login

Reminder: The OEIS is hiring a new managing editor, and the application deadline is January 26.

Numbers k such that 7*k! - 1 is prime.
8

%I #23 Jul 07 2024 22:32:47

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

%T 1924,3469,3990

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

%C a(26) > 12000. - _Michael S. Branicky_, Jul 07 2024

%H <a href="/index/Pri#primepop">Index entries for sequences related to numbers of primes of the form k*n!+-1</a>

%t fQ[n_] := PrimeQ[7 n! - 1]; k = 0; lst = {}; While[k < 1501, If[ fQ@k, AppendTo[lst, k]; Print@k]; k++ ]; lst

%t Select[Range[4000],PrimeQ[7#!-1]&] (* _Harvey P. Dale_, Apr 22 2024 *)

%o (PARI) is(k) = ispseudoprime(7*k!-1); \\ _Jinyuan Wang_, Feb 03 2020

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

%K nonn,more

%O 1,1

%A _Robert G. Wilson v_, Sep 13 2010

%E a(23) from _Jinyuan Wang_, Feb 03 2020

%E a(24)-a(25) from _Michael S. Branicky_, Apr 25 2023