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 A239613 #22 Aug 08 2018 04:31:49
%S A239613 1,16,96,384,960,1536,4032,8192,11664,15360,26400,36864,52416,64512,
%T A239613 92160,163840,156672,186624,246240,368640,387072,422400,534336,786432,
%U A239613 900000,838656,1259712,1548288,1364160,1474560,1785600,3145728
%N A239613 a(n) = Sum_{0 < x,y,z,t <= n and gcd(x^2 + y^2 + z^2 + t^2, n)=1} gcd(x^2 + y^2 + z^2 + t^2 - 1, n).
%C A239613 Related to Menon's identity. See Conclusions and further work section of the arXiv file linked.
%H A239613 Andrew Howroyd, <a href="/A239613/b239613.txt">Table of n, a(n) for n = 1..1000</a>
%H A239613 C. Calderón, J. M. Grau, A. Oller-Marcen, L. Toth, <a href="http://arxiv.org/abs/1403.7878">Counting invertible sums of squares modulo n and a new generalization of Euler totient function</a>, arXiv:1403.7878 [math.NT], 2014.
%t A239613 g4[n_] := Sum[If[GCD[x^2 + y^2+ z^2+ t^2, n] == 1, GCD[x^2 + y^2+ z^2+ t^2 - 1, n], 0], {x, 1, n}, {y, 1, n},{z,1,n},{t,1,n}]; Array[g4,100]
%o A239613 (PARI) a(n) = {s = 0; for (x=1, n, for (y=1, n, for (z=1, n, for (t=1, n, if (gcd(x^2+y^2+z^2+t^2,n) == 1, s += gcd(x^2+y^2+z^2+t^2-1,n)););););); s;} \\ _Michel Marcus_, Jun 29 2014
%o A239613 (PARI) a(n)={my(p=lift(Mod(sum(i=0, n-1, x^(i^2%n)), x^n-1)^4)); sum(i=0, n-1, if(gcd(i,n)==1, polcoeff(p,i)*gcd((i-1)%n,n)))} \\ _Andrew Howroyd_, Aug 07 2018
%Y A239613 Cf. A239611, A239612, A239614, A239615, A079458, A053191, A227499.
%K A239613 nonn,mult
%O A239613 1,2
%A A239613 _José María Grau Ribas_, Jun 25 2014
%E A239613 Keyword:mult added by _Andrew Howroyd_, Aug 07 2018