A238575 k-Lehmer numbers with two prime factors.

15, 51, 85, 91, 133, 247, 259, 451, 481, 511, 679, 703, 763, 771, 949, 1111, 1141, 1261, 1285, 1351, 1387, 1417, 1843, 1891, 2047, 2071, 2119, 2509, 2701, 2761, 3031, 3097, 3277, 3409, 3589, 3667, 4033, 4039, 4141, 4369, 4411, 4681, 5383, 5461, 5611, 5629

Offset

1,1

Comments

The first terms which are == 0 (mod 3) are 15, 51, 771, 196611, which are equal to 3*(5, 17, 257, 65537) = 3*(2^2+1, 2^4+1, 2^8+1, 2^16+1), i.e., 3 times the Fermat primes > 3. No other exceptions below 10^9. - Giovanni Resta

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Mathematica

rad[n_] := Times @@ Transpose[FactorInteger[n]][[1]]; Select[1 + \
Range[10000], Length[FactorInteger[#]] == 2 && Mod[# - 1,
   rad[EulerPhi[#]]] == 0 &]

Crossrefs

Cf. A187731, A238574.

Sequence in context: A227129 A238574 A333314 * A020144 A339880 A235040

Adjacent sequences: A238572 A238573 A238574 * A238576 A238577 A238578

Keyword

nonn

Author

José María Grau Ribas, Mar 01 2014

Status

approved