A067701 - OEIS

%I #15 Feb 24 2024 06:47:48

%S 5,7,25,77,217,305,3665,4645,5065,8815,12205,12545,19877,20435,24235,

%T 29545,37735,47995,60115,72085,73025,77045,87475,121145,126055,129245,

%U 149755,190267,234635,247855,273295,275723,419365,531235,553475,621725,774485,840085

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

%H Donovan Johnson, <a href="/A067701/b067701.txt">Table of n, a(n) for n = 1..200</a>

%e phi(25-1) + phi(25+1) = 8+12 = phi(2*25), so 25 is in the sequence.

%t Select[Range[10^5], EulerPhi[ # - 1] + EulerPhi[ # + 1] == EulerPhi[2# ] &]

%t Flatten[Position[Partition[EulerPhi[Range[850000]],3,1],_?(#[[1]]+#[[3]]== #[[2]]&),1,Heads->False]]+1 (* _Harvey P. Dale_, Mar 26 2023 *)

%Y Cf. A000010, A066231.

%K nonn

%O 1,1

%A _Joseph L. Pe_, Feb 05 2002

%E Edited by _Dean Hickerson_, Feb 12 2002