On conditional expectations in L^p(mu;L^q(nu;X))
Let $(A,\mathscr{A},\mu)$ and $(B,\mathscr{B},\nu)$ be probability spaces, let $\mathscr{F}$ be a sub-$\sigma$-algebra of the product $\sigma$-algebra $\mathscr{A}\times\mathscr{B}$, let $X$ be a Banach space, and let $1< p,q< \infty$. We obtain necessary and sufficient conditions in order that the conditional expectation with respect to $\mathscr{F}$ defines a bounded linear operator from $L^p(\mu;L^q(\nu;X))$ onto $L^p_{\mathscr{F}}(\mu;L^q(\nu;X))$, the closed subspace in $L^p(\mu;L^q(\nu;X))$ of all functions having a strongly $\mathscr{F}$-measurable representative.