A333101 Numbers k such that both k and k + 2 are noncototients (A005278).
50, 170, 266, 290, 344, 518, 532, 534, 650, 686, 722, 730, 872, 962, 1036, 1158, 1166, 1332, 1394, 1462, 1464, 1586, 1634, 1682, 1804, 1864, 1922, 1946, 1970, 2034, 2072, 2074, 2116, 2134, 2262, 2314, 2316, 2318, 2330, 2420, 2534, 2598, 2666, 2668, 2772, 2822
Offset: 1
Keywords
Examples
50 is a term since both 50 and 52 are noncototients.
Links
- Eric Weisstein's World of Mathematics, Noncototient.
- Wikipedia, Noncototient.
Programs
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Mathematica
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 *)