A066167 - OEIS

%I #16 May 05 2017 12:37:35

%S 5187,5313,273525,292005,494715,536055,657855,2379975,3045075,9960045,

%T 15091545,19420665,23977305,28292745,45864225,62361495,81758325,

%U 93794715,213205575,309227655,602444325,806687427,1375738845,1411639047,1538174925,1589814975,1628145057

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

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

%C 3 divides all known terms (up to 2*10^9) of the sequence. - _Farideh Firoozbakht_, Jan 01 2008

%H Giovanni Resta, <a href="/A066167/b066167.txt">Table of n, a(n) for n = 1..86</a> (terms < 10^13)

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

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

%K nonn

%O 1,1

%A _Joseph L. Pe_, Dec 13 2001

%E More terms from _Labos Elemer_, Oct 27 2004

%E More terms from _Farideh Firoozbakht_, Jan 01 2008

%E Missing a(25) from _Giovanni Resta_, May 05 2017