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 A070037 #20 Feb 11 2021 01:27:40
%S A070037 1,4,15,900,903,28611063
%N A070037 Nonprime numbers k such that sigma(k) == k+1 (mod phi(k)).
%C A070037 Nonprime numbers k such that Chowla(k)/phi(k) is an integer.
%C A070037 a(7) > 3.7*10^12. - _Giovanni Resta_, Jul 14 2013
%C A070037 3^30*13*852977547249259 and 3^36*13*621820631944710643 are also terms. - _Giovanni Resta_, Nov 14 2019
%e A070037 Below 30000000 only 5 composite numbers were found: C = {4,15,900,902,28611063}, Chowla(C) = {2,8,1920,504,17600976}, phi(C) = {2,8,240,504,17600976}, quotient = {1,1,8,1,1}.
%t A070037 s2[x_] := DivisorSigma[1, x]-x-1 e0[x_] := EulerPhi[x] Do[s=s2[n]/e0[n]; If[IntegerQ[s]&&!PrimeQ[n], Print[{n, s2[n], e0[n], s}]], {n, 1, 1000000}]
%o A070037 (PARI) is(n)=!isprime(n) && Mod(sigma(n),eulerphi(n))==n+1 \\ _Charles R Greathouse IV_, Jun 06 2013
%Y A070037 Cf. A000010, A000203, A048050.
%K A070037 more,nonn
%O A070037 1,2
%A A070037 _Labos Elemer_, Apr 18 2002