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 A028683 #16 Jul 14 2025 00:25:06
%S A028683 1,420,81496800,6988909668048000,264339188251171547754240000,
%T A028683 4409145118315866486641282521305984000000,
%U A028683 32432910584848683243891703579686352553931989191680000000,105209765057463921593261518265177017436422522866428383386348592435200000000
%N A028683 Pseudo Galois numbers for d=21.
%H A028683 Vincenzo Librandi, <a href="/A028683/b028683.txt">Table of n, a(n) for n = 0..27</a>
%F A028683 a(n) = 21^n * Product_{k=0..n-1} (21^n - 21^k).
%F A028683 a(n) ~ c * 21^(n^2+n), where c = Product_{k>=1} (1 - 1/21^k) = 0.950113624091... . - _Amiram Eldar_, Jul 14 2025
%t A028683 FoldList[ #1*21^#2 (21^#2-1)&, 1, Range[ 20 ] ]
%t A028683 a[n_] := 21^n * Product[21^n - 21^k, {k, 0, n-1}]; Array[a, 8, 0] (* _Amiram Eldar_, Jul 14 2025 *)
%o A028683 (PARI) a(n) = 21^n * prod(k = 0, n-1, 21^n - 21^k); \\ _Amiram Eldar_, Jul 14 2025
%Y A028683 Cf. A028668, A028670, A028671, A028672, A028674, A028676, A028677, A028678, A028680, A028682, A028684, A028686.
%K A028683 nonn
%O A028683 0,2
%A A028683 _Olivier Gérard_