  The phase space of $N$ damped linear oscillators is endowed with a bilinear
map under which the evolution operator is symmetric. This analog of
self-adjointness allows properties familiar from conservative systems to be
recovered, e.g., eigenvectors are "orthogonal" under the bilinear map and obey
sum rules, initial-value problems are readily solved and perturbation theory
applies to the_complex_ eigenvalues. These concepts are conveniently
represented in a biorthogonal basis.
