La. Caffarelli et Ce. Kenig, GRADIENT ESTIMATES FOR VARIABLE-COEFFICIENT PARABOLIC EQUATIONS AND SINGULAR PERTURBATION PROBLEMS, American journal of mathematics, 120(2), 1998, pp. 391-439
In this article we prove, via monotonicity formulas, interior and boun
dary gradient estimates for solutions to second order parabolic equati
ons, in divergence form, with Dini top order coefficients. We then pro
ve uniform Lipschitz estimates for solutions of singular perturbation
problems, using the previous results, and two phase monotonicity formu
las.