A289850 Primes of the form k!2 - 4, where k!2 is the double factorial number (A006882).

11, 101, 941, 10391, 135131, 2027021, 654729071, 7905853580621, 221643095476699771871, 79777941814291672401518892224505807820921910393015244140621, 6462013286957625464523030270184970433494674741834234775390621

Offset: 1

Author

Robert Price, Jul 13 2017

Keywords

• nonn

Links

• Robert Price, Table of n, a(n) for n = 1..13
• Henri and Renaud Lifchitz, PRP Records.Search for n!2-4.
• Joe McLean, Interesting Sources of Probable Primes.
• OpenPFGW Project, Primality Tester.

Crossrefs

Cf. A006882, A123910.

Programs

Mathematica

MultiFactorial[n_, k_] := If[n<1, 1, n*MultiFactorial[n-k, k]];
Select[Table[MultiFactorial[i, 2] - 4, {i, 4, 100}], PrimeQ[#]&]
Select[Range[4,100]!!-4,PrimeQ] (* Harvey P. Dale, Dec 16 2020 *)

Formula

a(n) = A006882(A123910(n)) - 4. - Elmo R. Oliveira, Apr 14 2025