# 2 complex analysis questions

• Nov 9th 2008, 08:32 PM
2 complex analysis questions
1.) Let G be a region and let f and g be analytic functions on G such that
f(z)g(z) = 0 on G. Show that either f(z) = 0 or g(z) = 0 on G.

2.) Let f be an entire function with constants 0<M, R>0 and positive integer n>1 such that |f(z)|<M|z|^n for all |z|<R. Prove that f is a polynomial of degree less than or equal to n.
• Nov 9th 2008, 08:50 PM
Rapha
Hey

Quote:

1.) Let G be a region and let f and g be analytic functions on G such that
f(z)g(z) = 0 on G. Show that either f(z) = 0 or g(z) = 0 on G.

I dont know

Quote:

2.) Let f be an entire function with constants 0<M, R>0 and positive integer n>1 such that |f(z)|<M|z|^n for all |z|<R. Prove that f is a polynomial of degree less than or equal to n.

f is an entire function, a holomorphic function on $D_\infty(0) = D_\infty(z_0)$

=> $f(x) = \sum^\infty_{n=0} a_n(z-z_0)^n = \sum^\infty_{n=0} a_n*z^n$

show that a_m = 0 if m > N

$|a_m| \le max_{\eta \in \partial D_R(0)} \frac{|f(\eta)|}{r^m}$

For $| \eta | = r \ge R$ is $|f(\eta )| \le M * r^N$

That means $|a_m| \le M*r^N / r^m = M * \frac{1}{r^{m-N}}$

$-> 0, r -> \infty$

$=> |a_m| = 0$

$=> f(z) = \sum^N_{n=0} a_n z^n$

=> f is a polynomial of degree less than or equal to N.
• Nov 11th 2008, 02:22 PM
ThePerfectHacker
Quote:

Hint: If $\{ z_n \}$ is a sequence of convergent distinct points in a region with limit point in the region and $h(z_n) = 0$ where $h$ is analytic on the region then $h$ is identically zero.