A247203 -id:A247203 - cp's OEIS frontend

A248796 Numbers k such that Product_{d|(k-2)} phi(d) = Product_{d|(k-1)} phi(d) where phi(x) = Euler totient function (A000010).

3, 5, 7, 17, 257, 65537, 2200696, 2619707, 6372796, 40588487, 76466987, 81591196, 118018096, 206569607, 470542487, 525644387, 726638836, 791937616, 971122516, 991172807, 1268457016, 1384822007, 1613055047, 1709460755, 1861556656, 1872619667, 2507927416, 2659263947

Offset: 1

Jaroslav Krizek, Nov 19 2014

nonn

Numbers k such that A029940(k-2) = A029940(k-1).

The first 5 known Fermat primes (A019434) are terms of this sequence.

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

Supersequence of A247164 and A247203.

Cf. A000010, A019434, A029940, A248795.

[n: n in [3..100000] | (&*[EulerPhi(d): d in Divisors(n-2)]) eq (&*[EulerPhi(d): d in Divisors(n-1)])];

a(n) = A248795(n) + 2.

A029940(a(n)) = a(n) - 1 if a(n) = prime.

a(7)-a(9) using A248795 by Jaroslav Krizek, Nov 19 2014
a(10)-a(20) using A248795 by Jaroslav Krizek, Nov 25 2014
More terms from Jinyuan Wang, Jul 27 2025

A247164 Primes p such that Product_{d|(p-2)} phi(d) = Product_{d|(p-1)} phi(d) where phi(x) = Euler totient function (A000010).

Original entry on oeis.org

3, 5, 7, 17, 257, 65537, 991172807, 1872619667, 4081364447

Offset: 1

Jaroslav Krizek, Nov 21 2014

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

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

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

Subsequence of A248796. Supersequence of A247203.

Cf. A000010, A019434, A029940, A248795.

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

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

a(8)-a(9) from Jinyuan Wang, Jul 27 2025

cp's OEIS Frontend

A248796 Numbers k such that Product_{d|(k-2)} phi(d) = Product_{d|(k-1)} phi(d) where phi(x) = Euler totient function (A000010).

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