A066167 Numbers n such that phi(n) = (phi(n+1) + phi(n-1))/2.

5187, 5313, 273525, 292005, 494715, 536055, 657855, 2379975, 3045075, 9960045, 15091545, 19420665, 23977305, 28292745, 45864225, 62361495, 81758325, 93794715, 213205575, 309227655, 602444325, 806687427, 1375738845, 1411639047, 1538174925, 1589814975, 1628145057

Offset: 1

Author

Joseph L. Pe, Dec 13 2001

Keywords

• nonn

Comments

Identical to the sequence of n such that phi(n-1), phi(n), phi(n+1) are in arithmetic progression.

3 divides all known terms (up to 2*10^9) of the sequence. - Farideh Firoozbakht, Jan 01 2008

Examples

			Phi(5313) = 2640 = (2656 + 2624)/2 = (phi(5314) + phi(5212))/2.
		

Links

• Giovanni Resta, Table of n, a(n) for n = 1..86 (terms < 10^13)

Programs

Mathematica

Select[ Range[ 2, 10^6 ], EulerPhi[ # ] == (EulerPhi[ #+1 ] + EulerPhi[ #-1 ])/2 & ]

Extensions

More terms from Labos Elemer, Oct 27 2004

More terms from Farideh Firoozbakht, Jan 01 2008

Missing a(25) from Giovanni Resta, May 05 2017