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

Author

José María Grau Ribas, Mar 01 2014

Keywords

• nonn

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

Crossrefs

Cf. A187731, A238574.

Programs

Mathematica

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