# Probability distribution of uAv…

Consider the complex domain ℂ. If U and V are 2 unitary random matrices and A is a deterministic matrix.

What is the distribution of $u^HAv$ ( or $||u^HAv||^2$)

where : u is a column vector of U. v is a column vector of V.

Simulations give that $||u^HAv||^2$ is exponential; is it true ? if yes, how to prove it ?

Have a nice day!

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a simple and exact answer follows if $A$ is a unitary $n\times n$ matrix; then $\tilde{v}=Av$ is a unit vector and since $u$ is a randomly oriented unit vector the distribution of the scalar product $u^HAv=(u,\tilde{v})$ does not depend on the orientation of $\tilde{v}$, so we may take it in the $x_1$ direction, hence

$$P(u^H Av)=P(u_1).$$

the distribution of a single element $u_1$ of the random unit vector $u$ follows upon integration over the other $n-1$ elements,

$$P(u_1)= \int du_2\int du_3\cdots\int du_n\;\delta\left(1-\sum_{i=1}^{n}|u_i|^2\right)={\rm constant}\times(1-|u_1|^2)^{n-2},$$

hence $\xi=|u^H Av|^2$ has the distribution

$$P(\xi)=(n-1)(1-\xi)^{n-2},\;\;0<\xi<1.$$

this is exact for any $n\geq 2$; for $n\gg 1$ the distribution becomes exponential to leading order in $1/n$,

$$P(\xi)\approx ne^{-n\xi}.$$

for non unitary $A$, the answer will depend on its singular values $a_1,a_2,\ldots a_n$; I doubt that you can find a simple closed-form expression for any $n$, but for $n\gg 1$ again an exponential distribution will appear.

To see this, define the unit vector $v'=|Av|^{-1}\,Av$ and first consider the distribution of $\xi'=|u^H v'|^2$. Here the same argument as above applies, so for $n\gg 1$ we have $P(\xi')\propto\exp(-n\xi')$. The quantity $\xi$ we want is related to $\xi'$ by $\xi=|Av|^2\xi'$. The factor $|Av|^2$ has a normal distribution in the large-$n$ limit, with a mean $$E[|Av|^2]=\frac{1}{n}{\rm Tr}\,AA^H$$ and a variance that is an order $1/n$ smaller; to leading order in $1/n$ we may therefore just equate

$$\xi\approx E[Av|^2]\xi'\Rightarrow P(\xi)\approx\frac{n^2}{{\rm Tr}\,AA^H}\exp\left(-\frac{n^2}{{\rm Tr}\,AA^H}\xi\right).$$

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Could you please give me some references for the cases you have explained. thank you very much –  mat Jun 1 '14 at 15:28
hmm, I don't really have references, if you tell me for which step you would like to have further explanations, I can try to fill you in. –  Carlo Beenakker Jun 1 '14 at 15:42
in the mean time, you might check your numerics that for unitary $A$ you find $P(\xi)\propto(1-\xi)^{n-2}$, exactly for small $n$ (the exponential distribution only follows in the large-$n$ limit) –  Carlo Beenakker Jun 1 '14 at 15:44
"for non unitary A and n>>1 I would think the same exponential distribution will appear.." if you don't have the answer, I will be grateful if you give me the explanation for the case where A is unitary (and n>>1). –  mat Jun 1 '14 at 15:52
added details at various steps of the derivation, and generalized it to the general case of nonunitary $A$. –  Carlo Beenakker Jun 1 '14 at 18:12