On the microscopic spacetime convexity principle for fully nonlinear   parabolic equations I: Spacetime convex solutions
  Spacetime convexity is a basic geometric property of the solutions of parabolic equations. In this paper, we study microscopic convexity properties of spacetime convex solutions of fully nonlinear parabolic partial differential equations and give a new simple proof of the constant rank theorem in \cite{CH13}. 
Chuanqiang Chen
math.AP