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The longest prime number ever found has been discovered by a computer in Missouri, and it's a doozy: 2^{74,207,281}–1 has 22,338,618 digits, the Guardian reports. The number also known as M74207281 was found by a computer tied to the Great Internet Mersenne Prime Search (GIMPS), a 20-year-old project that connects computers around the world looking for prime numbers. In addition to this one, professor Chris Cooper's computers at the University of Central Missouri have unearthed three other record prime numbers—numbers that can only be divided by themselves and one—and this one's a rare type of prime (only 49 of them exist, per the project site): a Mersenne prime, which can be written as one less than a power of two. It's a huge find not just scientifically, but also physically: Wired UK notes that a downloaded text copy of the number runs nearly 22MB.
Perhaps pointing out the obvious, the Mersenne site notes "this prime is too large to currently be of practical value," but computers toiling day and night to unearth primes can help gauge hardware functionality—for instance, GIMPS computers recently found a bug in Intel Skylake CPUs. Big prime numbers are also typically used for security encryption by banks and retailers, notes the BBC, though those numbers usually cap off at hundreds of digits. M74207281 was actually discovered by the Missouri computer in September, but a glitch caused the notification about the find to go unsent, and no one noticed it until the computer got a routine checkup, the website notes. As the Guardian points out, the quest to find the highest prime will go on indefinitely, "since there an infinite number of them"—and a nonprofit technology organization is offering a $150,000 prize to whomever finds a 100-million-digit prime first. Download the free program to search yourself. (The new number beats the old one by 5 million digits or so.)