A215780 -id:A215780 - cp's OEIS frontend

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

Michel Lagneau, Aug 23 2012

The corresponding primes are in A215780.

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

Cf. A006882, A215780.

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

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

A215777 Primes of the form 2*k!! + 1.

Original entry on oeis.org

3, 3, 5, 7, 17, 31, 97, 211, 769, 7681, 270271, 68918851, 371589121, 3923981107201, 2856658246041601, 85699747381248001, 639661973545755541631251, 5960455827486621749452458387843751

Offset: 1

Michel Lagneau, Aug 23 2012

nonn

The corresponding k are in A215775.

			68918851 =  2*17!! + 1 = 2*34459425 + 1 is in the sequence.

Vincenzo Librandi, Table of n, a(n) for n = 1..23

Cf. A006882, A215775, A215780.

lst={}; Do[If[PrimeQ[p = 2 n!! + 1], AppendTo[lst, p]], {n, 0, 60}]; lst
Select[Table[2 n!! + 1, {n, 0, 60}], PrimeQ] (* Vincenzo Librandi, Oct 01 2012 *)

Offset corrected from 0 to 1 by Vincenzo Librandi, Oct 01 2012

cp's OEIS Frontend

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

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