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Book with just one number -- with 23,249,425 digits -- sells out in Japan

A book without any words but all numbers.

Japanese publishing house Nanairosha's recent offering is a strange book that has become surprisingly popular.

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The book, "The Biggest Prime Number in 2017", contains just one thing -- a newly discovered prime number that has broken the record for the largest ever found, coming in at a whopping 23,249,425 digits that covers the 791 pages in the book.

The number, the 50th prime number of its type to be discovered, is 2 to the power of 77,232,917 minus 1.

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In just four days, some 1,500 copies of the book were sold and it is currently out of stock at Amazon.

As numbers get larger, prime numbers, which are divisible only by 1 and themselves, become difficult to find. They become further apart, and there's no pattern to their distribution.

Even the formula for finding the Mersenne prime numbers -- named after the 17th-century French monk who studied them, Marin Mersenne -- isn't a surefire method, it's simply a way to narrow down a likelier place to find them.

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2 to the power of 77,232,917 minus 1 is the largest Mersenne prime number discovered so far, obtained by multiplying 2 to the power of 77,232,917, and then subtracting 1.

It was discovered late last year by Jonathan Pace, an American electrical engineer, through a software called the Great Internet Mersenne Prime Search, a collaborative project of volunteers to search for such figures.

Looking for a Mersenne prime is very difficult, just like searching for a needle in a haystack, said Jordan Ellenberg, a mathematics professor at the University of Wisconsin-Madison.

It has made great contributions in the fields of computer science and mathematics. And because of the difficulties in finding them, the Mersenne primes appear mysterious and romantic.