R. Pyke, VIRIAL RELATIONS FOR NONLINEAR-WAVE EQUATIONS AND NONEXISTENCE OF ALMOST-PERIODIC SOLUTIONS, Reviews in mathematical physics, 8(7), 1996, pp. 1001-1039
We present a formalism for constructing integral identities involving
time almost periodic solutions of nonlinear wave equations. As an appl
ication we prove a nonexistence theorem that is applicable to a large
class of nonlinearities.