 We find that epistasis is the nonlinearity of the Green function, which corresponds to a sum over multiple scattering paths passing through the localized perturbations .
<meaning-changed>  We find that epistasis is the nonlinearity of the Green function, which corresponds to a sum over multiple scattering paths passing through the localized perturbations .
 We find that epistasis -- the interaction among mutations in the gene -- is related to the nonlinearity of the Green function, which corresponds to a sum over multiple scattering paths passing through the localized perturbations .