Numbers n for which there are no base-2 Fermat pseudoprimes x that have ord(2,x) = n.
1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 13, 17, 19, 27, 31, 49, 61, 77, 89, 107, 127
1
All terms in A000043 are in this sequence.
T. D. Noe, Plot of 21 terms
(Mma) t = {}; Do[d = Divisors[2^n - 1]; cnt = 0; Do[m = d[[i]]; If[! PrimeQ[m] && PowerMod[2, m, m] == 2 && MultiplicativeOrder[2, m] == n, cnt++; Break[]], {i, Length[d]}]; If[cnt == 0, AppendTo[t, n]], {n, 127}]; t
nonn,hard,more
T. D. Noe, Jan 08 2015