Paul R. Halmos (1916–2006) was a prominent American mathematician who taught at the University of Chicago, the University of Michigan, and other schools and made significant contributions to several areas of mathematics, including mathematical logic, ergodic theory, functional analysis, and probability theory. His other Dover books are Algebraic Logic, Finite-Dimensional Vector Spaces, Introduction to Hilbert Space and the Theory of Spectral Multiplicity, Lectures on Ergodic Theory, and Naive Set Theory.