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 A015013 #45 Aug 10 2025 02:41:58
%S A015013 1,1,-1,-3,15,165,-3465,-148995,12664575,2165642325,-738484032825,
%T A015013 -504384594419475,688484971382583375,1880252456845835197125,
%U A015013 -10268058666835106011499625,-112158004817839862963610403875,2450091615245711806440069272649375,107046952761700394535173066591323843125
%N A015013 q-factorial numbers for q=-2.
%H A015013 Vincenzo Librandi, <a href="/A015013/b015013.txt">Table of n, a(n) for n = 0..80</a>
%H A015013 <a href="/index/Fa#factorial">Index entries for sequences related to factorial numbers</a>.
%F A015013 a(n) = Product_{k=1..n} ((-2)^k - 1) / (-2 - 1).
%F A015013 a(1) = 1, a(n) = (((-2)^n - 1) * a(n-1))/(-3). - _Vincenzo Librandi_, Oct 26 2012
%F A015013 a(n) = (-1)^(floor((n mod 4)/2)) * Product_{k=1..n} A001045(k). - _Altug Alkan_, Apr 05 2016
%F A015013 a(n) ~ (-1)^floor(n/2) * c * 2^(n*(n+1)/2) / 3^n, where c = Product_{k>=1} (1 - 1/(-2)^k) = 1.21072413030105918013... (A330862). - _Amiram Eldar_, Aug 09 2025
%t A015013 RecurrenceTable[{a[1]==1, a[n]==(((-2)^n - 1) * a[n-1])/(-3)}, a, {n, 15}]
%t A015013 Table[QFactorial[n, -2], {n, 20}] (* _Vladimir Reshetnikov_, Sep 16 2016 *)
%o A015013 (Magma) I:=[1]; [n le 1 select I[n] else (((-2)^n - 1) * Self(n-1))/(-3): n in [1..18]]; // _Vincenzo Librandi_, Oct 26 2012
%o A015013 (PARI) a(n) = prod(k=1, n, ((-2)^k-1)/(-3)) \\ _Michel Marcus_, Apr 05 2016
%Y A015013 Column k=2 of A384454.
%Y A015013 Cf. A001045, A330862.
%K A015013 sign,easy
%O A015013 0,4
%A A015013 _Olivier Gérard_
%E A015013 a(0)=1 prepended by _Seiichi Manyama_, May 30 2025