Improved strong-coupling expansions and matrix Pade approximants for lattice theories

A generalization of Hamiltonian perturbation theory is presented. The method solves a low-energy sector of the theory exactly while systematically accounting for higher-energy states perturbatively. It produces an expansion for the mass matrix of the low-energy sector. In applications to lattice theories, the mass matrix can be extrapolated to the continuum limit using matrix Pade approximants. The method is illustrated for a lattice potential model and fast convergence to the continuum limit is verified numerically