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

Author

Labos Elemer, Oct 27 2004

Keywords

• nonn

Examples

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

Links

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

Crossrefs

Cf. A066167, A000010, A099631, A099632.

Programs

Mathematica

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

PARI

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

Extensions

Incorrect term 2 removed by Michel Marcus, Nov 19 2022