A028683 Pseudo Galois numbers for d=21.

1, 420, 81496800, 6988909668048000, 264339188251171547754240000, 4409145118315866486641282521305984000000, 32432910584848683243891703579686352553931989191680000000, 105209765057463921593261518265177017436422522866428383386348592435200000000

Offset: 0

Author

Olivier Gérard

Keywords

• nonn

Links

• Vincenzo Librandi, Table of n, a(n) for n = 0..27

Crossrefs

Cf. A028668, A028670, A028671, A028672, A028674, A028676, A028677, A028678, A028680, A028682, A028684, A028686.

Programs

Mathematica

FoldList[ #1*21^#2 (21^#2-1)&, 1, Range[ 20 ] ]
a[n_] := 21^n * Product[21^n - 21^k, {k, 0, n-1}]; Array[a, 8, 0] (* Amiram Eldar, Jul 14 2025 *)

PARI

a(n) = 21^n * prod(k = 0, n-1, 21^n - 21^k); \\ Amiram Eldar, Jul 14 2025

Formula

a(n) = 21^n * Product_{k=0..n-1} (21^n - 21^k).

a(n) ~ c * 21^(n^2+n), where c = Product_{k>=1} (1 - 1/21^k) = 0.950113624091... . - Amiram Eldar, Jul 14 2025