OFFSET
1,1
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..499
EXAMPLE
k=154, phi(152)=72, phi(156)=48, phi(154)=60.
MATHEMATICA
Flatten[Position[Partition[EulerPhi[Range[121000]], 5, 1], _?((#[[1]]+ #[[5]]) /2 == #[[3]]&), {1}, Heads->False]]+2 (* Harvey P. Dale, Jan 20 2015 *)
PROG
(PARI) isok(k) = (k>2) && (eulerphi(k-2)+eulerphi(k+2) == 2*eulerphi(k)); \\ Michel Marcus, Nov 18 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Oct 27 2004
EXTENSIONS
Incorrect term 2 removed by Michel Marcus, Nov 19 2022
STATUS
approved