Question

Determine whether each of the following statement is true or false. Justify your answer with a short proof or a counter example.

i) If z = a +ib , where a and b are integers, then n [1+Z+Z^{2}+......+Z^{n}]geq ]z]^{n} if a>0.

 

 

ii) If f (z) and ar{f(z)} are analytic functions in a domain, then f is necessarily a constant.

 

iii) A real-valued function u(x, y) is harmonic in D iff u(x, − y) is harmonic in D .

 

iv) lim_{n 	o infty }{(n!)}^{	frac{1}{n}}=infty .

 

v) The inequality [e^{a}-e^{b}]leq ]a-b] holds for [e^{a}-e^{b}]leq ]a-b] a, bepsilon D={w:Re leq 0left { ight }

 

vi) If f(z) sum_{n=0}^{infty }a_{n}(z-a)^{n} has the property that  sum_{n=0}^{infty } f^{(n)}(a)converges, then f is necessarily an entire function.

 

vii) If a power series sum_{n=0}^{infty } a_{n}z^{n}converges for left | z ight |< 1and if bn b_{n}in mathbb{C} is such thatleft | b_{n} ight |< n^{2}left | a_{n}ight | for all ngeq 0 , thensum_{n=0}^{infty }b_{n}z^{n} converges for left | z ight |< 1.

 

viii) If f is entire and f (z) = f (−z) for all z , then there exists an entire function g such that f(z)=g(z^{2}) for all zin mathbb{C}

ix) A mobius transformation which maps the upper half planeleft { z:Im z> 0 ight } onto itself and fixing 0,infty and no other points, must be of the form Tz =alphaz for some alpha > 0 0 and alpha eq 1..

 

x) If f is entire and Re f (z) is bounded as | z| → infty , then f is constant. 

20 Mar 2023
Answer :
Word Count : 740

i) False. The statement appears to have some typographical errors, so I'll address a corrected version: If z = a + ib, where a and b are integers, then |1 + Z + Z^2 + ... + Z^n| ≥ |z|^n if a > 0. Counterexample: Take z = 1 (a = 1, b = 0). Then, 1 + Z + Z^2 + ... + Z^n = 1 + 1 + ... + 1 = n + 1, and |z|^n = 1. For n > 1, |1 + Z + Z^2 + ... + Z^n| < |z|^n.

ii) False. Counterexample: Let f(z) = z and g(z) = z*. Both f(z) and g(z) are analytic, but f(z) is not a constant.

iii) True. If u(x, y) is harmonic, then it satisfies Laplace's equation: u_{xx}(x, y) + u_{yy}(x, y) = 0. Let v(x, y) = u(x, -y). Then, v_{xx}(x, y) = _________ ____ _________ _________ _____.
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