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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 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

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