Using Maximum Modulus Principle

In summary, according to the maximum modulus principle, if a function is analytic on a domain boundary, then it is analytic on the interior of that domain.
  • #1
d2j2003
58
0

Homework Statement



Suppose p is a polynomial of degree n and |p(z)|≤M if |z|=1

Show that |p(z)|≤M|z|[itex]^{n}[/itex] if |z|≥1

Homework Equations



Maximum Modulus Principle: If f is a nonconstant analytic function on a domain D, then |f| can have no local maximum on D.

The Attempt at a Solution



Book says to apply the maximum modulus principle to [itex]\frac{p(z)}{z^{n}}[/itex] on domain |z|>1 but I am unsure why?
 
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  • #2
p(z)/z^n is 'analytic at infinity'. Has the book gone over what that means?
 
  • #3
I believe f(z) is analytic at infinity if f(1/z) is analytic at z=0
 
  • #4
d2j2003 said:
I believe f(z) is analytic at infinity if f(1/z) is analytic at z=0

That's it. And p(z)/z^n is one of those functions, yes? It's just a polynomial in 1/z. If that's the case you can treat the function as analytic on the domain |z|>1.
 
  • #5
so we can say that since p(z)/z^n is analytic at infinity then it is analytic on the domain |z|>1 so according to the maximum modulus principle, p(z)/z^n can have no local maximum on domain (|z|>1) right?
 
  • #6
I'm thinking now I have to show that |p(z)/z^n|≤M if |z|≥1 is this correct?
 
  • #7
d2j2003 said:
so we can say that since p(z)/z^n is analytic at infinity then it is analytic on the domain |z|>1 so according to the maximum modulus principle, p(z)/z^n can have no local maximum on domain (|z|>1) right?

Yes, so |p(z)/z^n|<=M on |z|>1. Otherwise it would have a local maximum.
 
  • #8
and M is just a real number? it is not specified anywhere... and what about |z|=1 since it says on |z|≥1 and we have looked at |z|>1
 
  • #9
d2j2003 said:
and M is just a real number? it is not specified anywhere... and what about |z|=1 since it says on |z|≥1 and we have looked at |z|>1

Of course M is real. Saying |p(z)|<=M doesn't make any sense if M is complex. The whole point is that if |f(z)|<=M on the boundary of an analytic domain then |f(z)|<=M on the interior of that domain. That's the maximum principle.
 
  • #10
ok i thought so, just seemed like a weird variable for a real number.. but I think i understand it now, thank you so much
 

What is the Maximum Modulus Principle?

The Maximum Modulus Principle is a theorem in complex analysis that states that the maximum value of a holomorphic function on a closed and bounded set in the complex plane occurs either at a boundary point or at a point inside the set where the function is not analytic.

How is the Maximum Modulus Principle used?

The Maximum Modulus Principle is used to find the maximum value of a holomorphic function on a given set, by analyzing the behavior of the function at its boundary points. It is also used to prove the uniqueness of analytic functions, as well as to solve various problems in physics and engineering.

What are the limitations of the Maximum Modulus Principle?

The Maximum Modulus Principle only applies to holomorphic functions, which are functions that are complex differentiable at every point within a given set. It also only applies to closed and bounded sets in the complex plane, and cannot be used for unbounded sets or functions that are not analytic.

What are some real-world applications of the Maximum Modulus Principle?

The Maximum Modulus Principle has various applications in physics and engineering, particularly in the study of fluid dynamics, heat transfer, and electromagnetism. It is also used in the design and analysis of electronic circuits, as well as in the study of fluid flow in porous media.

Are there any alternative principles to the Maximum Modulus Principle?

Yes, there are several alternative principles in complex analysis that are similar to the Maximum Modulus Principle, such as the Minimum Modulus Principle, the Argument Principle, and the Cauchy Integral Theorem. These principles have different applications and may be used in different scenarios depending on the problem at hand.

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