Generalized Fermat

The Prime Pages keeps a list of the 5000 largest known primes, plus a few each of certain selected archivable forms and classes. These forms are defined in this collection's home page.

This page is about one of those forms.

Definitions and Notes

Any generalized Fermat number F_b,n = (with b an integer greater than one and n greater than zero) which is prime is called a generalized Fermat prime (because they are Fermat primes in the special case b=2).

Why is the exponent a power of two? Because if m is an odd divisor of n, then b^n/m+1 divides bⁿ+1, so for the latter to be prime, m must be one. Because the exponent is a power of two, it seems reasonable to conjecture that the number of Generalized Fermat primes is finite for every fixed b.

Record Primes of this Type

rank prime digits who when comment 1 19637361048576 + 1 6598776 L4245 Sep 2022 Generalized Fermat 2 19517341048576 + 1 6595985 L5583 Aug 2022 Generalized Fermat 3 10590941048576 + 1 6317602 L4720 Nov 2018 Generalized Fermat 4 9194441048576 + 1 6253210 L4286 Sep 2017 Generalized Fermat 5 81 · 220498148 + 1 6170560 L4965 Jun 2023 Generalized Fermat 6 25 · 213719266 + 1 4129912 L4965 Sep 2022 Generalized Fermat 7 81 · 213708272 + 1 4126603 L4965 Oct 2022 Generalized Fermat 8 81 · 213470584 + 1 4055052 L4965 Oct 2022 Generalized Fermat 9 4 · 55380542 + 1 3760839 L4965 Feb 2023 Generalized Fermat 10 6339004524288 + 1 3566218 L1372 Jun 2023 Generalized Fermat 11 5897794524288 + 1 3549792 x50 Dec 2022 Generalized Fermat 12 4896418524288 + 1 3507424 L4245 May 2022 Generalized Fermat 13 3638450524288 + 1 3439810 L4591 May 2020 Generalized Fermat 14 9 · 211366286 + 1 3421594 L4965 Mar 2020 Generalized Fermat 15 3214654524288 + 1 3411613 L4309 Dec 2019 Generalized Fermat 16 2985036524288 + 1 3394739 L4752 Sep 2019 Generalized Fermat 17 2877652524288 + 1 3386397 L4250 Jun 2019 Generalized Fermat 18 2788032524288 + 1 3379193 L4584 Apr 2019 Generalized Fermat 19 2733014524288 + 1 3374655 L4929 Mar 2019 Generalized Fermat 20 2312092524288 + 1 3336572 L4720 Aug 2018 Generalized Fermat

Related Pages

• Generalized Fermat Prime Search by Yves Gallot
• Smallest base values yielding Generalized Fermat Primes by Yves Gallot

References

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H. Riesel, "Common prime factors of the numbers A_n =a²ⁿ+1," BIT, 9 (1969) 264-269.  MR 41:3381