Paper 3 , Section I, 2H2 H

Topics in Analysis | Part II, 2021

State Runge's theorem on the approximation of analytic functions by polynomials.

Let Ω={zC,Rez>0,Imz>0}\Omega=\{z \in \mathbb{C}, \operatorname{Re} z>0, \operatorname{Im} z>0\}. Establish whether the following statements are true or false by giving a proof or a counterexample in each case.

(i) If f:ΩCf: \Omega \rightarrow \mathbb{C} is the uniform limit of a sequence of polynomials PnP_{n}, then ff is a polynomial.

(ii) If f:ΩCf: \Omega \rightarrow \mathbb{C} is analytic, then there exists a sequence of polynomials PnP_{n} such that for each integer r0r \geqslant 0 and each zΩz \in \Omega we have Pn(r)(z)f(r)(z)P_{n}^{(r)}(z) \rightarrow f^{(r)}(z).

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