.M., 470 model types, 83, 85 McGuire, T.W., 304, 469 modeling "tricks", 86 Mclntyre, J., 108, 456 Modigliani, F., 130, 153, 174, McNicoU, G., 304, 418 443, 457 McShane, E.J., 10 Moiseev, N.N., 457 measurement noise, 340 Monahan, G.E., 457 Meech, J.A., 445 Mond, B., 457 Mehlmann, A., 304, 305, 308, Moore, E.J., 476 429, 433, 441, 456 Morey, R.C., 454 Mehra, R.K., 304, 456 Morton, T.E., vi, 174, 182, 254, Mehrez, A., 456 259, 264, 453, 457, 458, Merton, R.C., 355, 456 469 Mesak, H.I., 456 Motta, M., 324, 458 Michel, P., 428, 435, 457 Muller, E., 430, 452, 454, 458 middle game, 321 Mulvey, J.M., 469 Miele, A., 196, 457 Murata, Y., 458 MiUer, M.H., 130, 457 Murray, D.M., 236, 458 MiUer, R.E., 457 Muth, J.F., 153, 443 Milyutin, A.A., 430 Muzicant, J., 322, 458 minimax solution, 308, 309 minimum fuel problem, 238 Naert, P.A., 315, 419, 422 Mirman, L.J., 452, 457 Nahorski, Z., 458 Mirrlees, J., 457 Naik, P.A., 345, 458 miscellaneous applications, 303 Nash solutions, 308 Mischenko, E.F., 9, 462 Naslund, B., 10, 58, 154, 241, MitcheU, A., 457 278, 287, 419, 458 Mitra, T., 454 natural resources, 267, 360 Mitter, S.K., 459 necessary condition, 31, 33, 228 mixed constraints, 59, 104, 106 Neck, R., 458 mixed inequality constraints, 3, needle-shaped variation, 394, 57, 58 395 mixed optimization technique, neighborhood, 15 258 Nelson, R.T., 304, 458 model type (a), 84, 85, 87, 243, Nepomiastchy, P., 304, 459 244 Nerlove, M., 186, 188, 459 Index 493

Nerlove-Love advertising model, optimal long-run stationary 186 equilibriimi, 82, 189 Neuman, C.P., 186, 477 optimal path, 25 Neustadt, L.W., 10, 455, 459 optimal thinning, 274 Newton, 8, 384 optimal trajectory, 25 Nguyen, D., 459 order of the constraint, 105 Nissen, G., 322, 420 Oren, S.S., 460 nonlinear programming, 217, Osayimwese, I., 460 218, 227 overdraft, 124 nonzero-simi games, 310 Ozga model, 214 , 15 Ozga, S., 214, 460 Norstrom, C.J., vi, 125, 153, 170, 459 Paiewonsky, B., 108, 456 Norton, F.E., 345, 422 Palda, K.S., 186, 460 notation, 10 Pantoja, J.F., 236, 460 Novak, A., 304, 433, 434, 441, parametric linear programming, 459 87 Oakland, W.H., 304, 422 Parlar, M., 437, 460 Oberle, H.J., 438, 459 Parrish, B., 417 objective function, 2, 25, 398 Parsons, L.J., 438 Oettli, W., 422 partial differential equations, Oguztoreli, M.N., 459 372 oil driller's problem, 324 partial fractions, 350 0ksendal, B.K., 344, 360, 422, particular integral, 367 459 particular solutions, 366, 367 Okuguchi, K., 426 path of least time, 8 Olsder, G.J., 419, 459 Pauwels, W., 460 one-sector model, 291 Pekelman, D., 154, 174, 182, one-sided constraints, 71 241, 259, 460, 461 Oniki, H., 460 Pepyne, D.L., 236, 461 open access fishery, 269 Pesch, H.J., 9, 450, 461 open-loop Nash solution, 310 pessimal solution, 146 optimal consumption of an ini­ Peterson, D.W., 461 tial investment, 89 Peterson, F.M., 461 optimal control problem, 25 Peterson, R.A., 454 optimal control theory, 1, 379 Petrov, lu.P., 461 optimal economic growth mod­ phase diagram, 192, 293, 301 els, 289, 293 Phelps, E.S., 437 optimal financing model, 129 Pierskalla, W.P., 241, 461 494 Index

Pindyck, R.S., 279, 360, 429, rank of a matrix, 20 461, 462, 478 Rao, R.C., 315, 463 Pitchford, J.D., 435, 452, 462 Rapoport, A., 464 Pliska, S.R., 357, 439 Rapp, B., 241, 464 Pohjola, M., 462 Rausser, G.C., 360, 461, 464 Polak, E., 108, 235, 423, 456, 462 Raviv, A., 304, 464 pollution control model, 299, 302 Ravn, H.F., 458 Pontryagin, L.S., 9, 10, 23, 27, Ray, A., 464 76, 106, 393, 462 reachable set, 3, 59 Powell, S.G., 460 Reeves, CM., 434 predator-prey relationships, 267 regional allocation of invest­ Prescott, E.G., 450 ment, 52 Presman, E., 462, 463 Reinganum, J.F., 464 price elasticity of demand, 187 Rempala, R., 174, 464 price shield, 174, 175, 177 Richard, S.F., 427, 464 principle of optimality, 27, 346, Ringbeck, J., 304, 464 358 Ripper, M., 304, 450 product rule for differentiation, Rishel, R.W., 346, 434, 465 14 Roberts, S.M., 33, 465 production fimction, 292 Robinett, R.D., 236, 430 production planning model, 153, Robinson, B., 465 339 Robson, A.J., 322, 465 production smoothing, 154 Rockafeller, R.T., 465 production-inventory model, 4, Roxin, E.G., 421, 423, 452 153, 154, 234 Russak, B., 465 Proth, J.-M., 173, 182, 419, 425 Russell, D.L., 427, 465 Prskawetz, A., 463 Riistem, B., 443 pure constraints, 116 Ruusunen, J., 315, 438 pure state variable inequality Ryu, Y., viii constraints, 3, 97, 98, 104 saddle point, 19, 309 Pytlak, R., 108, 463 Sage, A.P., 317, 465 Salukvadze, M.E., 465 quasiconcave fimction, 64 salvage value, 3, 25, 87 quasiconvex function, 64 Samaratunga, G., 466 Samuelson, P.A., 465 Rajagopalan, S., 154, 452 Sargent, T.J., 453 Raman, K., 360, 463 Sarma, V.V.S., 249, 265, 417, Rampazzo, F., 324, 458 465 Ramsey, P.P., 69, 289, 306, 463 Sasieni, M., 466 Index 495

sat fiinction, 16 476, 480 Sawyer, A., 345, 458 Sethi-Morton model, 254, 264, Scalzo, R.C., 466 331 Schaefer, M.B., 268, 466 shadow price, 10, 35, 219 Schijndel, G.-J.C.Th.,van, 304, Shapiro, A., 472 466 Shapiro, C, 103, 472 SchiUing, K., 466 Shell, K., 289, 425, 472 Schmalensee, R., 451, 452 Shipman, J.S., 33, 465 Scholes, M., 355, 421 shooting method, 182 Schubert, U., 303, 453 short-selling, 124 Schultz, R. L., 438 Shreve, S.E., 340, 344, 346, 347, Schwartz, N.L., 10, 248, 253, 359, 360, 420, 447 289, 303, 447 Shtub, A., 449 Schwodiauer, G., 476 Siebert, H., 472 second-order differential equa­ Silva, G.N., 324, 472 tions, 359 Simaan, M., 472 second-order variations, 388, 405 Simon, H.A., 153, 404, 443, 472 Segers, R., 430 Simon, L.S., 443 Seidman, T.I., 315, 466 simple cash balance problem, Seierstad, A., 10, 27, 45, 58, 62, 120 64, 81, 99, 113, 289, 466 simplest variational problem, Selten, R., 308, 466 379 Sen, S.K., 428, 447 Singh, M.G., 468, 472 Sengupta, J.K., 477 singular arcs, 407 separation principle, 403, 404 singular control, 42, 132, 140, Sethi, S.P., 10, 26, 27, 45, 58, 297, 407 62, 89, 99,100,105, 106, Skiba, A.K., 472 108, 113, 115, 119, 129, Skorohod, A.V., 344, 353, 437 130, 150, 153, 154, 173, Smith, B.L.R., 444 174, 182, 185, 190, 194- Smith, M., 445 196, 202, 206, 213-215, Smith, R.L., 182, 419 236, 239, 241, 247, 254, Smith, V.K., 447 259, 264, 270, 271, 276, Smith, V.L., 472 278, 279, 295, 298, 303, Snower, D.J., 472 304, 311, 315, 317, 322, sole owner fishery resource 337, 340, 347, 351, 352, model, 268 354, 357-360, 420-423, Soliman, M.A., 304, 475 425, 427-429, 433, 441, Solow, R.M., 279, 472, 473 442, 445, 447, 449, 450, Soner, H.M., 360, 434, 450 453, 461-464, 466-471, Sorenson, H., 341, 473 496 Index

Sorger, G., viii, 10, 182, 254, stochastic advertising problem, 289, 308, 315, 423, 425, 352 430, 433, 434, 453, 469, stochastic calculus, 347 473 stochastic differential equations, Sothmann, B., 459 339, 344 Southwick, L., 303, 473 stochastic manufacturing prob­ special topics, 401 lems, 360 Spence, M., 299, 448, 473 stochastic optimal control, 339, Speyer, J.L., 108, 444 345, 346 Spiegel, M.R., 375, 473 stochastic production planning Spremarm, K., 473 model, 347 Sprzeuzkouski, A.Y., 154, 473 stockout cost, 5 Spulber, P.F., 452, 457 Stoer, J., 422, 474 Srinivasan, V., 186, 473 stopping time, 357 Sriskandarajah, C, viii Stoppler, S., 10, 154, 429, 474 Staats, P.W., 295, 469 Strauss, A., 454 Stalford, H., 473 Streitferdt, L., 466 strengthened Jacobi condition, standard adjoint variables, 65 389 standard Hamiltonian, 65 strengthened Legendre condi­ standard Lagrangian, 65 tion, 389 standard multipliers, 65 strengthened Legendre-Clebsch Starr, A.W., 308, 311, 473 condition, 406 starting correction, 158 strictly concave function, 18, 64 state equation, 24 strictly convex function, 64 state trajectory, 2, 24 strong forecast horizon, 174, state variable, 2, 24 177, 179 static efficiency condition, 291 strong maximum, 389, 390 stationarity assumption, 81 sufficiency conditions, 44, 46, 64, Stein, R.B., 459 113, 228 Steinberg, R., 431 sufficiency transversality condi­ Steindl, A., 453, 473 tion, 159 Steiner, P.O., 430 summary of transversality con­ Stepan, A., 473 ditions, 75 Stern, L.E., 473 Suo, W., viii, 462, 469, 470 Stiglitz, J.E., 474 surveys of appfications, 10 Stirling numbers of the first Sutinen, J.G., 428, 452 kind, 378 Swan, G.W., 295, 474 StirUng numbers of the second Sweeney, D.J., 417, 474 kind, 376 switching curves, 78 Index 497

switching point, 138 Tintner, G., 477 switching time, 80 TitU, A., 472 Sydsaeter, K., 10, 27, 45, 58, 62, Tolwinski, B., 308, 442, 477 64, 81, 99,113, 289, 466, total contribution, 35 474 Tou, J.T., 341, 404, 446 synthesis of optimal controls, 76, Toussaint, S., 477 133 TPBVP, 33, 48, 50, 182, 291 system, 2 Tracz, G.S., 477 system noise, 340 Tragler, G., 477 Szego, G.P., 472 transition matrix, 396 transversality condition, 32, 62, Takayama, A., 36, 289, 431, 474 67, 69, 75, 81 Taksar, M.I., 449, 450, 469, 470 Treadway, A.B., 303, 477 Tan, K.C., 474 Troch, I., 477 Tapiero, C.S., 10, 254, 264, 304, Tsurumi, H., 214, 477 322, 360, 420, 470, 474, Tsurumi, Y., 214, 477 475 Tu, P.N.V., 10, 303, 477 Taylor, J.G., 108, 304, 475 Tuominen, M.P.T., 452 Teichroew, D., 10, 427 Turner, R.E., 185, 477 Teng, J.-T., 182, 475, 476 Turnovsky, S.J., 435, 452, 462, Teo, K.L., 108, 445, 476 477 Terborgh, G., 241, 476 turnpike, 82, 158, 189, 207 terminal conditions, 32, 69 two person zero-sum games, 308 terminal inequality constraints, two-point bovmdary value prob­ 59 lem, 33, 48, 372 terminal time, 4, 25, 62 two-reservoir system, 116 Thepot, J., 304, 451, 476 Tzafestas, S.G., 322, 432, 477 Thisse, J., 444 Thompson's maintenance Udayabhanu, V., 470 model, 331 Uhler, R.S., 478 Thompson, G.L., 45, 119, 153, utility of consiunption, 7, 289, 165, 174, 182, 234, 241, 290, 355 249, 259, 264, 304, 311, 315, 317, 322, 331, 337, Vaisanen, U., 273, 274, 448 347, 351, 418, 428, 429, Valentine, F.A., 10, 478 436, 444, 449, 470, 475, value function, 27, 346 476 Van Hilten, O., 10, 289, 478 Tihomirov, V.M., 444 Van Loon, P.J.J.M., 10, 289, 478 time-optimal control problem, Vanthienen, L., 174, 478 76 Varaiya, P.P., 304, 308, 450, 478 498 Index variational equations, 395 Weinstein, M.C., 279, 479 Veinott, A.F., 418 Weitz, B., 463 Venezia, I., 475 Weizsacker, C.C. von, 479 Verheyen, P.A., 304, 478 Welam, U.P., 479 Verma, B., 445 WeU, K.H., 438 Vickson, R.G., 26, 27, 58, 62, 99, Wensley, R., 463 100, 105, 106, 108, 113, Westphal, L.C., 479 115, 154, 280, 420, 435, wheat trading model with no 442, 460, 478, 480 short-selling, 170 Vidal, R.V.V., 458 Whitin, T.M., 153, 254, 479 Vidale, M.L., 186, 194, 195, 478 Whittle, P., 479 Vidale-Wolfe advertising model, Wickwire, K., 10, 295, 479 194, 353 Wiegand, M., 118, 455 Vilcassim, N.J., 315, 426 Wiener process, 344, 346, 356 Villas-Boas, J.M., 478 Wiener, N., 479 Vincent, T.L., 267, 437 Wind, Y., 430, 447, 454, 475 Vinokurov, V.R., 478 Wirl, F., 434, 480 Vinter, R.B., 103, 108, 324, 434, Wolfe, H.B., 186, 194, 195, 478 463, 472, 478 Wong, K.H., 108, 476 Voelker, J.A., 241, 461 Wonham, W.M., 480 Vousden, N., 303, 452, 479 Wright, C, 303, 480 Wright, S.J., 236, 480 Wagner, H.M., 153, 254, 479 Wunderlich, H.J., 479 Wagner-Whitin framework, 258 Wagner-Whitin solution, 262 Yakowitz, S.J., 236, 458 Wan, F.Y., 473 Yan, H., viii, 469, 480 Wan, Jr., H.Y., 427 Yang, J., 480 Wang, C.-S., 304, 431 Yang, T.H., 108, 462 Wang, P.K.C., 479 Yeh, D., viii warehousing constraint, 175 Yin, G., 360, 462, 468, 470, 471, Warga, J., 479 480 Warnecke, H.J., 479 Young, L.C., 379, 399, 480 Warschat, J., 479 weak forecast horizon, 174, 175 Zaccour, G., 154, 446 weak maximum, 389 Zalkin, J.H., 461 Weierstrass, 9 Zarrop, M.B., 443 Weierstrass necessary condition, Zeckhauser, R.J., 279, 299, 448, 389, 391 479 Weierstrass-Erdmann corner Zeidan, V., 480 conditions, 388 Zemel, E., 457 Index 499

Zhang, H., viii, 462, 463, 470, 471 Zhang, Q., viii, 340, 360, 453, 462, 463, 468, 470, 471, 480 Zhou, X., 466, 471, 480 Ziemba, W.T., 469, 480 Zimin, I.N., 304, 480 Ziolko, M., 480 Zionts, S., 303, 471, 473 Zoltners, A.A., 445, 481 Zowe, J., 103, 450 Zuckermann, D., 475 List of Figures

1.1 The Brachistochrone Problem 9 1.2 A Concave Function 18 1.3 An Illustration of a Saddle Point 19

2.1 An Optimal Path in the State-Time Space 28 2.2 Optimal State and Adjoint Trajectories for Example 2.1 . 38 2.3 Optimal State and Adjoint Trajectories for Example 2.2 . 39 2.4 Optimal Trajectories for Examples 2.3 and 2.4 41 2.5 Optimal Control for Example 2.5 44 2.6 Solution of TPBVP by EXCEL 50 2.7 Water Reservoir of Example 2.12 53

3.1 State and Adjoint Trajectories in Example 3.3 73 3.2 Minimum Time Optimal Response for Problem (3.63) . . 79

4.1 State and Adjoint Trajectories in Example 4.1 102 4.2 Infeasible State Space and Optimal State Trajectory for Example 4.3 109 4.3 Adjoint Trajectory for Example 4.3 Ill 4.4 Two-Reservoir System of Exercise 4.6 116

5.1 Optimal Policy Shown in (Ai, A2) Space 123 5.2 Optimal Policy Shown in (^, A2/A1) Space 124 5.3 Adjoint Variables and Lagrange Multipliers for Example 5.1 128 5.4 Case A: g r 134 5.6 Optimal Path for Case A: g r 143 5.8 Solution for Exercise 5.10 150 5.9 Adjoint Trajectories for Exercise 5.11 151 502 List of Figures

6.1 Optimal Production and Inventory Levels 161 6.2 Solution of Example 6.1 with IQ = 10 163 6.3 Solution of Example 6.1 with IQ = 50 164 6.4 Solution of Example 6.1 with Io=^30 165 6.5 The Price Trajectory (6.49) 168 6.6 Adjoint Variable, Optimal Policy and Inventory in the Wheat Trading Model 169 6.7 Adjoint Trajectory and Optimal Policy for the Wheat Trading Model 173 6.8 Decision Horizon and Optimal Policy for the Wheat Trading Model 175 6.9 Optimal Policy and Horizons for the Wheat Trading Model with Warehouse Constraint 177 6.10 Optimal Policy and Horizons for Example 6.3 179 6.11 Optimal Policy and Horizons for Example 6.4 180 6.12 The Flow Chart for Exercise 6.9 183

7.1 Optimal Policies in the Nerlove-Arrow Model 189 7.2 A Case of a Time-Dependent Turnpike and the Nature of Optimal Control 190 7.3 Phase Diagram of System (7.18) for Problem (7.13) .... 192 7.4 Feasible Arcs in (^, a;)-Space 197 7.5 Optimal Trajectory for Case 1: XQ < x^ and x^ > XT - - • 200 7.6 Optimal Trajectory for Case 2: XQ < x^ and x^ < XT - - - 200 7.7 Optimal Trajectory for Case 3: XQ > x^ and x^ > XT - - - 201 7.8 Optimal Trajectory for Case 4: XQ > x^ and x^ < XT • • - 201 7.9 Optimal Trajectory (Solid Lines) 202 7.10 Optimal Trajectory When T Is Small in Case 1: XQ < x^ 8indxT x^ andxTx 209

8.1 Shortest Distance from a Point to a Semi-Circle 224 8.2 Graph of Example 8.5 224 8.3 Kuhn-Tucker Constraint Qualification 226 8.4 Discrete-Time Conventions 229 8.5 Sketch of x^ and A^ 233 List of Figures 503

9.1 Optimal Maintenance and Machine Resale Value 247 9.2 Sat Function Optimal Control 249

10.1 Optimal Policy for the Sole Owner Fishery Model 271 10.2 Singular Usable Timber Volume x{t) 275 10.3 Optimal Policy for the Forest Thinning Model when XQ < x{to) 276 10.4 Optimal Policy for the Chain of Forests Model when T>i 277 10.5 Optimal Policy for the Chain of Forests Model when T f 285 10.9 Optimal Price Trajectory for T < f 285

11.1 Phase Diagram for the Optimal Growth Model 293 11.2 Optimal Trajectory when XT > x^ 298 11.3 Optimal Trajectory when XT

12.1 Region D with Boundaries Fi and r2 316 12.2 A Partition of Region D 320 12.3 Solution of Equation 12.52 323 12.4 Boundary of No-Drilling Region 328 12.5 Drilling Time 329 12.6 Value of-Q + A(t+)[l-x(t)] 330 12.7 Optimal Maintenance Policy 333 12.8 Replacement Time ti and Maintenance Policy 335 12.9 The Case ^1-^2 336

13.1 Autocorrelation Function for a Scalar Process 343 13.2 A Sample Path of Xt with Xo = a;o > 0 and 5 > 0 . . . . 352

B.l Examples of Admissible Functions for the Problem .... 380 B.2 Variation about the Solution Function 381 B.3 A Broken Extremal with Corner at r 387 504 List of Figures

C.l Needle-Shaped Variation 394 C.2 Trajectories x*{t) and x{t) in a One-Dimensional Case. . . 394 List of Tables

1.1 The Product ion-Inventory Model of Example 1.1 4 1.2 The Advertising Model of Example 1.2 6 1.3 The Consumption Model of Example 1.3 8

3.1 Summary of the Transversality Conditions 75 3.2 State Trajectories and Switching Curves 78 3.3 Objective, State, and Adjoint Equations for Various Model Types 85

5.1 Characterization of Optimal Controls 135

A.l Examples of Homogeneous Equations of Order Two . . . 364 A.2 General Solution Forms for Second-Order Linear Homogeneous Equations, Constant Coefficients 365 A.3 Examples of Homogeneous E]quations of Order n 366 A.4 General Solution Forms for Multiple Roots of Auxiliary Equation 366 A.5 Particular Solution Forms for Various Forcing Functions . 367 A.6 Particular Integrals in Specific Examples 368 A.7 General Solution Forms for Some Homogeneous Partial Differential Equations 374