0,4

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.

Eric Weisstein, Link to a section of The World of Mathematics. Double Factorial.

a(n) = A006882(n) - 1.

Table[ n!! - 1, {n, 0, 35} ]

Cf. A006882, A002145, A117141, A093173, A007749, A091415.

Sequence in context: A018542 A018577 A121118 this_sequence A018281 A018592 A123752

Adjacent sequences: A128879 A128880 A128881 this_sequence A128883 A128884 A128885

nonn

Alexander Adamchuk (alex(AT)kolmogorov.com), Apr 18 2007