David Rees ScD Cantab, FIMA, FRS was an emeritus professor of pure mathematics at the University of Exeter, having been head of the Mathematics / Mathematical Sciences Department at Exeter for many years. During the Second World War, Rees was active on Enigma research in Hut 6 at Bletchley Park. Rees initially worked on semigroup theory, and Rees factor semigroup is named after him. He also gave a characterization of completely simple and completely 0-simple semigroups, nowadays known as Rees' theorem. The matrix-based semigroups used in this characterization are called Rees matrix semigroups. At the behest of Douglas Northcott he switched his research focus to commutative algebra. In 1954, in a joint paper with Northcott, Rees introduced the Northcott-Rees theory of reductions and integral closures which has subsequently been influential in commutative algebra. In 1993 Rees was awarded the P�lya Prize by the London Mathematical Society. In August 1998 a conference on Commutative Algebra was held at Exeter in honour of David Rees' 80th Year. He was an Honorary Fellow of Downing College, Cambridge 

May 28th, 1918 in England (Age 105)