# Analytic problem

• Dec 16th 2011, 04:13 AM
fareastmovement
Analytic problem
Let $f(z)$ be analytic in the disk $|z| <1$. If $f(z)$ has a zero of order 2 at the origin and $|f(z)| \le 1$ in that disk. Prove that $|f(z)|\le|z|^2$ in $|z|<1$

I have no idea where to start. Thank you.
• Dec 16th 2011, 04:35 AM
FernandoRevilla
Re: Analytic problem
Hint: In the series expansion $f(z)=\sum_{n=0}^{+\infty}a_nz^n$ we have $a_0=a_1=0$ , so $f(z)/z^2$ is analytic in $|z|<1$ .
• Dec 16th 2011, 06:07 AM
fareastmovement
Re: Analytic problem
Thank you for you reply, I could do what you said but whats the next step? Thanks:)
• Dec 16th 2011, 07:31 AM
FernandoRevilla
Re: Analytic problem
By hypothesis $|f(z)|\leq 1$ so, $|f(z)/z^2|\leq 1/r^2$ for $|z|=r$ . This equality is also valid for $|z|\leq r$ according to the Maximum Modulus Principle. If we fix $z$ in $|z|<1$ we have $|f(z)|\leq |z|^2/r^2$ for all $r\geq |z|$ and $<1$ . You can conclude.
• Dec 16th 2011, 08:17 AM
fareastmovement
Re: Analytic problem
Quote:

Originally Posted by FernandoRevilla
By hypothesis $|f(z)|\leq 1$ so, $|f(z)/z^2|\leq 1/r^2$ for $|z|=r$ . This equality is also valid for $|z|\leq r$ according to the Maximum Modulus Principle. If we fix $z$ in $|z|<1$ we have $|f(z)|\leq |z|^2/r^2$ for all $r\geq |z|$ and $<1$ . You can conclude.

Sorry but I didn't get it. How can I deduce $|f(z)/z^2|\leq 1/r^2$for $|z|\leq r$ according to the Maximum Modulus Principle? Thanks!!!
• Dec 16th 2011, 08:25 AM
FernandoRevilla
Re: Analytic problem
Quote:

Originally Posted by fareastmovement
Sorry but I didn't get it. How can I deduce $|f(z)/z^2|\leq 1/r^2$for $|z|\leq r$ according to the Maximum Modulus Principle? Thanks!!!

What does the Maximum Modulus Principle say?
• Dec 16th 2011, 08:48 AM
fareastmovement
Re: Analytic problem
Suppose $f$ is analytic on $|z-z_0|<\epsilon$. If $|f(z)| \le |f(z_0)|$ for $z$ on this region then $f(z) \equiv f(z_0)$ on the region.

but $|f(z)/z^2|\leq 1/r^2$. How can I make sure that $f(z_0)=1/r^2$? Or I am in the wrong direction? Thank you (Itwasntme)
• Dec 16th 2011, 12:34 PM
FernandoRevilla
Re: Analytic problem
Better use this version: Let $D \subset \mathbb{C}$ be a bounded domain, and let $f$ be a continuous function on the closed set $\overline{D}$ that is analytic on $D$. Then the maximum value of $|f|$ on $\overline{D}$ (which always exists) occurs on the boundary $\partial D$ . So, in our case, it is not possible $|f(z)/z^2|>1/r^2$ if $|z| .