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))).

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.

Links

Table of n, a(n) for n=1..52.

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

Adjacent sequences: A143186 A143187 A143188 * A143190 A143191 A143192

Keyword

nonn,uned

Author

Roger L. Bagula, Oct 17 2008

Status

approved