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These are counted by the double factorial 15 = (6 − 1)‼. In mathematics, the double factorial of a number n, denoted by n‼, is the product of all the positive integers up to n that have the same parity (odd or even) as n. [1] That is,
Multiplicative partitions of factorials are expressions of values of the factorial function as products of powers of prime numbers. They have been studied by Paul Erdős and others. [1] [2] [3] The factorial of a positive integer is a product of decreasing integer factors, which can in turn be factored into prime numbers.
Since a Mersenne number M p can be prime only if p is prime, (see Mersenne prime for a proof), a double Mersenne number can be prime only if M p is itself a Mersenne prime. For the first values of p for which M p is prime, M M p {\displaystyle M_{M_{p}}} is known to be prime for p = 2, 3, 5, 7 while explicit factors of M M p {\displaystyle M_{M ...
(resulting in 24 factorial primes - the prime 2 is repeated) No other factorial primes are known as of December 2024 [update] . When both n ! + 1 and n ! − 1 are composite , there must be at least 2 n + 1 consecutive composite numbers around n !, since besides n ! ± 1 and n ! itself, also, each number of form n ! ± k is divisible by k for 2 ...
These numbers have been proved prime by computer with a primality test for their form, for example the Lucas–Lehmer primality test for Mersenne numbers. “!” is the factorial, “#” is the primorial, and () is the third cyclotomic polynomial, defined as + +.
the k given prime numbers p i must be precisely the first k prime numbers (2, 3, 5, ...); if not, we could replace one of the given primes by a smaller prime, and thus obtain a smaller number than n with the same number of divisors (for instance 10 = 2 × 5 may be replaced with 6 = 2 × 3; both have four divisors);
That is, the product of the positive integers less than m and relatively prime to m is one less than a multiple of m when m is equal to 4, or a power of an odd prime, or twice a power of an odd prime; otherwise, the product is one more than a multiple of m.
Since ! is the product of the integers 1 through n, we obtain at least one factor of p in ! for each multiple of p in {,, …,}, of which there are ⌊ ⌋.Each multiple of contributes an additional factor of p, each multiple of contributes yet another factor of p, etc. Adding up the number of these factors gives the infinite sum for (!