This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A128882 #13 Feb 16 2025 08:33:05
%S A128882 0,0,1,2,7,14,47,104,383,944,3839,10394,46079,135134,645119,2027024,
%T A128882 10321919,34459424,185794559,654729074,3715891199,13749310574,
%U A128882 81749606399,316234143224,1961990553599,7905853580624,51011754393599
%N A128882 a(n) = n!! - 1.
%C A128882 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.
%C A128882 p^2 divides a(p-1) for odd prime p = 71.
%C A128882 p^2 divides a(p+1) for odd prime p = 23.
%C A128882 a(n) is prime for n = {3, 4, 6, 8, 16, 26, 64, 82, 90, 118, 194, 214, ...} = A007749; A007749(n) = 2*A091415(n-1) for n > 1. Corresponding primes of the form n!! - 1 are listed in A117141, cf. also A093173.
%H A128882 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/DoubleFactorial.html">Double Factorial</a>
%F A128882 a(n) = A006882(n) - 1.
%t A128882 Table[ n!! - 1, {n,0,35} ]
%Y A128882 Cf. A006882, A002145, A117141, A093173, A007749, A091415.
%K A128882 nonn
%O A128882 0,4
%A A128882 _Alexander Adamchuk_, Apr 18 2007