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%I #4 Mar 07 2020 20:19:07
%S 50,170,266,290,344,518,532,534,650,686,722,730,872,962,1036,1158,
%T 1166,1332,1394,1462,1464,1586,1634,1682,1804,1864,1922,1946,1970,
%U 2034,2072,2074,2116,2134,2262,2314,2316,2318,2330,2420,2534,2598,2666,2668,2772,2822
%N Numbers k such that both k and k + 2 are noncototients (A005278).
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Noncototient.html">Noncototient</a>.
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Noncototient">Noncototient</a>.
%e 50 is a term since both 50 and 52 are noncototients.
%t 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 Cf. A005278, A072296, A231964, A306952, A333100.
%K nonn
%O 1,1
%A _Amiram Eldar_, Mar 07 2020