Michael Atiyah, former president of the Royal Society, was widely seen as the greatest living British mathematician before God divided him by zero. His two most important discoveries were the Atiyah–Singer index theorem (which proved that for any elliptic differential operator on a compact manifold, the analytical index is equal to the topological index) and the Atiyah–Segal completion theorem (pi over X times EG is always greater than X, when G is a compact lie group and X is a G-CW complex). Don’t ask me, I was an English major.
Michael Atiyah
22 April 1929 – 11 January 2019
Dead at 89 (unique pick)