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A143189
a(n) = if(EulerPhi(2*n - 1) - EulerPhi(2*n) == 0, n, if(DivisorSigma(1, 2*n - 1) - DivisorSigma(1, 2*n) == 0, n, if(Length(Divisors(2*n - 1)) - Length(Divisors(2*n)) == 0, n))).
0
1, 2, 8, 11, 17, 29, 38, 43, 47, 67, 68, 71, 73, 74, 86, 89, 95, 101, 103, 107, 109, 116, 122, 127, 128, 143, 151, 188, 191, 194, 197, 215, 223, 227, 241, 248, 251, 254, 269, 277, 283, 302, 305, 317, 323, 332, 349, 359, 368, 371, 373, 397
OFFSET
1,2
COMMENTS
Inspired by A134922.
MATHEMATICA
a[n_] =
If[EulerPhi[2*n - 1] - EulerPhi[2*n] == 0, n,
If[DivisorSigma[1, 2*n - 1] - DivisorSigma[1, 2*n] == 0, n,
If[Length[Divisors[2*n - 1]] - Length[Divisors[2*n]] == 0, n, {}]]];
Flatten[Table[a[n], {n, 1, 400}]]
CROSSREFS
Sequence in context: A064105 A129516 A220269 * A056550 A074263 A295070
KEYWORD
nonn,uned
AUTHOR
Roger L. Bagula, Oct 17 2008
STATUS
approved