Remarks on the maximum principle for parabolic equations and its applications
Avner Friedman · Pacific Journal of Mathematics · 1958
FRIEDMANin which P° is a boundary point of D. We may assume that P° is the origin.Let t = φ(x) be the equation of the boundary of D near P°.Assume that £=0 is the tangent hyperplane to the boundary of D at P°. Therefore dφldXi\ P o~0.