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 A211173 #28 Mar 04 2024 01:14:42
%S A211173 0,2,1,6,0,10,1,0,1,18,0,22,0,0,1,30,0,0,1,0,1,42,0,46,0,0,1,0,0,58,1,
%T A211173 0,0,66,0,70,1,0,0,78,0,82,0,0,1,0,0,0,1,0,1,102,0,106,1,0,1,0,0,0,0,
%U A211173 0,0,126,0,130,0,0,1,138,0
%N A211173 (2n)!^n (modulo 2n+1).
%C A211173 a(n)= 0, 1 or 2n. In fact, a(n) = 0 iff n belongs to A047845, a(n) = 1 iff n belongs to A104636 and a(n) = 2n iff n belongs to A104635.
%H A211173 Charles R Greathouse IV, <a href="/A211173/b211173.txt">Table of n, a(n) for n = 0..10000</a>
%H A211173 Larry Riddle, <a href="http://www.agnesscott.edu/lriddle/women/germain-flt/sgandflt.htm"> Sophie Germain and Fermat's Last Theorem</a>, Agnes Scott College, Math. Dept. Jul, 1999.
%F A211173 a(n) = (2n)!^n (modulo 2n+1).
%t A211173 f[n_] := Mod[((2 n)!)^n, 2 n + 1]; Array[f, 70]
%t A211173 Table[PowerMod[(2n)!,n,2n+1],{n,0,70}] (* _Harvey P. Dale_, Nov 02 2019 *)
%o A211173 (PARI) a(n)=if(isprime(2*n+1),if(n%2,2*n,1),0) \\ _Charles R Greathouse IV_, Feb 07 2013
%Y A211173 Cf. A047845, A104635, A104636.
%K A211173 nonn,easy
%O A211173 0,2
%A A211173 Larry Riddle (LRiddle(AT)AgnesScott.edu) and _Robert G. Wilson v_, Jan 31 2013