Alexander Joseph Nagel (born 13 September 1945 in New York City) is an American mathematician, specializing in harmonic analysis, functions of several complex variables, and linear partial differential equations.
He wrote a series of major papers on fundamental operators in complex analysis, in particular on the question of how the geometry of the underlying domains relates to concrete estimates of the kernels.
A deep paper by Nagel, Stein and Wainger with the unassuming title "Balls and metrics defined by vector fields: Basic properties" had tremendous impact on many branches of analysis and geometry.
Alex was a major contributor to the theory of singular Radon transforms, in various stages of development.
He is also well known for a fundamental estimate on Fourier transforms of surface carried measures and related applications.