sentences:  His Ph.D. advisor was Alfred Tarski at Berkeley; his dissertation is "Ultraproducts and Elementary Classes" (1961).Following Abraham Robinson's work resolving what had long been thought to be inherent logical contradictions in the literal interpretation of Leibniz's notation that Leibniz himself had proposed, that is, interpreting "dx" as literally representing an infinitesimally small quantity, Keisler published "", a first-year calculus textbook conceptually centered on the use of infinitesimals, rather than the epsilon, delta approach, for developing the calculus.He is also known for extending the Henkin construction (of Leon Henkin) to what are now called Henkin–Keisler models.