A068481 Numbers k such that gcd(k!+1, 2^k+1) > 1.

5, 9, 21, 33, 65, 81, 89, 113, 173, 209, 221, 245, 261, 281, 285, 309, 341, 345, 369, 393, 473, 509, 525, 545, 561, 593, 645, 725, 749, 785, 789, 833, 861, 873, 933, 953, 965, 1001, 1065, 1101, 1113, 1173, 1185, 1265, 1289, 1329, 1341, 1401, 1409, 1469

Offset: 1

Author

Benoit Cloitre, Mar 10 2002

Keywords

• easy
• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1000 from G. C. Greubel)

Crossrefs

Cf. A000051 (2^n+1), A038507 (n!+1).

Cf. A068480, A068482, A068483.

Programs

GAP

Filtered([1..1470],n->Gcd(Factorial(n)+1,2^n+1)>1); # Muniru A Asiru, Oct 16 2018

Maple

select(n->gcd(factorial(n)+1,2^n+1)>1,[$1..1470]); # Muniru A Asiru, Oct 16 2018

Mathematica

Select[Range[2500], GCD[#! + 1, 2^# + 1] > 1 &] (* G. C. Greubel, Oct 15 2018 *)

PARI

isok(n) = gcd(n!+1,2^n+1) > 1; \\ Michel Marcus, Oct 16 2018