A232720 Numbers such that 2*phi(n-2) = n-1.

3, 5, 17, 257, 65537, 83623937, 4294967297, 6992962672132097

Offset: 1

Author

Michel Marcus, Nov 28 2013

Keywords

• nonn
• more

Comments

Numbers such that A074057(n) = 0.

Note that the first 5 terms are Fermat primes (A019434), while a(6) is prime but is not a Fermat number, and a(7) is (F5 from A000215) but is not prime. Note also that F6, F7, F8 and F9 do not belong to this sequence.

Crossrefs

Cf. A000010, A074057.

Programs

PARI

is_A232720(n) = 2*eulerphi(n-2) == n-1

Formula

a(n) = A050474(n) + 2. - Jaroslav Krizek, Feb 25 2015

Extensions

a(8) from Jaroslav Krizek, Feb 25 2015