%I #26 Sep 08 2022 08:46:12
%S 3,4,5,8,17,32,257,512,527,992,1952,2522,5252,6512,7412,10376,23432,
%T 23717,26732,27302,35114,36632,37442,45872,47027,49022,51092,65537,
%U 78899,84242,92432,98432,98672,114767,115292,131072,227222,231167,240977,328352,369272
%N Numbers n such that phi(n) = 2*phi(n-2).
%C Conjecture: a prime p is in the sequence iff p is a Fermat prime (A019434).
%C This is not correct: the first non-Fermat prime term is 83623937 = 2^18*11*29 + 1. - _Joerg Arndt_, Oct 11 2015
%e phi(32) = 16 = 2*phi(30) = 2*8, so 32 is in the sequence.
%t Select[Range@ 400000, EulerPhi@ # == 2 EulerPhi[# - 2] &] (* _Michael De Vlieger_, Sep 25 2015 *)
%o (Magma) [n: n in [3..10000000] | EulerPhi(n) eq 2*EulerPhi(n-2)]
%o (PARI) for(n=1, 1e6, if(eulerphi(n) == 2*eulerphi(n-2), print1(n", "))); \\ _Altug Alkan_, Sep 26 2015
%Y Cf. A171271, A171262.
%K nonn
%O 1,1
%A _Jaroslav Krizek_, Sep 24 2015