A139200 Numbers k such that (k!-5)/5 is prime.

5, 11, 12, 16, 36, 41, 42, 47, 127, 136, 356, 829, 1863, 2065, 2702, 4509, 7498

Offset: 1

Author

Artur Jasinski, Apr 11 2008

Keywords

• hard
• more
• nonn

Comments

a(16) > 3000. - Ray G. Opao, Oct 05 2008

a(18) > 25000. - Robert Price, Nov 20 2016

Crossrefs

Cf. A139189, A139190, A139191, A139192, A139193, A139194, A139195, A139196, A139197, A139198.

Cf. n!/m-1 is a prime: A002982, A082671, A139056, A139199-A139205; n!/m+1 is a prime: A002981, A082672, A089085, A139061, A139058, A139063, A139065, A151913, A137390, A139071 (1<=m<=10).

Programs

Magma

[n: n in [5..500] | IsPrime((Factorial(n)-5) div 5)]; // Vincenzo Librandi, Nov 21 2016

Mathematica

a = {}; Do[If[PrimeQ[(n! - 5)/5], Print[a]; AppendTo[a, n]], {n, 1, 300}]; a (* Artur Jasinski *)

Extensions

a(13)-a(15) from Ray G. Opao, Oct 05 2008

a(16) from Serge Batalov, Feb 18 2015

a(17) from Robert Price, Nov 20 2016