This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A066167 #16 May 05 2017 12:37:35 %S A066167 5187,5313,273525,292005,494715,536055,657855,2379975,3045075,9960045, %T A066167 15091545,19420665,23977305,28292745,45864225,62361495,81758325, %U A066167 93794715,213205575,309227655,602444325,806687427,1375738845,1411639047,1538174925,1589814975,1628145057 %N A066167 Numbers n such that phi(n) = (phi(n+1) + phi(n-1))/2. %C A066167 Identical to the sequence of n such that phi(n-1), phi(n), phi(n+1) are in arithmetic progression. %C A066167 3 divides all known terms (up to 2*10^9) of the sequence. - _Farideh Firoozbakht_, Jan 01 2008 %H A066167 Giovanni Resta, <a href="/A066167/b066167.txt">Table of n, a(n) for n = 1..86</a> (terms < 10^13) %e A066167 Phi(5313) = 2640 = (2656 + 2624)/2 = (phi(5314) + phi(5212))/2. %t A066167 Select[ Range[ 2, 10^6 ], EulerPhi[ # ] == (EulerPhi[ #+1 ] + EulerPhi[ #-1 ])/2 & ] %K A066167 nonn %O A066167 1,1 %A A066167 _Joseph L. Pe_, Dec 13 2001 %E A066167 More terms from _Labos Elemer_, Oct 27 2004 %E A066167 More terms from _Farideh Firoozbakht_, Jan 01 2008 %E A066167 Missing a(25) from _Giovanni Resta_, May 05 2017