Maths fan discovers largest known prime number

The world's largest prime number, with 23 million digits, has been discovered by a maths enthusiast from the United States as part of a collaborative prime-number hunting project.

Jonathan Pace, a 51-year-old electrical engineer from Germantown in the southern US state of Tennessee, uncovered the elusive number after running a special software for six full days. Pace is a volunteer for a collaborative computational project called the Great Internet Mersenne Prime Search (GIMPS). Thousands of volunteers armed with "reasonably modern" computers and GIMPS software have joined the project, according to GIMPS website.

The number, known simply as M77232917, is the 50th "Mersenne prime" to be discovered. The number is more than 23 million digits long -- 1 million digits more than the previous record holder discovered in January 2016.

The figure is arrived at by calculating two to the power of 77,232,917 and then subtracting one, leaving a massive string of 23,249,425 digits.

The number belongs to a rare group of the so-called Mersenne prime numbers, named after the 17th century French monk Marin Mersenne. Like any prime number, a Mersenne prime is divisible only by itself and one, but is derived by multiplying many twos together and then taking away one.

This is Pace's first discovery since he began volunteering for the GIMPS project 14 years ago. He is now eligible for a US$3,000 award.

Prime numbers have been an interest of mathematicians for more than 2,000 years. The numbers seem to occur sporadically because researchers don't fully understand the pattern they follow, said Chris Caldwell, a professor of mathematics and statistics at the University of Tennessee at Martin.

"In the long run, we hope to use these data to predict how often these occur," he said.

GIMPS has discovered the last 16 Mersenne primes since its founding in 1996.