Solving a Complex Problem: Proving a Function Reduces to a Polynomial

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Discussion Overview

The discussion centers around the problem of proving that a function which is analytic in the whole plane and satisfies the inequality |f(z)| < |z|^n for some n and sufficiently large |z| reduces to a polynomial. Participants are seeking clarification on the requirements for this proof and the methods that can be employed.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant expresses confusion about what needs to be shown to prove that the function reduces to a polynomial.
  • Another participant references a general theorem related to entire functions and suggests looking into Liouville's theorem for context.
  • A different participant proposes writing the function as a power series and suggests that the inequality |f(z)| < |z|^n implies that coefficients a_m must be zero for sufficiently large m.
  • Another suggestion involves applying the Cauchy Integral Formula to demonstrate that certain derivatives of the function vanish, which could support the claim of polynomial reduction.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the approach to take or the specific requirements for the proof. Multiple viewpoints and methods are presented without resolution.

Contextual Notes

Some assumptions about the nature of the function and the implications of the inequality are not fully explored. The discussion includes various mathematical techniques that may or may not be applicable depending on the specific conditions of the problem.

Who May Find This Useful

Individuals interested in complex analysis, particularly those studying properties of entire functions and polynomial approximations, may find this discussion relevant.

AlbertEinstein
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Hi all.

The problem is "Prove that a function which is analytic in the whole plane and satisfies an inequality |f(z)| < |z|^n for some n and sufficiently large |z| reduces to a polynomial." I do not understand what I need to show that the function reduces to a polynomial.

Any help will be appreciated.
Thanks.
 
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A more general theorem holds,that no entire function dominates another.. Check out :
http://en.wikipedia.org/wiki/Liouville's_theorem_(complex_analysis )
 
Last edited by a moderator:
AlbertEinstein said:
Hi all.

The problem is "Prove that a function which is analytic in the whole plane and satisfies an inequality |f(z)| < |z|^n for some n and sufficiently large |z| reduces to a polynomial." I do not understand what I need to show that the function reduces to a polynomial.

Any help will be appreciated.
Thanks.

Write [tex]f(z) = \sum_{m=0}^{\infty} a_m\ z^{m}[/tex]

(which you can do since it's entire).

Show that |f(z)| < |z|^n implies for some natural number N,

[tex]a_m = 0[/tex]

for any m >= N.
 
Last edited:
You can apply Cauchy Integral Formula to show that f^(n+m)(a) = 0 for any complex number a in C and m>=1.
 

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