A215779 Numbers k such that 2*k!! - 1 is prime.

2, 3, 5, 9, 11, 13, 25, 35, 46, 56, 68, 69, 71, 84, 92, 103, 110, 121, 193, 259, 308, 368, 406, 426, 620, 627, 1206, 1631, 1845, 2108, 2210, 2786, 7931, 8008, 8367, 12181, 12291

Offset: 1

Author

Michel Lagneau, Aug 23 2012

Keywords

• nonn
• hard
• more

Comments

The corresponding primes are in A215780.

Examples

			5 is in the sequence because 2*5!! - 1 = 2*15 - 1 = 29 is a prime.
		

Crossrefs

Cf. A006882, A215780.

Programs

Mathematica

lst={};Do[If[PrimeQ[2*n!!-1],AppendTo[lst,n]],{n,0,1500}];lst

Extensions

Offset changed to 1 by and a(27)-a(32) from Jinyuan Wang, May 15 2021

a(33)-a(37) from Michael S. Branicky, Aug 09 2024