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Factorization of composite Mersenne numbers

Since they are prime numbers, Mersenne primes are divisible only by 1 and themselves. However, not all Mersenne numbers are Mersenne primes. Mersenne numbers are very good test cases for the special number field sieve algorithm, so often the largest number factorized with this algorithm has been a Mersenne number. As of June 2019, 2 − 1 is the record-holder, having been factored with a variant of the special number field sieve that allows the factorization of several numbers at once. See integer factorization records for links to more information. The special number field sieve can factorize numbers with more than one large factor. If a number has only one very large factor then other algorithms can factorize larger numbers by first finding small factors and then running a primality test on the cofactor. As of September 2022, the largest completely factored number (with probable prime factors allowed) is 2 − 1 = 1,119,429,257 × 175,573,124,547,437,977 × 8,480,999,878,421,106,991 × q, where q is a 3,829,294-digit probable prime. It was discovered by a GIMPS participant with nickname "Funky Waddle". As of September 2022, the Mersenne number M is the smallest composite Mersenne number with no known factors; it has no prime factors below 2, and is very unlikely to have any factors below 10 (~2).

The table below shows factorizations for the first 20 composite Mersenne numbers (sequence A244453 in the OEIS).

The number of factors for the first 500 Mersenne numbers can be found at (sequence A046800 in the OEIS).