Parabolic Boundary Harnack Inequalities with Right-Hand Side.

We prove the parabolic boundary Harnack inequality in parabolic flat Lipschitz domains by blow-up techniques, allowing, for the first time, a non-zero right-hand side. Our method allows us to treat solutions to equations driven by non-divergence form operators with bounded measurable coefficients, and a right-hand side $f\in L^q$ for $q>n+2$ . In the case of the heat equation, we also show the optimal $C^{1-\epsilon }$ regularity of the quotient. As a corollary, we obtain a new way to prove that flat Lipschitz free boundaries are $C^{1,\alpha }$ in the parabolic obstacle problem and in the parabolic Signorini problem.