Can you please help to understand how to solve this question:
Let $f^{ij}(x)$ be a positive definite matrix smoothly varying with $x$ and define
$\square=D_{t}^{2}-\sum_{i,j}f^{ij}(x)D_{x_{i}}D_{x_{j}}$
suppose $\square$u is smooth, find an upper bound on WF(u).
Thank you!