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 A015017 #30 Aug 10 2025 02:41:16
%S A015017 1,1,-3,-39,1989,407745,-333943155,-1094331718935,14343405840081045,
%T A015017 752010424789609108305,-157707866234752874148183075,
%U A015017 -132294978377551030760819002477575,443908259132104963309174796130361690725,5958035886896289829709072930982993020807155425
%N A015017 q-factorial numbers for q=-4.
%H A015017 Vincenzo Librandi, <a href="/A015017/b015017.txt">Table of n, a(n) for n = 0..50</a>
%H A015017 <a href="/index/Fa#factorial">Index entries for sequences related to factorial numbers</a>.
%F A015017 a(n) = Product_{k=1..n} ((-4)^k - 1) / (-4 - 1).
%F A015017 a(1) = 1, a(n) = ((-4)^n - 1)*a(n-1)/(-5). - _Vincenzo Librandi_, Oct 26 2012
%F A015017 a(n) ~ (-1)^floor(n/2) * c * 4^(n*(n+1)/2) / 5^n, where c = Product_{k>=1} (1 - 1/(-4)^k) = 1.1864623436704848646... . - _Amiram Eldar_, Aug 09 2025
%t A015017 RecurrenceTable[{a[1]==1, a[n]==(((-4)^n-1) * a[n-1])/(-5)}, a, {n, 18}] (* _Vincenzo Librandi_, Oct 26 2012 *)
%o A015017 (Magma) [n le 1 select 1 else ((-4)^n - 1)*Self(n-1)/(-5): n in [1..18]]; // _Vincenzo Librandi_, Oct 26 2012
%Y A015017 Column k=4 of A384454.
%Y A015017 Cf. A015002.
%K A015017 sign,easy
%O A015017 0,3
%A A015017 _Olivier Gérard_
%E A015017 a(0)=1 prepended by _Seiichi Manyama_, May 30 2025