A080778 Double factorial primes; values k for which k!! + 1 is prime.

0, 1, 2, 518, 33416, 37310, 52608, 123998, 220502

Offset

1,3

Comments

Numbers k such that A006882(k) + 1 is prime.

a(1) and a(2) correspond to the same prime, 1+1. For n >= 3, a(n) is even (because (2j+1)!! + 1 is even), and then a(n)/2 equals A256594(n-1). - Jeppe Stig Nielsen, Apr 01 2023

Links

Table of n, a(n) for n=1..9.

Ken Davis, Status of Search for Multifactorial Primes.

Ken Davis, Results for n!2+1.

Eric Weisstein's World of Mathematics, Double Factorial Prime

Eric Weisstein's World of Mathematics, Integer Sequence Primes

Example

518 is included because 518*516*514*...*4*2 + 1 is prime. - Jeppe Stig Nielsen, Apr 01 2023

Crossrefs

Cf. A006882, A007749, A256594.

Sequence in context: A196290 A182446 A218436 * A007513 A352852 A283756

Adjacent sequences: A080775 A080776 A080777 * A080779 A080780 A080781

Keyword

hard,nonn,more

Author

Steven Harvey, Mar 11 2003

Extensions

a(7) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 03 2008

a(8) from Sou Fukui, Jun 05 2015

a(9) from Sou Fukui, Jan 17 2017

Status

approved