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

Author

Jaroslav Krizek, Nov 19 2014

Keywords

• nonn

Comments

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

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

Examples

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

Crossrefs

Supersequence of A247164 and A247203.

Cf. A000010, A019434, A029940, A248795.

Programs

Magma

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

Formula

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

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

Extensions

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