Some results on the boundary condition model t-matrix

It is shown that the pure boundary condition model (BCM) t-matrix obtained from the Hoenig-Lomon pseudopotential does not satisfy the analyticity requirements suggested by Brayshaw, since the t-matrix possesses unphysical poles. A closed form expression is derived for the t-matrix which arises when the external potential in the BCM is taken to be a square well. A separable expansion for the t-matrix is also derived and compared numerically with the closed form result. The rate of convergence is found to be fast enough to be of practical value in calculations on the three-nucleon system.