A271655 Primes p such that phi(p+1) = phi(phi(p-1)+1).

5, 11, 17, 257, 401, 2789, 7481, 15401, 24443, 37517, 51521, 65537, 85793, 646271, 719813, 891047, 900293, 2535473, 2841851, 3167569, 3260809, 3516109, 4356749, 5111261, 5914369, 7056293, 9832271, 9838769, 10309253, 12026603, 12231311, 14599097, 16509617

Offset

1,1

Comments

The first 4 known Fermat primes > 3 from A019434 are in the sequence.

Links

Table of n, a(n) for n=1..33.

Example

257 is a term because phi(257+1) = phi(258) = 84 = phi(phi(257-1)+1) = phi(phi(256)+1) = phi(128+1) = phi(129).

Mathematica

Select[Prime@ Range[10^6], EulerPhi[# + 1] == EulerPhi[EulerPhi[# - 1] + 1] &] (* Michael De Vlieger, Apr 11 2016 *)

Prog

(Magma) [n: n in [2..10^7] | IsPrime(n) and EulerPhi(n+1) eq EulerPhi(EulerPhi(n-1) +1)]

Crossrefs

Cf. A019434, A271656, A271657, A271658, A271659, A271660.

Sequence in context: A265850 A185365 A282669 * A294135 A265851 A326716

Adjacent sequences: A271652 A271653 A271654 * A271656 A271657 A271658

Keyword

nonn

Author

Jaroslav Krizek, Apr 11 2016

Status

approved