A factorial prime is a number that is one less or one more than a factorial and is also a prime number. The first few factorial primes are:
2, 3, 5, 7, 23, 719, 5039, 39916801, 479001599, 87178291199, ...
Factorial primes are of interest to number theorists because they sometimes signal the end or the beginning of an extraordinarily lengthy run of consecutive composite numbers. For example, the prime following 479,001,599 is 479,001,629.
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