[ I think 'Mersenne Primes' is why I'm on the maths-fun list. Thanks Richard. ] The record M(57,885,161), discovered by Prof. Cooper et al, is a prime with 17,425,170 decimal digits. It is the 48th known prime M(p). It may yet not be the 48th in size: GIMPS have only just confirmed the '42nd by size' position of the prime M(p) discovered in 2005. http://www.mersenne.org/report_milestones/ Its discovery ends the longest wait - nearly 4 years - for the 'largest prime' record to be broken since the Great Internet Mersenne Prime Search (GIMPS) began. Curt Noll kindly credited my Mersenne work 30 years ago as being part of the reason GIMPS began, hence Chris Caldwell's note at http://primes.utm.edu/glossary/xpage/GIMPS.html Dan Shanks, whose wonderful book first interested me in Mersenne Numbers in 1967, included an amusing piece about that work in the 3rd edition. GIMPS is the oldest continuously running crowd-sourced programme running on the web and has clocked over 100 TFLOPS. The prime-indexes, circa 100,000, that my ICL colleagues were working with then seem so small now! The back-history to 1987, and our modest results of the 1980s, can be consulted here: Back-history: http://centaur.reading.ac.uk/4571/1/1987_H_Mersenne_Numbers.pdf ICL DAP results: http://centaur.reading.ac.uk/5948/1/1986_H_Mp_Consolidated_Results.pdf AMS Abstracts note 1: http://centaur.reading.ac.uk/4555/1/1983_Abstracts_of_the_AMS_v4.2_p196.pdf AMS Abstracts note 2: http://centaur.reading.ac.uk/4572/1/1986_Abstracts_of_the_AMS_v7.2_pp224-5.p... Huge congratulations to Prof. Cooper and team. It is the third prime M(p) that they have found. M(M(31)) remains the largest Double Mersenne Number whose primality status is known and that record may stand for 31 years (1983-2014) ... see 'Back History' above ... :-) Guy