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 A238575 k-Lehmer numbers with two prime factors. 2
 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 (list; graph; refs; listen; history; text; internal format)
 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: A127643 A227129 A238574 * A020144 A235040 A075933 Adjacent sequences:  A238572 A238573 A238574 * A238576 A238577 A238578 KEYWORD nonn AUTHOR José María Grau Ribas, Mar 01 2014 STATUS approved

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Last modified October 17 08:36 EDT 2019. Contains 328107 sequences. (Running on oeis4.)