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A099633
Numbers k such that (phi(k-2) + phi(k+2))/2 = phi(k); 2-phi/balanced numbers.
1
5, 10, 14, 20, 25, 50, 80, 154, 200, 434, 520, 610, 815, 1205, 1345, 2680, 3200, 3560, 4669, 7330, 9290, 10130, 11705, 17630, 24410, 25090, 33385, 34205, 39754, 39785, 40870, 48470, 55979, 59090, 73165, 75470, 78035, 95990, 107680, 115685, 120230
OFFSET
1,1
LINKS
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