  We obtain approximation bounds for products of quasimodes for the
Laplace-Beltrami operator, on compact Riemannian manifolds of all dimensions
without boundary. We approximate the products of quasimodes $uv$ by a
low-degree vector space $B_{n}$, and we prove that the size of the space
$\dim(B_{n})$ is small. In our paper, we first study bilinear quasimode
estimates of all dimensions $d = 2, 3$, $d = 4,5$ and $d \ge 6$, respectively,
to make the highest frequency disappear from the right hand. Furthermore, the
result of the case $\lambda=\mu$ of bilinear quasimode estimates improves
$L^{4}$ quasimodes estimates of Sogge-Zelditch in \cite{sogge6} when $d \ge 8$.
And on this basis, we give approximation bounds in $H^{-1}$ norm. We also prove
approximation bounds for the products of quasimodes in $L^{2}$ norm using the
results of $L^{p}$-estimates for quasimodes in \cite{sogge3}. We extend the
results of Lu-Steinerberger in \cite{lu} to quasimodes.

Analysis of PDEs