Hans Ivar Riesel (born 1929 in Stockholm) is a Swedish mathematician who discovered the 18th known Mersenne prime in 1957, using the computer BESK: this prime is 23217-1 and consists of 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. 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.
- Riesel, Hans (1994). Prime numbers and computer methods for factorization (2nd ed.). Birkhauser. ISBN 0-8176-3743-5.