A265850 Numbers n such that phi(n-2) = phi(n+5) - 2.

5, 11, 17, 160, 257, 65537

Offset

1,1

Comments

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

The next term, if it exists, must be greater than 2*10^7.

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

Prime terms are in the sequence A265851.

The next term if it exists is greater than 10^9. - Michel Marcus, Dec 20 2015

The next term if it exists is greater than 10^11. - Dana Jacobsen, Dec 24 2015

Links

Table of n, a(n) for n=1..6.

Example

160 is in the sequence because phi(158) = phi(165) - 2 = 78.

Mathematica

aa={}; Do[If[EulerPhi[n - 2] == EulerPhi[n + 5] - 2, AppendTo[aa, n]], {n, 1, 2 10^5}]; aa (* Vincenzo Librandi, Dec 17 2015 *)
Flatten[Position[Partition[EulerPhi[Range[66000]], 8, 1], _?(#[[1]]== #[[-1]] -2&), 1, Heads->False]]+2 (* Harvey P. Dale, Dec 31 2021 *)

Prog

(Magma) [n: n in [3..100000] | EulerPhi(n-2) eq EulerPhi(n+5)-2];
(PARI) for(n=3, 1e4, if(eulerphi(n-2) == eulerphi(n+5) - 2, print1(n, ", "))) \\ Altug Alkan, Dec 16 2015

Crossrefs

Cf. A000010, A019434, A051953, A265851.

Sequence in context: A182482 A075582 A059960 * A185365 A282669 A271655

Adjacent sequences: A265847 A265848 A265849 * A265851 A265852 A265853

Keyword

nonn,more

Author

Jaroslav Krizek, Dec 16 2015

Status

approved