A180628 Numbers k such that 7*k! - 1 is prime.

2, 3, 4, 5, 6, 7, 8, 12, 23, 25, 31, 57, 74, 86, 140, 240, 310, 703, 713, 796, 1028, 1102, 1924, 3469, 3990

Offset: 1

Author

Robert G. Wilson v, Sep 13 2010

Keywords

• nonn
• more

Comments

a(26) > 12000. - Michael S. Branicky, Jul 07 2024

Links

• Index entries for sequences related to numbers of primes of the form k*n!+-1

Crossrefs

Cf. A002982, A076133, A076134, A099350, A099351, A180627, A180629, A180630, A180631.

Programs

Mathematica

fQ[n_] := PrimeQ[7 n! - 1]; k = 0; lst = {}; While[k < 1501, If[ fQ@k, AppendTo[lst, k]; Print@k]; k++ ]; lst
Select[Range[4000],PrimeQ[7#!-1]&] (* Harvey P. Dale, Apr 22 2024 *)

PARI

is(k) = ispseudoprime(7*k!-1); \\ Jinyuan Wang, Feb 03 2020

Extensions

a(23) from Jinyuan Wang, Feb 03 2020

a(24)-a(25) from Michael S. Branicky, Apr 25 2023