A constant rank theorem for quasiconcave solutions of fully nonlinear partial differential equations Baojun BianPengfei GuanXinan MaLu Xu 35J92535J6535B05level-setconvexityfully nonlinear equations We prove a constant rank theorem for the second fundamental form of the convex level surfaces of solutions to equations $F(D^2u,Du,u,x) = 0$ under a structural condition introduced by Bianchini-Longinetti-Salani in [C. Bianchini, M. Longinetti and P. Salani, \textit{Quasiconcave solutions to elliptic problems in convex rings}, Indiana Univ. Math. J. \textbf{58} (2009), 1565--1590]. Indiana University Mathematics Journal 2011 text pdf 10.1512/iumj.2011.60.4222 10.1512/iumj.2011.60.4222 en Indiana Univ. Math. J. 60 (2011) 101 - 120 state-of-the-art mathematics http://iumj.org/access/