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Advanced Complex Analysis Advanced Complex Analysis Course Notes | Harvard University | Math 213a C. McMullen January 8, 2021 Contents 1 Basic complex analysis . 2 2 The simply-connected Riemann surfaces . 39 3 Entire and meromorphic functions . 59 4 Conformal mapping . 81 5 Elliptic functions and elliptic curves . 109 Forward Complex analysis is a nexus for many mathematical fields, including: 1. Algebra (theory of fields and equations); 2. Algebraic geometry and complex manifolds; 3. Geometry (Platonic solids; flat tori; hyperbolic manifolds of dimen- sions two and three); 4. Lie groups, discrete subgroups and homogeneous spaces (e.g. H= SL2(Z); 5. Dynamics (iterated rational maps); 6. Number theory and automorphic forms (elliptic functions, zeta func- tions); 7. Theory of Riemann surfaces (Teichm¨ullertheory, curves and their Ja- cobians); 8. Several complex variables and complex manifolds; 9. Real analysis and PDE (harmonic functions, elliptic equations and distributions). This course covers some basic material on both the geometric and analytic aspects of complex analysis in one variable. Prerequisites: Background in real analysis and basic differential topology (such as covering spaces and differential forms), and a first course in complex analysis. Exercises (These exercises are review.) 3 2 2 2 1. Let T ⊂ R be the spherical triangle defined by x + y + z = 1 and x; y; z ≥ 0. Let α = z dx dz. 3 (a) Find a smooth 1-form β on R such that α = dβ. (b) Define consistent orientations for T and @T . R R (c) Using your choices in (ii), compute T α and @T β directly, and check that they agree. (Why should they agree?) 1 2. Let f(z) = (az + b)=(cz + d) be a M¨obius transformation. Show the number of rational maps g : Cb ! Cb such that g(g(g(g(g(z))))) = f(z) is 1, 5 or 1. Explain how to determine which alternative holds for a given f. P n 3. Let anz be the Taylor series for tanh(z) at z = 0. (a) What is the radius of convergence of this power series? (b) Show that a5 = 2=15. PN (c) Give an explicit value of N such that tanh(1) and 0 an agree to 1000 decimal places. Justify your answer. 4. Let f : U ! V be a proper local homeomorphism between a pair of open sets U; V ⊂ C. Prove that f is a covering map. (Here proper means that f −1(K) is compact whenever K ⊂ V is compact.) 5. Let f : C ! C be given by a polynomial of degree 2 or more. Let 0 V1 = ff(z): f (z) = 0g ⊂ C −1 be the set of critical values of f, let V0 = f (V1), and let Ui = C − Vi for i = 0; 1. Prove that f : U0 ! U1 is a covering map. 6. Give an example where U0=U1 is a normal (or Galois) covering, i.e. where f∗(π1(U0)) is a normal subgroup of π1(U1). 1 Basic complex analysis We begin with an overview of basic facts about the complex plane and analytic functions. Some notation. The complex numbers will be denoted C. We let ∆; H and Cb denote the unit disk jzj < 1, the upper half plane Im(z) > 0, and the 1 1 Riemann sphere C [ f1g. We write S (r) for the circle jzj = r, and S for the unit circle, each oriented counter-clockwise. We also set ∆∗ = ∆ − f0g ∗ and C = C − f0g. 2 1.1 Algebraic and analytic functions The complex numbers are formally defined as the field C = R[i], where i2 = −1. They are represented in the Euclidean plane by z = (x; y) = x+iy. There are two square-roots of −1 in C; the number i is the one with positive imaginary part. An important role is played by the Galois involution z 7! z. We define 2 2 2 jzj = N(z) =pzz = x + y . (Compare the case of a real quadratic field, where N(a + b d) = a2 − db2 gives an indefinite form.) Compatibility of jzj 2 with the Euclidean metric justifies the identification of C and R . We also see that z is a field: 1=z = z=jzj. It is also convenient to describe complex numbers by polar coordinates z = [r; θ] = r(cos θ + i sin θ): Here r = jzj and θ = arg z 2 R=2πZ. (The multivaluedness of arg z requires care but is also the ultimate source of powerful results such as Cauchy's integral formula.) We then have [r1; θ1][r2; θ2] = [r1r2; θ1 + θ2]: In particular, the linear maps f(z) = az + b, a 6= 0, of C to itself, preserve angles and orientations. This formula should be proved geometrically: in fact, it is a consequence of the formula jabj = jajjbj and properties of similar triangles. It can then be used to derive the addition formulas for sine and cosine (in Ahflors the reverse logic is applied). Algebraic closure. A critical feature of the complex numbers is that they are algebraically closed; every polynomial has a root. (A proof will be reviewed below). Classically, the complex numbers were introducing in the course of solv- ing real cubic equations. Staring with x3 + ax + b = 0 one can make a Tschirnhaus transformation so a = 0. This is done by introducing a new 2 P P 2 variable y = cx + d such that yi = yi = 0; even when a and b are real, it may be necessary to choose c complex (the discriminant of the equation for c is 27b2 + 4a3.) It is negative when the cubic has only one real root; this can be checked by looking at the product of the values of the cubic at its max and min. Analytic functions. Let U be an open set in C and f : U ! C a function. We say f is analytic if f(z + t) − f(z) f 0(z) = lim t!0 t 3 exists for all z 2 U. It is crucial here that t approaches zero through arbitrary 1 values in C. Remarkably, this condition implies that f is a smooth (C ) function. For example, polynomials are analytic, as are rational functions away from their poles. Note that any real linear function φ : C ! C has the form φ(v) = av+bv. 0 The condition of analytic says that Dfz(v) = f (z)v; in other words, the v part is absent. 1 To make this point systematically, for a general C function F : U ! C we define dF 1 dF 1 dF dF 1 dF 1 dF = + and = − · dz 2 dx i dy dz 2 dx i dy We then have dF DF DF (v) = v + v: z dz dz We can also write complex-valued 1-form dF as dF dF dF = @F + @F = dz + dz dz dz Thus F is analytic iff @F = 0; these are the Cauchy-Riemann equations. We note that (d=dz)zn = nzn−1; a polynomial p(z; z) behaves as if these variables are independent. Sources of analytic functions. 1. Polynomials and rational functions. Using addition and multipli- Pn n cation we obtain naturally the polynomial functions f(z) = 0 anz : C ! C. The ring of polynomials C[z] is an integral domain and a unique factorization domain, since C is a field. Indeed, since C is algebraically closed, fact every polynomial factors into linear terms. It is useful to add the allowed value 1 to obtain the Riemann sphere Cb = C [ f1g. Then rational functions (ratios f(z) = p(z)=q(z) of rel- atively prime polynomials, with the denominator not identically zero) determine rational maps f : C ! C. The rational functions C(z) are the same as the field of fractions for the domain C[z]. We set f(z) = 1 if q(z) = 0; these points are called the poles of f. 2. Algebraic functions. Beyond the rational and polynomial functions, pthe analytic functions include algebraic functions such that f(z) = 2 PN n z + 1. A general algebraic function f(z) satisfies P (f) = 0 an(z)f(z) = 0 for some rational functions an(z); these arise, at least formally, when 4 one forms algebraic extension of C(z). Such functions are generally multivalued, so we must choose a particular branch to obtain an ana- lytic function. 3. Differential equations. Analytic functions also arise when one solves differential equations. Even equations with constant coefficients, like y00 + y = 0, can give rise to transcendental functions such as sin(z), cos(z) and ez. Here are some useful facts about these familiar functions when extended to C: j exp(z)j = exp Re z cos(iz) = cosh(z) sin(iz) = i sinh(z) cos(x + iy) = cos(x) cosh(y) − i sin(x) sinh(y) sin(x + iy) = sin(x) cosh(y) + i cos(x) sinh(y): In particular, the apparent boundedness of sin(z) and cos(z) fails badly as we move away from the real axis, while jezj is actually very small in the halfplane Re z 0. 4. Integration. A special case of course is integration. While R (x2+ax+ b)−1=2 dx can be given explicitly in terms of trigonometric functions, already R (x3 + ax + b)−1=2 dx leads one into elliptic functions; and higher degree polynomials lead one to hyperelliptic surfaces of higher genus. Note that the `periodicity' of the function in increases from Z 2 1 (trigonometric) to Z (elliptic) to H (Σg; Z) (hyperelliptic). 5. Power series. Analytic functions can be given concretely, locally, by P n power series such as anz . Conversely, suitable coefficients deter- mine analytic functions; for example, ez = P zn=n!.
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