Hitoshi Ishii (石井 仁 司 Ishii Hitoshi, born 12 October 1947) is a Japanese mathematician, specializing in partial differential equations. Ishii first studied physics and then mathematics at Waseda University in Tokyo with a master's degree in 1972 and a doctorate in 1975 with dissertation "偏微分方程式の初期値問題のLP可解性及び一意性" (translation: "Lp solvability and uniqueness of the initial value problem for partial differential equations"). He became an assistant professor at Chūō University in Tokyo in 1976 and in 1989 a full professor. In 1996, he became a professor at Tokyo Metropolitan University and in 2001, he became a professor at Waseda University. He studies nonlinear partial differential equations (PDEs) such as the Hamilton–Jacobi equation, viscosity solutions of PDEs, optimal control theory, diff