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 A015018 #32 Aug 10 2025 02:41:01
%S A015018 1,1,-4,-84,8736,4551456,-11851991424,-154324780331904,
%T A015018 10047160498728278016,3270561732706527788046336,
%U A015018 -5323179358400075453935368658944,-43320145405426340445710562789228109824,1762701221841919957075369153792221868461981696,358622481951075194907281490606356664886183644743663616
%N A015018 q-factorial numbers for q=-5.
%H A015018 Vincenzo Librandi, <a href="/A015018/b015018.txt">Table of n, a(n) for n = 0..50</a>
%H A015018 <a href="/index/Fa#factorial">Index entries for sequences related to factorial numbers</a>.
%F A015018 a(n) = Product_{k=1..n} ((-5)^k - 1) / (-5 - 1).
%F A015018 a(1) = 1, a(n) = ((-5)^n - 1)*a(n-1)/(-6). - _Vincenzo Librandi_, Oct 26 2012
%F A015018 a(n) ~ (-1)^floor(n/2) * c * 5^(n*(n+1)/2) / 6^n, where c = Product_{k>=1} (1 - 1/(-5)^k) = 1.1596671959367684201... . - _Amiram Eldar_, Aug 09 2025
%t A015018 RecurrenceTable[{a[1]==1, a[n]==(((-5)^n - 1) * a[n-1])/(-6)}, a, {n, 15}] (* _Vincenzo Librandi_, Oct 26 2012 *)
%o A015018 (Magma) [n le 1 select 1 else ((-5)^n - 1)*Self(n-1)/(-6): n in [1..18]]; // _Vincenzo Librandi_, Oct 26 2012
%Y A015018 Column k=5 of A384454.
%Y A015018 Cf. A015004.
%K A015018 sign,easy
%O A015018 0,3
%A A015018 _Olivier Gérard_
%E A015018 a(0)=1 prepended by _Seiichi Manyama_, May 30 2025