Probability distribution of a stochastic variable

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SUMMARY

The discussion focuses on the probability distribution of a stochastic variable V, as defined by a generator functional G[h]. The functional is expressed as G[h] = ⟨exp(i∫ V(𝑟,t)h(𝑟,t)d𝑟dt)⟩, where h is an arbitrary function and ⟨⟩ denotes expectation values. The author equates this to a specific form involving a double integral, G[h] = exp(-1/2∬ h(𝑟,t)h(𝑟′,t)f(𝑟−𝑟′)d𝑟d𝑟′dt). The discussion seeks clarification on the mathematical steps involved in this derivation and the justification provided by the author.

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Ravi Mohan
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I am studying an article which involves stochastic variables http://www.rmki.kfki.hu/~diosi/prints/1985pla112p288.pdf.

The author defines a probability distribution of a stochastic potential V by a generator functional
<br /> G[h] = \left&lt;exp\left(i\int V(\vec{r},t)h(\vec{r},t)d\vec{r}dt\right)\right&gt;,<br />
where h is an arbitrary function and \langle\rangle stands for expectation values evaluated by means of the probabil-
ity distribution of V.

He, then equates it to (equation 1 in the article)
<br /> G[h] = exp\left(-\frac{1}{2}\iint h(\vec{r},t)h(\vec{r}^{\prime},t)f(\vec{r}-\vec{r}^{\prime})d\vec{r}d\vec{r}^{\prime}dt\right).<br />

How do we mathematically work out the steps? Any relevant reference or hint will be of great help. Thanks.
 
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I don't understand the author's justification. However it looks like something of the form:

&lt;\sqrt(A,A*)&gt;, where A is the exponential integral.
 

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