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A080778 Double factorial primes; values k for which k!! + 1 is prime. 24
0, 1, 2, 518, 33416, 37310, 52608, 123998, 220502 (list; graph; refs; listen; history; text; internal format)
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
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
Sequence in context: A196290 A182446 A218436 * A007513 A352852 A283756
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

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Last modified April 20 05:55 EDT 2024. Contains 371799 sequences. (Running on oeis4.)