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0, 0, 1, 2, 7, 14, 47, 104, 383, 944, 3839, 10394, 46079, 135134, 645119, 2027024, 10321919, 34459424, 185794559, 654729074, 3715891199, 13749310574, 81749606399, 316234143224, 1961990553599, 7905853580624, 51011754393599
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OFFSET
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0,4
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COMMENTS
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n divides a(n-1) and a(n+1) for n = {1,2,8,11,16,19,23,31,32,43,64,67,71,...} which include all powers of 2 except 2^2 and some odd primes of the form 4n+3 belonging to A002145. p^2 divides a(p-1) for odd prime p = 71. p^2 divides a(p+1) for odd prime p = 23. a(n) is prime for n = {3,4,6,8,16,26,64,82,90,118,194,214,...} = A007749(n) = 2*A091415(n-1) for n>1. Corresponding primes of the form n!! - 1 are listed in A117141(n) = {2,7,47,383,10321919,51011754393599,...} = A093173(n-1) for n>1.
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LINKS
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Table of n, a(n) for n=0..26.
Eric Weisstein, Link to a section of The World of Mathematics. Double Factorial.
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FORMULA
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a(n) = A006882(n) - 1.
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MATHEMATICA
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Table[ n!! - 1, {n, 0, 35} ]
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CROSSREFS
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Cf. A006882, A002145, A117141, A093173, A007749, A091415.
Sequence in context: A018542 A018577 A121118 * A018281 A018592 A123752
Adjacent sequences: A128879 A128880 A128881 * A128883 A128884 A128885
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KEYWORD
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nonn
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AUTHOR
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Alexander Adamchuk, Apr 18 2007
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STATUS
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approved
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