Elementary Complex Functions I Textbook Sections 17.6-17.7

Elementary Complex Functions I textbook sections 17.6-17.7

MATH 241

March 15, 2012

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 Elementary Real Functions n n−1 polynomials anx + an−1x + ··· + a1x + a0 exponential functions ex , bx trigonometric functions sin x, cos x 1 x −x hyperbolic functions sinh x = 2 (e − e ), 1 x −x cosh x = 2 (e + e ) logarithmic functions ln x inverse trigonometric functions arcsin x, arccos x

These are all continuous and diﬀerentiable to all orders.

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 Complex polynomials

Complex polynomials:

n n−1 αnz + αn−1z + ··· + α1z + α0

where αi are complex numbers and z is a complex variable

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 Complex exponential

Deﬁnition The real exponential function g(x) = ex is the unique solution of the IVP g 0 = g, g(0) = 1.

x x d The function f (x + iy) = e cos y + ie sin y has dz f = f and f (0) = 1.

Deﬁnition The complex exponential function is given by:

exp(x + iy) = ex cos y + iex sin y

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 If z = x + 0i is real, then exp(z) = ex . If z = 0 + iy is purely imaginary, then exp(z) = eiy = cos(y) + i sin(y) exp(z + w) = exp(z) exp(w) exp(z + 2πi) = exp(z)

Complex exponential

Properties of exp(z)

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 If z = 0 + iy is purely imaginary, then exp(z) = eiy = cos(y) + i sin(y) exp(z + w) = exp(z) exp(w) exp(z + 2πi) = exp(z)

Complex exponential

Properties of exp(z) If z = x + 0i is real, then exp(z) = ex .

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 exp(z + w) = exp(z) exp(w) exp(z + 2πi) = exp(z)

Complex exponential

Properties of exp(z) If z = x + 0i is real, then exp(z) = ex . If z = 0 + iy is purely imaginary, then exp(z) = eiy = cos(y) + i sin(y)

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 exp(z + 2πi) = exp(z)

Complex exponential

Properties of exp(z) If z = x + 0i is real, then exp(z) = ex . If z = 0 + iy is purely imaginary, then exp(z) = eiy = cos(y) + i sin(y) exp(z + w) = exp(z) exp(w)

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 Complex exponential

Properties of exp(z) If z = x + 0i is real, then exp(z) = ex . If z = 0 + iy is purely imaginary, then exp(z) = eiy = cos(y) + i sin(y) exp(z + w) = exp(z) exp(w) exp(z + 2πi) = exp(z)

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 Complex exponential function

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 Complex logarithm

Recall If x and y are real numbers,

ey = x y = ln x

Since ey is invertible, ln x is a function.

If we set exp(w) = z and solve for w, we get

w = ln |z| + iarg(z)

Note ln z is not a function!

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 Complex logarithm

Deﬁnition The principal logarithm of z is

Ln(z) = ln |z| + iArg(z)

Lnz is analytic except where arg(z) = π.

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 Other exponentials

Deﬁnition If b is a complex number, we deﬁne

bz = exp(z ln b)

Note bz may not be a function.

Deﬁnition The principal value of bz is exp(zLnb).

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 Trigonometric functions

Deﬁnition The real trigonometric functions c(x) = cos(x) and s(x) = sin(x) are the unique solution of the IVP

s0 c s(0) 0 = , = c0 −s c(0) 1

Deﬁnition The complex trigonometric functions are:

sin(x + iy) = sin x cosh y + i cos x sinh y cos(x + iy) = cos x cosh y − i sin x sinh y

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 Trigonometric functions

Euler’s formulae

1 cos(z) = eiz + e−iz 2 1 sin(z) = eiz − e−iz 2i eiz = cos(z) + i sin(z)

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 Trigonometric functions

Zeros sin z = 0 when z = nπ

π cos z = 0 when z = 2 + nπ

The trigonmetric functions are 2π-periodic.

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7 sin(z)

MATH 241 Elementary Complex Functions I textbook sections 17.6-17.7