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 A015020 #31 Aug 10 2025 02:40:19
%S A015020 1,1,-6,-258,77400,162617400,-2391451484400,-246183190158589200,
%T A015020 177399606828279377520000,894839274162813664365761520000,
%U A015020 -31596243236160097777438406008857120000,-7809512095098558650122990001755435531352160000,13511712475016607822799577492128185918816231362544000000
%N A015020 q-factorial numbers for q=-7.
%H A015020 Vincenzo Librandi, <a href="/A015020/b015020.txt">Table of n, a(n) for n = 0..40</a>
%H A015020 <a href="/index/Fa#factorial">Index entries for sequences related to factorial numbers</a>.
%F A015020 a(n) = Product_{k=1..n} ((-7)^k - 1) / (-7 - 1).
%F A015020 a(1) = 1, a(n) = ((-7)^n - 1)*a(n-1)/(-8). - _Vincenzo Librandi_, Oct 26 2012
%F A015020 a(n) ~ (-1)^floor(n/2) * c * 7^(n*(n+1)/2) / 8^n, where c = Product_{k>=1} (1 - 1/(-7)^k) = 1.1223882662358546529... . - _Amiram Eldar_, Aug 09 2025
%t A015020 RecurrenceTable[{a[1]==1, a[n]==(((-7)^n - 1) * a[n-1])/(-8)}, a, {n, 15}] (* _Vincenzo Librandi_, Oct 26 2012 *)
%o A015020 (Magma) [n le 1 select 1 else ((-7)^n - 1)*Self(n-1)/(-8): n in [1..13]]; // _Vincenzo Librandi_, Oct 26 2012
%Y A015020 Column k=7 of A384454.
%Y A015020 Cf. A015006.
%K A015020 sign,easy
%O A015020 0,3
%A A015020 _Olivier Gérard_
%E A015020 a(0)=1 prepended by _Seiichi Manyama_, May 30 2025