# What are some general properties of $F(x, z) = \int f(x, y) f(y, z) dy$?

What are some general properties of $F(x, z) = \int f(x, y) f(y, z) dy$ ?

For example, what can be said about the relation between $F(x,z)$ and $f(x,z)$?

Optical theorem in physics of wave scattering has this form. What other theorems in natural sciences and mathematics also have this form?

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I will instead consider the operation taking $f$ and $g$ to $F(x,z)=\int f(x,y) g(y,z)d y$. Everything said below is under mild assumptions, measurability or continuity.

• The operation is associative.

• As Noah observed, when the integral is a sum, it is matrix product.

• For symmetric functions: it induces inner product on space of continuous symmetric bivariate real functions.

• The formula reminds me of Möbius transform and incidence algebra, I think it is a continuous form of this.

• It reminds me also of composition of relations and composition of profunctors in category theory

• Also faintly reminds me of convolution

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It is hard to give a detailed answer without knowing which kind of properties you're seeking. One idea: it may be helpful to think of the analogous definition for $x$, $y$, and $z$ discrete, taking values in $\{1,\ldots, n\}$. In this case $f$ is a matrix and $F$ is just $f^2$, the matrix product of $f$ with itself. Depending on your situation this may yield some insights. For example, if $f$ is a symmetric matrix then $f^2$ is positive semidefinite, and a similar statement should hold for the case of continuous variables if the terms are reinterpreted appropriately.

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