OFFSET
1,2
COMMENTS
Let p2(n) = phi(phi(n)). This list shows numbers n such that p2(n) = p2(n+1) = p2(n+2) = x for some x.
Here phi is Euler's totient function.
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..500
FORMULA
EXAMPLE
p2(1) = p2(2) = p2(3) = 1, p2(7) = p2(8) = p2(9) = 2.
MATHEMATICA
Select[Range[100], EulerPhi[EulerPhi[#]] == EulerPhi[EulerPhi[# + 1]] && EulerPhi[EulerPhi[#]] == EulerPhi[EulerPhi[# + 2]] &] (* G. C. Greubel, Jun 23 2016 *)
PROG
(PARI) isok(n) = (eulerphi(eulerphi(n)) == eulerphi(eulerphi(n+1))) && (eulerphi(eulerphi(n)) == eulerphi(eulerphi(n+2))) \\ Michel Marcus, Jul 12 2013
(Magma) [n: n in [1..2*10^5] | EulerPhi(EulerPhi(n)) eq EulerPhi(EulerPhi(n + 1)) and EulerPhi(EulerPhi(n)) eq EulerPhi(EulerPhi(n + 2))]; // Vincenzo Librandi, Jun 24 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Fred Schneider, Nov 11 2009
EXTENSIONS
Edited by N. J. A. Sloane, Nov 12 2009
Extended by R. J. Mathar, Nov 12 2009
STATUS
approved