This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A238575 #18 Oct 02 2019 07:28:34 %S A238575 15,51,85,91,133,247,259,451,481,511,679,703,763,771,949,1111,1141, %T A238575 1261,1285,1351,1387,1417,1843,1891,2047,2071,2119,2509,2701,2761, %U A238575 3031,3097,3277,3409,3589,3667,4033,4039,4141,4369,4411,4681,5383,5461,5611,5629 %N A238575 k-Lehmer numbers with two prime factors. %C A238575 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_ %H A238575 Amiram Eldar, <a href="/A238575/b238575.txt">Table of n, a(n) for n = 1..10000</a> %t A238575 rad[n_] := Times @@ Transpose[FactorInteger[n]][[1]]; Select[1 + \ %t A238575 Range[10000], Length[FactorInteger[#]] == 2 && Mod[# - 1, %t A238575 rad[EulerPhi[#]]] == 0 &] %Y A238575 Cf. A187731, A238574. %K A238575 nonn %O A238575 1,1 %A A238575 _José María Grau Ribas_, Mar 01 2014