OFFSET
0,4
COMMENTS
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 4k+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.
LINKS
Eric Weisstein's World of Mathematics, Double Factorial
FORMULA
a(n) = A006882(n) - 1.
MATHEMATICA
Table[ n!! - 1, {n, 0, 35} ]
CROSSREFS
KEYWORD
nonn
AUTHOR
Alexander Adamchuk, Apr 18 2007
STATUS
approved