login
Primes p such that Product_{d|(p-2)} phi(d) = Product_{d|(p-1)} phi(d) where phi(x) = Euler totient function (A000010).
2

%I #14 Sep 08 2022 08:46:09

%S 3,5,7,17,257,65537,991172807

%N Primes p such that Product_{d|(p-2)} phi(d) = Product_{d|(p-1)} phi(d) where phi(x) = Euler totient function (A000010).

%C Primes p such that A029940(p-2) = A029940(p-1).

%C First 5 known terms of Fermat primes (A019434) are terms of this sequence.

%C Subsequence of A248796. Supersequence of A247203.

%F A029940(a(n)) = a(n)-1.

%e Prime 17 is in the sequence because A029940(15) = A029940(16) = 64.

%o (Magma) [n: n in [3..100000] | IsPrime(n) and (&*[EulerPhi(d): d in Divisors(n-2)]) eq (&*[EulerPhi(d): d in Divisors(n-1)])]

%Y Cf. A000010, A019434, A029940, A248795, A248796.

%K nonn,more

%O 1,1

%A _Jaroslav Krizek_, Nov 21 2014