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A066167
Numbers n such that phi(n) = (phi(n+1) + phi(n-1))/2.
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
1,1
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
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..86 (terms < 10^13)
EXAMPLE
Phi(5313) = 2640 = (2656 + 2624)/2 = (phi(5314) + phi(5212))/2.
MATHEMATICA
Select[ Range[ 2, 10^6 ], EulerPhi[ # ] == (EulerPhi[ #+1 ] + EulerPhi[ #-1 ])/2 & ]
CROSSREFS
Sequence in context: A175745 A190464 A031570 * A247265 A252050 A028549
KEYWORD
nonn
AUTHOR
Joseph L. Pe, Dec 13 2001
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
STATUS
approved