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
KEYWORD
nonn
AUTHOR
José María Grau Ribas, Mar 01 2014
STATUS
approved