OFFSET
1,1
COMMENTS
Conjecture: this sequence is the union {9} and the Mersenne primes A000668.
EXAMPLE
Number 9 is in this sequence because phi(9)/2 + 1 = 6/2 + 1 = 3 + 1 = 4 is equal to phi(9 + 1) = phi(10) = 4.
MATHEMATICA
Select[Range[550000], EulerPhi[#]/2+1==EulerPhi[#+1] &] (* Stefano Spezia, Sep 22 2024 *)
PROG
(Magma) [k: k in [3..5*10^6] | ((EulerPhi(k) div 2) + 1) eq EulerPhi(k + 1)];
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Juri-Stepan Gerasimov, Sep 20 2024
STATUS
approved