Martensitic crystals which can undergo a cubic to tetragonal phase transformation have a non-convex energy density with three symmetry-related, rotationally invariant energy wells. We give estimates for the numerical approximation of a rst-order laminate for such martensitic crystals. We give bounds for the L 2 convergence of directional derivatives in the twin" plane, for the L 2 convergence of the deformation, for the weak convergence of the deformation gradient, for the convergence of the

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... rostructure, and for the convergence of nonlinear integrals of the deformation gradient.