 Bilinear correlation bound for the box norm

With uniform expectations and real or complex functions,Apply the Cauchy-Schwarz inequality first in $y$. Expand the remaining square, and apply the Cauchy-Schwarz inequality in $(x,x^{\prime })$ to $u(x)u(x^{\prime })$ and ${\mathbb{E}}_{y}f(x,y)f(x^{\prime },y)$. Their squared averages are $\Vert u{\Vert }_2^4$ and $\Vert f{\Vert }_{\square }^4$, respectively.