John William Scott Cassels

J. W. S. Cassels
Born	11 July 1922 Durham, North East England
Died	27 July 2015 (aged 93) England
Other names	Ian Cassels
Nationality	British
Fields	Mathematics
Institutions	University of Cambridge
Alma mater	University of Edinburgh (MA) Trinity College, Cambridge (PhD)
Doctoral advisor	Louis Mordell
Doctoral students	Bryan John Birch Antonia Jones José Felipe Voloch Victor Flynn
Notable awards	De Morgan Medal (1986) Royal Society Sylvester Medal (1973) Fellow of the Royal Society (1963)

John William Scott "Ian" Cassels, FRS (11 July 1922 – 27 July 2015) was a British mathematician.

Cassels was educated at Neville's Cross Council School in Durham and George Heriot's School in Edinburgh. He went on to study at the University of Edinburgh and graduated with an undergraduate Master of Arts (MA) degree in 1943.

His academic career was interrupted in World War II when he was involved in cryptography at Bletchley Park. After the war he became a research student of Louis Mordell at Trinity College, Cambridge; he received his PhD in 1949 and was elected a fellow of Trinity in the same year.

Cassels then spent a year lecturing in mathematics at the University of Manchester before returning to Cambridge as a lecturer in 1950. He was appointed Reader in Arithmetic in 1963, the same year he was elected as a fellow of the Royal Society of London. In 1967 he was appointed as Sadleirian Professor of Pure Mathematics at Cambridge. In 1969 he became Head of the Department of Pure Mathematics and Mathematical Statistics. He retired in 1984.

He initially worked on elliptic curves. After a period when he worked on geometry of numbers and diophantine approximation, he returned in the later 1950s to the arithmetic of elliptic curves, writing a series of papers connecting the Selmer group with Galois cohomology and laying some of the foundations of the modern theory of infinite descent. His best-known single result may be the proof that the Tate-Shafarevich group, if it is finite, must have order that is a square; the proof being by construction of an alternating form.