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Relevant operators in exactly solvable lattice Hamiltonians.

A recent suggestion that the Hamiltonian of lattice QCD may be rendered exactly solvable by quadrature after addition of an irrelevant operator is critically analyzed. It is shown that a precisely analogous procedure may be applied to the nonlinear O({ital N}) {sigma} model. An exact solution may be performed in the large-{ital N} limit in this latter case, and the modifying operator found to be a relevant perturbation of the original theory, but only after summing to all orders of perturbation theory.