Complex Analysis: Showing f is a Polynomial of Degree n

In summary, for a function f that is analytic on C with a given condition of |f(z)|<=M|z|^n, it can be shown that f is a polynomial function of degree less than n by using the Cauchy integral formula and proving that f^{(k)}(0) = 0 for k>=n. This can be done by taking the limit as z approaches 0.
  • #1
AcC
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Homework Statement



Let f be analytic throught C, suppose that |f(z)|<=M|z|^n for a real constant M and positive integer n. Show that f is a polynomial function of degree less than n.
 
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  • #2
If you can show that [itex]f^{(k)}(0) = 0[/itex] for [itex] k\geq n[/itex] that would establish that f(z) is a polynomial of degree less than n. Try using the Cauchy integral formula
 
  • #3
praharmitra said:
If you can show that [itex]f^{(k)}(0) = 0[/itex] for [itex] k\geq n[/itex] that would establish that f(z) is a polynomial of degree less than n. Try using the Cauchy integral formula

Thanks a lot for hint..:)

I have solved this as below;

since f is analytic on C so f is differentiable on C, if we use Cauchy's Estimate then
we find
|f[tex]^{(k)}[/tex](z)|<=M.|z|^n.k!/|z|^k for k>=n

if take lim as z→0
then we find f[tex]^{(k)}[/tex](0)=0 for k>=n

is it true!
 

FAQ: Complex Analysis: Showing f is a Polynomial of Degree n

1. What is complex analysis?

Complex analysis is a branch of mathematics that deals with the study of functions of complex numbers. It involves the analysis of complex-valued functions, which are functions that map complex numbers to other complex numbers.

2. How is a polynomial of degree n defined in complex analysis?

A polynomial of degree n in complex analysis is a function that can be written as f(z) = anzn + an-1zn-1 + ... + a1z + a0, where an, an-1, ..., a1, a0 are complex numbers and n is a non-negative integer.

3. What does it mean for a function to be a polynomial of degree n?

A function f(z) is a polynomial of degree n if it satisfies the definition mentioned in the previous question. This means that the highest power of z in the function is n, and all the coefficients (an, an-1, ..., a1, a0) are complex numbers.

4. How can we show that a function f(z) is a polynomial of degree n using complex analysis?

To show that a function f(z) is a polynomial of degree n using complex analysis, we need to prove that it satisfies the definition mentioned in question 2. This involves showing that the function can be written in the form f(z) = anzn + an-1zn-1 + ... + a1z + a0, where an, an-1, ..., a1, a0 are complex numbers and n is a non-negative integer.

5. Why is it important to show that a function f(z) is a polynomial of degree n in complex analysis?

Showing that a function f(z) is a polynomial of degree n in complex analysis is important because it allows us to understand the behavior of the function better. Polynomials are relatively simple functions and their properties are well-known, making them easier to analyze. Additionally, many complex functions can be approximated by polynomials, so studying polynomials can provide insights into more complex functions.

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