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A080778
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Double factorial primes; values k for which k!! + 1 is prime.
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25
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OFFSET
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1,3
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COMMENTS
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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
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LINKS
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EXAMPLE
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518 is included because 518*516*514*...*4*2 + 1 is prime. - Jeppe Stig Nielsen, Apr 01 2023
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CROSSREFS
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KEYWORD
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hard,nonn,more
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AUTHOR
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EXTENSIONS
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a(7) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 03 2008
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STATUS
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approved
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