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 A131827 #7 Mar 22 2020 01:59:10
%S A131827 4,7,9,11,13,15,36,37,44,46,54,56,70,80,84,88,90,92,94,112,118,138,
%T A131827 142,152,158,160,162,164,166,174,176,182,184,188,198,210,212,214,228,
%U A131827 230,234,236,252,272,276,278,282,304,312,316,318,320,322,328,352,354,364
%N A131827 Numbers k such that cototient(x) = k has exactly 2 solutions.
%e A131827 4 = cototient(6) = cototient(8) and there are no other solutions.
%o A131827 (PARI) lista(nn) = {my(v=vector(nn^2, i, i - eulerphi(i))); for(k=0, nn, if(sum(i=1, k*k, k==v[i])==2, print1(k, ", "))); } \\ _Jinyuan Wang_, Mar 21 2020
%Y A131827 Cf. A051953, A063742, A131825, A131826.
%K A131827 nonn
%O A131827 1,1
%A A131827 _Franz Vrabec_, Jul 19 2007
%E A131827 More terms from _Jinyuan Wang_, Mar 21 2020