A099633 Numbers k such that (phi(k-2) + phi(k+2))/2 = phi(k); 2-phi/balanced numbers.

5, 10, 14, 20, 25, 50, 80, 154, 200, 434, 520, 610, 815, 1205, 1345, 2680, 3200, 3560, 4669, 7330, 9290, 10130, 11705, 17630, 24410, 25090, 33385, 34205, 39754, 39785, 40870, 48470, 55979, 59090, 73165, 75470, 78035, 95990, 107680, 115685, 120230

Offset

1,1

Links

Donovan Johnson, Table of n, a(n) for n = 1..499

Example

k=154, phi(152)=72, phi(156)=48, phi(154)=60.

Mathematica

Flatten[Position[Partition[EulerPhi[Range[121000]], 5, 1], _?((#[[1]]+ #[[5]]) /2 == #[[3]]&), {1}, Heads->False]]+2 (* Harvey P. Dale, Jan 20 2015 *)

Prog

(PARI) isok(k) = (k>2) && (eulerphi(k-2)+eulerphi(k+2) == 2*eulerphi(k)); \\ Michel Marcus, Nov 18 2022

Crossrefs

Cf. A066167, A000010, A099631, A099632.

Sequence in context: A313627 A313628 A313629 * A313630 A313631 A313632

Adjacent sequences: A099630 A099631 A099632 * A099634 A099635 A099636

Keyword

nonn

Author

Labos Elemer, Oct 27 2004

Extensions

Incorrect term 2 removed by Michel Marcus, Nov 19 2022

Status

approved