Microlocal Analysis and Nonlinear Waves (The Ima Volumes in Mathematics and Its Applications) (Reprint)

If P( t, x, Dt,x) is a strictly hyperbolic operator or system then the singular support of f gives an upper bound for the singular support of u (Courant-Lax, Lax, Ludwig), namely singsupp u C the union of forward rays passing through the singular support of f.