login
A376337
Numbers k such that phi(k)/2 + 1 = phi(k + 1) where phi = A000010.
1
3, 7, 9, 31, 127, 8191, 131071, 524287
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
STATUS
approved