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

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

Offset

1,1

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.

Links

Table of n, a(n) for n=1..8.

Formula

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

Prog

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

Crossrefs

Cf. A000010, A074057.

Sequence in context: A260476 A070592 A254576 * A272061 A247203 A262534

Adjacent sequences: A232717 A232718 A232719 * A232721 A232722 A232723

Keyword

nonn,more

Author

Michel Marcus, Nov 28 2013

Extensions

a(8) from Jaroslav Krizek, Feb 25 2015

Status

approved