A271657 Primes p such that phi(p-3) = phi(phi(p-1)-1).

5, 17, 257, 1277, 3853, 6203, 8663, 16481, 65537, 131519, 257953, 265961, 269573, 380201, 510449, 512927, 846563, 904793, 1056707, 1503329, 1538057, 2675753, 2756153, 2952413, 3333893, 3837347, 4457753, 4643213, 4783873, 5068937, 6874949, 7536917, 13248227

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(257-3) = phi(254) = 126 = phi(phi(257-1)-1) = phi(phi(256)-1) = phi(128-1) = phi(127).

Mathematica

Rest@ Select[Prime@ Range[10^6], EulerPhi[# - 3] == EulerPhi[EulerPhi[# - 1] - 1] &] (* Michael De Vlieger, Apr 11 2016 *)

Prog

(Magma) [n: n in [4..5*10^7] | IsPrime(n) and EulerPhi(n-3) eq EulerPhi(EulerPhi(n-1)-1)]

Crossrefs

Cf. A019434, A271655, A271656, A271658, A271659, A271660.

Sequence in context: A235461 A271660 A273948 * A273999 A222008 A274000

Adjacent sequences: A271654 A271655 A271656 * A271658 A271659 A271660

Keyword

nonn

Author

Jaroslav Krizek, Apr 11 2016

Status

approved