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 4 · 511786358 + 1 8238312 A2 Oct 2024 Generalized Fermat 2 19637361048576 + 1 6598776 L4245 Sep 2022 Generalized Fermat 3 19517341048576 + 1 6595985 L5583 Aug 2022 Generalized Fermat 4 10590941048576 + 1 6317602 L4720 Nov 2018 Generalized Fermat 5 9194441048576 + 1 6253210 L4286 Sep 2017 Generalized Fermat 6 81 · 220498148 + 1 6170560 L4965 Jun 2023 Generalized Fermat 7 4 · 58431178 + 1 5893142 A2 Jan 2024 Generalized Fermat 8 4 · 311279466 + 1 5381674 A2 Sep 2024 Generalized Fermat 9 25 · 213719266 + 1 4129912 L4965 Sep 2022 Generalized Fermat 10 81 · 213708272 + 1 4126603 L4965 Oct 2022 Generalized Fermat 11 81 · 213470584 + 1 4055052 L4965 Oct 2022 Generalized Fermat 12 4 · 55380542 + 1 3760839 L4965 Feb 2023 Generalized Fermat 13 11937916524288 + 1 3710349 L6080 Oct 2024 Generalized Fermat 14 10913140524288 + 1 3689913 L6043 Jun 2024 Generalized Fermat 15 9332124524288 + 1 3654278 L5025 Jun 2024 Generalized Fermat 16 8630170524288 + 1 3636472 L5543 Apr 2024 Generalized Fermat 17 4 · 37578378 + 1 3615806 A2 Sep 2024 Generalized Fermat 18 6339004524288 + 1 3566218 L1372 Jun 2023 Generalized Fermat 19 5897794524288 + 1 3549792 x50 Dec 2022 Generalized Fermat 20 4896418524288 + 1 3507424 L4245 May 2022 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. Dubner and W. Keller, "Factors of generalized Fermat numbers," Math. Comp., 64 (1995) 397--405.  MR 95c:11010

Dubner86

H. Dubner, "Generalized Fermat primes," J. Recreational Math., 18 (1985-86) 279--280.  MR 2002j:11156

Morimoto86

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H. Riesel and A. Börn, Generalized Fermat numbers.  In "Mathematics of Computation 1943-1993: A Half-Century of Computational Mathematics," W. Gautschi editor, Proc. Symp. Appl. Math. Vol, 48, Amer. Math. Soc., Providence, RI, 1994.  pp. 583-587, MR 95j:11006

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H. Riesel, "Some factors of the numbers G_n = 6²ⁿ + 1 and H_n = 10²ⁿ + 1," Math. Comp., 23:106 (1969) 413--415.  MR 39:6813

Riesel69b

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