Multifactorial numbers are of the form n!k

If k is even then n!k is odd for all odd n. Therefore n!k+/-1 are both even and hence not prime. In this case we can look for primes of the form n!k+/-2. However as these numbers are not +/-1 from a number that can be easily factored Brillhart-Lehmer-Selfridge is not a suitable test to prove primality. So other than for the small primes I have used Marcel Martin's Primo which implements the elliptical curve primality proving (ECPP) algorithm to prove numbers prime.

Certificates for Primo proofs are available on request.

Type |
n_{max}tested |
Digits of largest proven prime |
Digits of largest prp |
n is prime for: |

n!2+2 | 10000 | 5617 | 12896 | primes |

n!2-2 | 10000 | 2914 | 16153 | primes |

n!4+2 | 20000 | 2831 | 18109 | primes |

n!4-2 | 20000 | 3918 | 18642 | primes |

n!6-2 | 10000 | 1486 | 5113 | primes |

n!10+2 | 26329 | 4031 | 10498 | primes |

Last Modified: 7th February 2012

Address questions about this web page to Ken Davis at "kraDenken(at)yahoo(dot)com(dot)au"