LARGE-U EXPANSIONS FOR THE HUBBARD-MODEL AT T = 0

We report results for the ground-state energy of the Hubbard model in the form of perturbation expansions in t/U, i.e., expansions about the strong-coupling or "atomic" limit. Expansions to order (t/U)40 are obtained for finite cells with number of sites less-than-or-equal-to 10, for the half-filled case, for both the linear chain and two-dimensional (2D) square lattice. Analysis of the series by Pade approximant methods yields, in the 1D case, results consistent with the known exact results. For the largest cell, N = 10, on the square lattice there is an indication of a singularity on the real t axis, which may indicate a phase transition at U/t congruent-to 4.