OFFSET
1,1
COMMENTS
Conjecture: a prime p is in the sequence iff p is a Fermat prime (A019434).
This is not correct: the first non-Fermat prime term is 83623937 = 2^18*11*29 + 1. - Joerg Arndt, Oct 11 2015
EXAMPLE
phi(32) = 16 = 2*phi(30) = 2*8, so 32 is in the sequence.
MATHEMATICA
Select[Range@ 400000, EulerPhi@ # == 2 EulerPhi[# - 2] &] (* Michael De Vlieger, Sep 25 2015 *)
PROG
(Magma) [n: n in [3..10000000] | EulerPhi(n) eq 2*EulerPhi(n-2)]
(PARI) for(n=1, 1e6, if(eulerphi(n) == 2*eulerphi(n-2), print1(n", "))); \\ Altug Alkan, Sep 26 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Sep 24 2015
STATUS
approved