A265851 Primes p such that phi(p-2) = phi(p+5) - 2.

5, 11, 17, 257, 65537

Offset

1,1

Comments

Also primes p such that cototient(p-2) = cototient(p+5) - 5, where cototient(x) = A051953(x).

The next term, if it exists, must be greater than 5*10^8.

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

Primes from A265850.

Links

Table of n, a(n) for n=1..5.

Example

Prime 17 is in the sequence because phi(15) = phi(22) - 2 =  8.

Mathematica

Select[Prime@ Range[10^4], EulerPhi[# - 2] == EulerPhi[# + 5] - 2 &] (* Michael De Vlieger, Dec 17 2015 *)

Prog

(Magma) [n: n in [3..10^7] | IsPrime(n) and EulerPhi(n-2) eq EulerPhi(n+5) - 2]
(PARI) is(n)=eulerphi(n-2)==eulerphi(n+5)-2 \\ Anders Hellström, Dec 16 2015
(Magma) [p: p in PrimesInInterval(3, 2*10^5) | EulerPhi(p-2) eq EulerPhi(p+5)-2]; // Vincenzo Librandi, Dec 17 2015

Crossrefs

Cf. A000010, A019434, A015953, A265850.

Sequence in context: A282669 A271655 A294135 * A326716 A118122 A314256

Adjacent sequences: A265848 A265849 A265850 * A265852 A265853 A265854

Keyword

nonn,more

Author

Jaroslav Krizek, Dec 16 2015

Status

approved