Decreasing sequence of subharmonic functions

In summary, the conversation discusses a problem where u(z) is a continuous function from D to [-inf, inf) in the extended field sense. The problem involves showing that a decreasing sequence of subharmonic functions, u_n (z), converges to a subharmonic function, v(z). The conversation mentions using the maximum principle to solve the problem, but notes that finding a function r(z) that satisfies the conditions is difficult.
  • #1
jian1
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Homework Statement


Let u(z) be a continuous function from D to [-inf, inf) (In the extended field sense which includes -inf). Suppose u_n (z) is a decreasing sequence of subharmonic functions on D such that u_n converges to a function v pointwisely. Show that v(z) is subharmonic.


Homework Equations





The Attempt at a Solution



I have proved that for such u_n (z) are increasing sequences , then v(z) is subharmonic, but encountered problem when the sequence is decreasing.
 
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  • #2
My thought is that, since the maximum principle holds for subharmonic functions, and we know v(z) is a pointwise limit of u_n (z), then if we can find a function r(z) which is smaller than or equal to each u_n (z) and greater than or equal to v(z), then by the maximum principle, v(z) is subharmonic. But I don't think there is such a r(z).
 

1. What is a decreasing sequence of subharmonic functions?

A decreasing sequence of subharmonic functions is a sequence of functions where each function is subharmonic, meaning the Laplacian of the function is non-negative, and the functions decrease as the sequence progresses.

2. What is the significance of studying decreasing sequences of subharmonic functions?

Studying decreasing sequences of subharmonic functions is important in the field of partial differential equations, as it can help in understanding the behavior and properties of solutions to certain types of equations.

3. How do you determine if a function is subharmonic?

A function is subharmonic if its Laplacian is non-negative, meaning the sum of its second-order partial derivatives is always greater than or equal to 0. This can be checked by calculating the Hessian matrix of the function and checking its eigenvalues.

4. Are there any real-world applications of decreasing sequences of subharmonic functions?

Yes, decreasing sequences of subharmonic functions have applications in various fields such as physics, engineering, and economics. For example, in physics, they can be used to model heat flow and diffusion processes.

5. What are some properties of decreasing sequences of subharmonic functions?

Some properties of decreasing sequences of subharmonic functions include monotonicity, meaning the functions either decrease or remain constant as the sequence progresses, and convergence, meaning the sequence of functions converges to a limit function as the number of functions in the sequence increases.

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