# UMD Analysis Qualifying Exam/Jan08 Complex

## Problem 2

 Prove there is an entire function $f\!\,$ so that for any branch $g\!\,$ of ${\sqrt {z}}\!\,$ $\sin ^{2}(g(z))=f(z)\!\,$ for all $z\!\,$ in the domain of definition of $g\!\,$ ## Solution 2

Key steps

• $\sin ^{2}(\theta )={\frac {1-\cos(2\theta )}{2}}\!\,$
• $\cos(z)=\sum _{n=1}^{\infty }{\frac {(-1)^{n}z^{2n}}{(2n)!}}\!\,$
• ratio test

## Problem 4

 Let $H\!\,$ be the domain $\{z:-{\frac {\pi }{2}}<\Re (z)<{\frac {\pi }{2}},\Im (z)>0\}\!\,$ . Prove that $g=\sin(z)\!\,$ is a 1:1 conformal mapping of $H\!\,$ onto a domain $D\!\,$ . What is $D\!\,$ ?

## Solution 4

### Showing G 1:1 conformal mapping

First note that

{\begin{aligned}(1)\quad |g^{\prime }(z)|&=|\sin ^{\prime }(z)|\\&=|\cos(z)|\\&=\left|{\frac {e^{-iz}+e^{iz}}{2}}\right|\\&\geq {\frac {1}{2}}(|e^{-ix}||e^{y}|-|e^{ix}||e^{-y}|)\\&\geq {\frac {1}{2}}(e^{y}-e^{-y})\\&>0\quad {\mbox{since }}y>0\end{aligned}}

Also, applying a , we have for all $z_{1},z_{2}\in H\!\,$ ,

$(2)\quad \sin z_{1}-\sin z_{2}=2\sin \left({\frac {z_{1}-z_{2}}{2}}\right)\cos \left({\frac {z_{1}+z_{2}}{2}}\right)\!\,$

Hence if $\sin z_{1}=\sin z_{2}\!\,$ , then

$\sin \left({\frac {z_{1}-z_{2}}{2}}\right)=0\!\,$

or

$\cos \left({\frac {z_{1}+z_{2}}{2}}\right)=0\!\,$

The latter cannot happen in $H\!\,$  since $|g^{\prime }(z)|=|\cos(z)|>0\!\,$  so

$\sin \left({\frac {z_{1}-z_{2}}{2}}\right)=0\!\,$

i.e.

$z_{1}=z_{2}\!\,$

Note that the zeros of $\sin(z)=\sin(x+iy)\!\,$  occur at $x=k\pi ,k\in \mathbb {Z} \!\,$ . Similary the zeros of $\cos(z)=\cos(x+iy)\!\,$  occur at $x={\frac {\pi }{2}}+k\pi ,k\in \mathbb {Z} \!\,$ .

Therefore from $(1)\!\,$  and $(2)\!\,$ , $g\!\,$  is a $1:1\!\,$  conformal mapping.

### Finding the domain D

To find $D\!\,$ , we only need to consider the image of the boundaries.

Consider the right hand boundary, $C_{3}=\{z=x+iy|x={\frac {\pi }{2}},y>0\}\!\,$

{\begin{aligned}g(C_{3})&=g\left({\frac {\pi }{2}}+yi\right)\\&=\sin \left({\frac {\pi }{2}}+yi\right)\\&={\frac {e^{i({\frac {\pi }{2}}+yi)}-e^{-i({\frac {\pi }{2}}+yi})}{2i}}\\&={\frac {e^{i{\frac {\pi }{2}}}e^{-y}-e^{-i{\frac {\pi }{2}}}e^{y}}{2i}}\\&={\frac {e^{i{\frac {\pi }{2}}}e^{-y}+e^{i{\frac {\pi }{2}}}e^{y}}{2i}}\\&={\frac {e^{i{\frac {\pi }{2}}}(e^{-y}+e^{y})}{2i}}\\&={\frac {i(e^{-y}+e^{y})}{2i}}\\&={\frac {e^{-y}+e^{y}}{2}}\\\end{aligned}}

Since $y>0\!\,$ ,

$g(C_{3})=(1,\infty )\!\,$

Now, consider the left hand boundary $C_{2}=\{z=x+iy|x=-{\frac {\pi }{2}},y>0\}\!\,$ .

{\begin{aligned}g(C_{2})&=g\left(-{\frac {\pi }{2}}+yi\right)\\&=\sin \left(-{\frac {\pi }{2}}+yi\right)\\&={\frac {e^{i(-{\frac {\pi }{2}}+yi)}-e^{-i(-{\frac {\pi }{2}}+yi})}{2i}}\\&={\frac {e^{-i{\frac {\pi }{2}}}e^{-y}-e^{i{\frac {\pi }{2}}}e^{y}}{2i}}\\&={\frac {-e^{i{\frac {\pi }{2}}}e^{-y}-e^{i{\frac {\pi }{2}}}e^{y}}{2i}}\\&={\frac {e^{i{\frac {\pi }{2}}}(-e^{-y}-e^{y})}{2i}}\\&=-{\frac {i(e^{-y}+e^{y})}{2i}}\\&=-{\frac {e^{-y}+e^{y}}{2}}\\\end{aligned}}

Since $y>0\!\,$ ,

$g(C_{2})=(-\infty ,-1)\!\,$

Now consider the bottom boundary $C_{1}=\{z=x+iy|-{\frac {\pi }{2}}<=x<={\frac {\pi }{2}},y=0\}\!\,$ .

{\begin{aligned}g(C_{1})&=g(x)\\&=\sin(x)\end{aligned}}

Since $-{\frac {\pi }{2}}<=x<={\frac {\pi }{2}}\!\,$ ,

$g(C_{2})=[-1,1]\!\,$

Hence, the boundary of $H\!\,$  maps to the real line. Using the test point $z=i\!\,$ , we find

{\begin{aligned}g(i)&=\sin(i)\\&={\frac {e^{i(i)}-e^{-i(i)}}{2i}}\\&={\frac {e^{-1}-e^{1}}{2i}}\\&={\frac {-i(e^{-1}-e^{1})}{2}}\\&={\frac {i(e^{1}-e^{-1})}{2}}\\&={\frac {i}{2}}(e-{\frac {1}{e}})\\&\in {\mbox{Upper Half Plane}}\end{aligned}}

We then conclude $D=g(H)={\mbox{Upper Half Plane}}\!\,$

## Problem 6

 Suppose that for a sequence $a_{n}\in R\!\,$ and any $z,\Im (z)>0\!\,$ , the series $h(z)=\sum _{n=1}^{\infty }a_{n}\sin(nz)\!\,$ is convergent. Show that $h\!\,$ is analytic on $\{\Im (z)>0\}\!\,$ and has analytic continuation to $C\!\,$ ## Solution 6

### Summation a_n Convergent

We want to show that $\sum _{n=1}^{\infty }a_{n}\!\,$  is convergent. Assume for the sake of contradiction that $\sum _{n=1}^{\infty }a_{n}\!\,$  is divergent i.e.

$\sum _{n=1}^{\infty }a_{n}=\infty \!\,$

Since $h(z)\!\,$  is convergent in the upper half plane, choose $z=i\!\,$  as a testing point.

{\begin{aligned}h(i)&=\sum _{n=1}^{\infty }a_{n}\sin(ni)\\&=\sum _{n=1}^{\infty }a_{n}{\frac {e^{-n}-e^{n}}{2i}}\\&=\sum _{n=1}^{\infty }a_{n}{\frac {-i(e^{-n}-e^{n})}{2}}\\&=\sum _{n=1}^{\infty }a_{n}{\frac {i(e^{n}-e^{-n})}{2}}\end{aligned}}

Since $h(i)\!\,$  converges in the upper half plane, so does its imaginary part and real part.

$\Im (h(i))={\frac {1}{2}}\sum _{n=1}^{\infty }a_{n}\underbrace {(e^{n}-e^{-n})} _{E_{n}}\!\,$

The sequence $\{E_{n}\}\!\,$  is increasing ($E_{1} ) since $e^{n+1}>e^{n}\!\,$  and $e^{-n}>e^{-(n+1)}\!\,$  e.g. the gap between $e^{n}\!\,$  and $e^{-n}\!\,$  is grows as $n\!\,$  grows. Hence,

{\begin{aligned}\Im (h(i))&\geq {\frac {1}{2}}(e^{1}-e^{-1})\underbrace {\sum _{n=1}^{\infty }a_{n}} _{=\infty }\\\\\\\Im (h(i))&\geq \infty \end{aligned}}

This contradicts that $h(i)\!\,$  is convergent on the upper half plane.

### Show that h is analytic

In order to prove that $h\!\,$  is analytic, let us cite the following theorem

Theorem Let ${h_{n}}\!\,$  be a sequence of holomorphic functions on an open set $U\!\,$ .   Assume that for each compact subset $K\!\,$  of $U\!\,$  the sequence converges uniformly on $K\!\,$ , and let the limit function be $h\!\,$ .   Then $h\!\,$  is holomorphic.

Proof See Theorem 1.1 in Chapter V, Complex Analysis Fourth Edition, Serge Lang.

Now, define $h_{n}(z)=\sum _{k=1}^{n}a_{k}\sin(kz)\!\,$ .   Let $K\!\,$  be a compact set of $U=\{\Im {z}>0\}\!\,$ .   Since $\sin(kz)\!\,$  is continuous