Paper 2, Section II, F

Topics in Analysis
Part II, 2012

(a) State Runge's theorem about uniform approximability of analytic functions by complex polynomials.

(b) Let KK be a compact subset of the complex plane.

(i) Let Σ\Sigma be an unbounded, connected subset of C\K\mathbb{C} \backslash K. Prove that for each ζΣ\zeta \in \Sigma, the function f(z)=(zζ)1f(z)=(z-\zeta)^{-1} is uniformly approximable on KK by a sequence of complex polynomials.

[You may not use Runge's theorem without proof.]

(ii) Let Γ\Gamma be a bounded, connected component of C\K\mathbb{C} \backslash K. Prove that there is no point ζΓ\zeta \in \Gamma such that the function f(z)=(zζ)1f(z)=(z-\zeta)^{-1} is uniformly approximable on KK by a sequence of complex polynomials.