  Finding a diagonal matrix congruent to $A - cI$ for constants $c$, where $A$
is the adjacency matrix of a graph $G$ allows us to quickly tell the number of
eigenvalues in a given interval. If $G$ has clique-width $k$ and a
corresponding $k$-expression is known, then diagonalization can be done in time
$O(\text{poly}(k) n)$ where $n$ is the order of $G$.
