Elias M. Stein (1931–2018) was a mathematician at Princeton University, widely regarded as one of the leading figures in harmonic analysis and singular integral operators in the twentieth century. His collaboration with his brother Jeremy C. Stein on stochastic volatility is an unusual bridge between pure mathematics and financial economics: the closed-form solution in Stein and Stein (1991) relies on techniques from the heat equation on the Heisenberg group — a structure central to Stein's work on nilpotent Lie groups.