A207575 - OEIS

%I #31 Nov 06 2023 11:28:23

%S 1,390,10374,2283934267736070,7316037865689066623729670

%N Numbers k such that phi(k) + 2 divides k + 2 and k is not twice a prime.

%C Contains 2 * terms t of A350777 such that (t-3)/phi(t) = 2. - _Max Alekseyev_, Oct 26 2023

%t Select[Range[20000000], !PrimeQ[#/2] && Divisible[#+2, EulerPhi[#]+2]&]

%o (PARI) for(n=1,1e5,if((n+2)%(eulerphi(n)+2)==0&&(n%2||!isprime(n/2)), print1(n", "))) \\ _Charles R Greathouse IV_, Mar 02 2012

%Y Cf. A000010, A207574, A202855, A203966, A350777.

%K nonn,more,hard

%O 1,2

%A _José María Grau Ribas_, Feb 19 2012

%E a(4)-a(5) from _Max Alekseyev_, Nov 06 2023