Hans Riesel

Hans Ivar Riesel (28 May 1929 in Stockholm – 21 December 2014) was a Swedish mathematician who discovered the 18th Mersenne prime in 1957 using the computer BESK: 23217-1, comprising 969 digits. He held the record for the largest known prime from 1957 to 1961, when Alexander Hurwitz discovered a larger one. Riesel also discovered the Riesel numbers as well as developing the Lucas–Lehmer–Riesel test. After having worked at the Swedish Board for Computing Machinery, he was awarded his Ph.D. from Stockholm University in 1969 for his thesis Contributions to numerical number theory, and in the same year joined the Royal Institute of Technology as a senior lecturer and associate professor.

Career and research

After completing an engineering degree at Kungliga Tekniska högskolan (KTH) in 1953, Riesel joined the state-run BESK computer project. Using nights and weekends on the machine he coded a self-checking Lucas–Lehmer routine in machine language, feeding exponents from punched paper tape; on 24 September 1957 the program halted with a zero residue for p = 3,217, identifying 23217 − 1 (969 digits) as the largest known prime of the day.