Arne Beurling
Arne Beurling (1905-1986) was a mathematician and professor of mathematics at Uppsala university and later at Institute for Advanced Study at Princeton, USA.Arne Beurling pioneered and worked extensively in harmonics, complex analysis and potential theory.
In 1940 he single-handedly deciphered and reverse-engineereded the Geheimfernschreiber (or "Fish cyphers") used by Nazi Germany, and created a decryption device that enabled Sweden to decrypt German teleprinter traffic passing through Sweden from Norway. In this way, Swedish authorities knew about Operation Barbarossa before it occurred.
