A227575 Largest prime factor of 7^n + 1.

2, 2, 5, 43, 1201, 191, 181, 911, 169553, 117307, 4021, 10746341, 1201, 228511817, 13564461457, 6568801, 47072139617, 29078814248401, 13841169553, 4058036683, 810221830361, 309079, 83960385389, 3421093417510114543, 33232924804801, 79787519018560501

Offset: 0

Author

Michel Marcus, Aug 22 2013

Keywords

• nonn

Examples

			7^12 + 1 = 2*73*193*409*1201, so a(12) = 1201.
		

Links

• Table of n, a(n) for n = 0..387
• S. S. Wagstaff, Jr., The Cunningham Project

Crossrefs

Cf. A006530, A034491.

Cf. A002587, A074249, A074476, A074478.

Programs

Magma

[Maximum(PrimeDivisors(7^n+1)): n in [0..30]]; // Bruno Berselli, Aug 23 2013

Mathematica

Table[FactorInteger[7^n + 1][[-1, 1]], {n, 0, 30}] (* Bruno Berselli, Aug 23 2013 *)

PARI

a(n) = f = factor(7^n + 1); f[#f~, 1]; \\ Michel Marcus, Aug 22 2013

Formula

a(n) = A006530(A034491(n)). - Vincenzo Librandi, Jul 12 2016

Extensions

Terms to a(100) in b-file from Vincenzo Librandi, Jul 12 2016

a(101)-a(372) in b-file from Amiram Eldar, Feb 02 2020

a(373)-a(387) in b-file from Max Alekseyev, Apr 25 2022, Aug 30 2023