Date: Mar 8, 2013 3:55 AM
Author: quasi
Subject: Re: Elementary complex analysis
William Hughes wrote:

>David C. Ullrich wrote:

>>Paul wrote:

>> >

>> >I suspect there's a theorem about entire complex function

>> >f which have the property that the absolute value of f(z)

>> >tends to infinity as the absolute value of z tends to

>> >infinity. What does this theorem say? I don't know of any

>> >such functions besides polynomials of degree >= 1. Is it

>> >the case that the set of functions which have this

>> >property is just the set of polynomials of degree >= 1.

>>

>> Yes.

>>

>> Non-elementary proof: Look up the Piicard theorems. This is

>> immediate even from the "Little" Picard theorem.

>>

>> Elementary proof: Let g = 1/f. Since f has only finitely many

>> zeroes, g is entire except for finitely many poles. Let R be

>> a rational function with the same poles as g, and with the

>> same principal part at each pole. Then g - R is an entire

>> function that tends to 0 at infinity, so g = R.

>

>Ok, I see why g-R is entire but not why it tends to 0

>at infinity. What am I missing?

I think the following variation of David Ullrich's argument

repairs the flaw.

Let R be the rational function consisting of the sum of all

the principal parts of g at its poles.

Then R approaches to 0 at infinity, hence since g also

approaches 0 at infinity, so does g - R.

As in David's argument, g - R is entire, hence, since g - R

appoaches 0 at infinity, it follows that g = R.

Write R = p/q as a quotient of polynomials where p,q have

no common zeros.

Then f = 1/g = 1/R = q/p.

Since f is entire, p has no zeros, hence p is a nonzero

constant.

Therefore f is a polynomial, as was to be shown.

quasi