Paper 2, Section II, B

Complex Analysis or Complex Methods | Part IB, 2021

(a) Let f:CCf: \mathbb{C} \rightarrow \mathbb{C} be an entire function and let a>0,b>0a>0, b>0 be constants. Show that if

f(z)azn/2+b|f(z)| \leqslant a|z|^{n / 2}+b

for all zCz \in \mathbb{C}, where nn is a positive odd integer, then ff must be a polynomial with degree not exceeding n/2\lfloor n / 2\rfloor (closest integer part rounding down).

Does there exist a function ff, analytic in C\{0}\mathbb{C} \backslash\{0\}, such that f(z)1/z|f(z)| \geqslant 1 / \sqrt{|z|} for all nonzero z?z ? Justify your answer.

(b) State Liouville's Theorem and use it to show the following.

(i) If uu is a positive harmonic function on R2\mathbb{R}^{2}, then uu is a constant function.

(ii) Let L={zz=ax+b,xR}L=\{z \mid z=a x+b, x \in \mathbb{R}\} be a line in C\mathbb{C} where a,bC,a0a, b \in \mathbb{C}, a \neq 0. If f:CCf: \mathbb{C} \rightarrow \mathbb{C} is an entire function such that f(C)L=f(\mathbb{C}) \cap L=\emptyset, then ff is a constant function.

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