
Conditional expectation
Let $\displaystyle \xi_1,\xi_2,\xi_3$ be three independent, identically distributed and bounded random variables with density $\displaystyle p(x)$.
What is the distribution of: $\displaystyle \mathbb{E}\big(\max(\xi_1,\xi_2,\xi_3)\min(\xi_1, \xi_2,\xi_3)\big)$, as a function of $\displaystyle p$ ?
One can easily obtain the distribution of $\displaystyle \max(\xi_1,\xi_2,\xi_3)$ and $\displaystyle \min(\xi_1,\xi_2,\xi_3)$. But how do you get the joint distribution?
Unless there is a better way to proceed.
Thanks in advance for your help.

Let $\displaystyle M=\max(\xi_1,\xi_2,\xi_3)$ and $\displaystyle m=\min(\xi_1,\xi_2,\xi_3)$. You can start from there:
$\displaystyle P(t\leq m\mbox{ and }M\leq u)=P(t\leq \xi_1\leq u)^3$.
This gives the distribution of $\displaystyle (m,M)$.

you can use the 'multinomial' formula for joint densities of order stats
$\displaystyle f(x_1,x_3)=3!f(x_1)[F(x_3)F(x_1)]f(x_3)$
where I'm using $\displaystyle x_1$ as the first order stat and $\displaystyle x_3$ as the third.

This is what you get using the joint probability suggested by Laurent.
I am not familiar with the multinomial formula you mention. Is there any reference you could suggest where I can find it?
Thanks for your answer.

It's in many books, Hogg and Craig for one.
I googled it the other day and I only found it online at another forum.
It was incorrect initially but someone finally fixed it.
sadly wikipedia only has this for uniforms...
http://en.wikipedia.org/wiki/Order_statistic
but from that you can see the full formula.
You can obtain the joint density of any subset of order stats directly.