Paul Richard Halmos (born 1916) is a Hungarian-born American mathematician who has done research in the fields of probability theory, statistics, operator theory, ergodic theory, and functional analysis (in particular Hilbert spaces).

He is noted for a number of expository books, viewed by many to be well written, including Naive Set Theory, Introduction to Hilbert Space and the Theory of Spectral Multiplicity, Lectures on Boolean Algebrass, and Finite-Dimensional Vector Spaces.

The use of "iff" to abbreviate "if and only if" is sometimes mistakenly credited to Halmos, however he has said that he borrowed this notation. The use of the "tombstone" notation to signify the end of a proof is also credited to him; the tombstone symbol ∎ is sometimes called a halmos.

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