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A167767
First of 3 or more consecutive integers with equal values of phi(phi(n)).
3
1, 2, 7, 8, 20, 31, 32, 33, 146, 211, 314, 384, 626, 674, 1754, 2694, 2695, 5186, 11714, 12242, 17329, 17613, 19310, 25544, 35774, 36728, 38018, 40227, 42626, 56834, 65731, 91106, 97724, 110971, 117536, 131071, 131072, 155821, 161734, 164174
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
FORMULA
{n: A010554(n) = A010554(n+1) = A010554(n+2)}. - R. J. Mathar, Nov 12 2009
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
Cf. A167768.
Sequence in context: A015617 A306903 A026579 * A054601 A279847 A291629
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