Abstract
A first‐order differential equation is found for the quasiphase parameter which describes the half‐off‐shell scattering matrix. The quasiphase is given as Δ(∞), where Δ(r) obeys an equation involving the potential but not the wavefunction and Δ(0) = 0. The equation is used to develop low‐momentum expansions for Δ, and to derive upper and lower bounds for the expansion parameters.