A functional that depends on an integral?

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    Functional Integral
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Discussion Overview

The discussion revolves around the possibility of finding extrema of an integral equation where the integrand involves a function and the integral of that function. Participants explore the applicability of the calculus of variations to such cases, particularly focusing on a specific integral expression.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant questions whether the integral in the form of f(x) * g(integral(x)) constitutes a "nonlocal" functional and expresses difficulty in finding references on this topic.
  • Another participant reformulates the question, asking if the calculus of variations can be applied to the integral expression involving f(y,x) and its logarithm.
  • A third participant attempts to apply the Euler-Lagrange equations to the problem but reports being unsuccessful, suggesting that a specific case might be solvable but expressing uncertainty about the general approach.
  • A fourth participant questions the equivalence of two integral expressions involving f(y,x) and proposes a potential simplification, though they express uncertainty about the correctness of their reasoning.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the applicability of the calculus of variations to the problem, and there are differing views on the equivalence of the integral expressions discussed.

Contextual Notes

Participants express uncertainty regarding the definitions and properties of the functionals involved, as well as the applicability of standard techniques like the Euler-Lagrange equations to the specific case presented.

jfitz
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Is it possible to find the extrema of an integral equation if the integral depends on a variable and an integral of that variable, i.e. the integrand is f(x) * g(integral(x)).

I'm not sure if this is a "nonlocal" functional, or not a functional at all, but I can't find any references that deal with this type of problem.

If anybody has any ideas of where I could find more information about this, please let me know.
 
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In case the question wasn't clear, here it is in a different way:

Does the calculus of variations apply to situations where you're looking for some function, y(x), that extremizes (in my particular case)

\int{ f(y,x) \ln{ \left[ \int{ f(y,x) dx } \right] } dx }

?
 
I tried to apply the quick-and-dirty physicist's derivation of the Euler-Lagrange equations to this case but came up empty handed. Maybe it is possible to solve for the particular case of the function f you are interested in, but otherwise I am at a loss for how to proceed.
 
jfitz said:
\int{ f(y,x) \ln{ \left[ \int{ f(y,x) dx } \right] } dx }

?

Maybe I'm being stupid but isn't that equal to

\int{ f(y,x) \ln{ \left[ \int{ f(y,x') dx' } \right] } dx }=\ln{ \left[ \int{ f(y,x') dx' } \right]\int{ f(y,x) } dx }=\ln{ \left[ \int{ f(y,x) dx } \right]\int{ f(y,x) } dx }

...?
 

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