We study the inverse problem of recovery a non-linearity $f(x,u)$, which is compactly supported in $x$, in the semilinear wave equation $u_{tt}-Delta u+ f(x,u)=0$. We probe the medium with either complex or real-valued harmonic waves of wavelength $sim h$ and amplitude $sim 1$. They propagate in a regime where the non-linearity affects the subprincipal but not the principal term, except for the zeroth harmonics. We measure the transmitted wave when it exits $text{supp}_x f$. We show that one can recover $f(x,u)$ when it is an odd function of $u$, and we can recover $alpha(x)$ when $f(x,u)=alpha(x)u^{2m}$. This is done in an explicit way as $hto0$.
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