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 A290044 #9 Jul 24 2017 11:56:41
%S A290044 1,1,7,41,343,3497,42583,602537,9719383,176075177,3539816023,
%T A290044 78209790377,1883780763223,49127973630377,1379201149390423,
%U A290044 41470672541233577,329725285558734423,45290937289840177577,1633052915424520654423,62144013487721190961577
%N A290044 a(n) = |Sum_{k=0..n} (-1)^k*(2k)!!|.
%C A290044 a(20) = 19 * 47 * 73 * 54470291 * 700938921577. Thus a(n) are multiples of 19 for n >= 20.
%t A290044 Table[Abs@ Sum[(-1)^k*(2 k)!!, {k, 0, n}], {n, 0, 19}] (* _Michael De Vlieger_, Jul 19 2017 *)
%Y A290044 Cf. A006882, A138525.
%K A290044 easy,nonn
%O A290044 0,3
%A A290044 _XU Pingya_, Jul 19 2017