This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A247203 #22 Jul 27 2025 10:40:36
%S A247203 3,5,17,257,65537,991172807,1872619667,4081364447
%N A247203 Primes p such that phi(p-2) = phi(p-1) and simultaneously Product_{d|(p-2)} phi(d) = Product_{d|(p-1)} phi(d) where phi(x) = Euler totient function (A000010).
%C A247203 Primes p such that A000010(p-2) = A000010(p-1) and simultaneously A029940(p-2) = A029940(p-1).
%C A247203 The first 5 known Fermat primes (A019434) are terms of this sequence.
%e A247203 17 is in the sequence because phi(15) = phi(16) = 8 and simultaneously Product_{d|15} phi(d) = Product_{d|16} phi(d) = 64.
%o A247203 (Magma) [p: p in PrimesInInterval(3, 10^7) | (&*[EulerPhi(d): d in Divisors(p-2)]) eq (&*[EulerPhi(d): d in Divisors(p-1)]) and EulerPhi(p-2) eq EulerPhi(p-1)];
%o A247203 (Magma) [n: n in [A248796(n)] | IsPrime(n) and EulerPhi(n-2) eq EulerPhi(n-1)];
%o A247203 (Magma) [n: n in [A247164(n)] | IsPrime(n) and EulerPhi(n-2) eq EulerPhi(n-1)];
%Y A247203 Subsequence of A247164 and A248796.
%Y A247203 Cf. A000010, A029940.
%K A247203 nonn,more
%O A247203 1,1
%A A247203 _Jaroslav Krizek_, Nov 25 2014
%E A247203 a(7)-a(8) from _Jinyuan Wang_, Jul 27 2025