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Florida man stumbles on biggest prime number after working plucky i5 CPU for 12 days straight

Processor thrashed by GIMPS

The largest known prime number, made up over 24 million digits, has been discovered by a lone IT professional quietly crunching numbers with an Intel-powered computer in December.

Patrick Laroche, a 35-year-old from Ocala, Florida, chanced upon the goody by running the free Great Internet Mersenne Prime Search (GIMPS) software on a four-core Intel Core i5-4590T processor over 12 days.

Many enthusiasts have tried searching for the elusive numbers for years often with no luck. Laroche, however, only spent four months before he cracked it on his fourth attempt. For comparison, Jonathan Pace, who found the 50th Mersenne prime last year in January, had been hunting the beasts for 14 years. It involves a lot of brute-force searching for the right numbers, hence why it takes so long.

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The new addition, named M82589933, belongs to a special class of prime numbers known for being particularly large. Mersenne primes equal 2n - 1, where n is the exponent needed to generate the prime and is used to form the name.

Thus, M82589933 equals 282,589,933 - 1. As you can appreciate, it's much easier to quote the M-string considering the full prime number contains a whopping 24,862,048 digits.

It's a rare find. Only 51 Mersenne primes have been discovered so far, and the latest one is is over half a million digits larger than than the previous number, 277,232,917 - 1. The math behind GIMPS is explained here: if 2n - 1 is prime, then n is prime, so the goal is to run through values of n that satisfy both sides of the theorem. That's when you know you've got a very large prime number: n is prime and 2n - 1 is prime. In this latest case, n is 82,589,933, and the prime number is 282,589,933 - 1.

Laroche struck gold on 7 December, though once a candidate has been found, it has to be painstakingly verified with other independent software running on different hardware. M82589933 was checked by Andreas Höglund, who ran two tests. One using the CUDALucas program on an Nvidia V100 GPU in 21 hours, and the Mlucas test on 16 cores of an AWS cloud instance in 72 hours. Aaron Blosser, another Mersenne prime expert, examined the new number using the Prime95 software on an Intel 7700K processor in six days and eight hours.

Laroche is eligible for the $3,000 GIMPS research discovery award cash prize. It gets more and more difficult to hunt for larger Mersenne primes as it demands increased computational effort. There may be smaller ones out there, however, and it's unknown if there are a finite or infinite number of Mersenne primes. ®

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