  A pair of scattering potentials are called $\alpha$-equivalent if they have
identical scattering properties for incident plane waves with wavenumber
$k\leq\alpha$ (energy $k^2\leq\alpha^2$.) We use a recently developed
multidimensional transfer-matrix formulation of scattering theory to obtain a
simple criterion for $\alpha$-equivalence of complex potentials in two and
three dimensions.
