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 A333101 #5 Feb 16 2025 08:33:59 %S A333101 50,170,266,290,344,518,532,534,650,686,722,730,872,962,1036,1158, %T A333101 1166,1332,1394,1462,1464,1586,1634,1682,1804,1864,1922,1946,1970, %U A333101 2034,2072,2074,2116,2134,2262,2314,2316,2318,2330,2420,2534,2598,2666,2668,2772,2822 %N A333101 Numbers k such that both k and k + 2 are noncototients (A005278). %H A333101 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Noncototient.html">Noncototient</a>. %H A333101 Wikipedia, <a href="https://en.wikipedia.org/wiki/Noncototient">Noncototient</a>. %e A333101 50 is a term since both 50 and 52 are noncototients. %t A333101 nmax = 3000; cototientQ[n_?EvenQ] := (x = n; While[test = x - EulerPhi[x] == n ; Not[test || x > 2*nmax], x++]; test); cototientQ[n_?OddQ] = True; nonc = Select[Range[nmax], !cototientQ[#]&]; nonc[[Flatten[Position[Differences[nonc], 2]]]] (* after _Jean-François Alcover_ at A005278 *) %Y A333101 Cf. A005278, A072296, A231964, A306952, A333100. %K A333101 nonn %O A333101 1,1 %A A333101 _Amiram Eldar_, Mar 07 2020