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 A072993 Numbers n such that phi(n) divides C(2n,n). 1

%I

%S 1,2,3,4,5,6,7,10,12,14,15,18,21,22,26,29,30,33,35,41,42,43,44,45,52,

%T 56,60,66,67,69,70,73,74,75,76,77,78,82,86,87,88,89,92,95,99,100,105,

%U 106,110,115,116,119,122,123,124,126,127,129,133,135,138,142,143,146

%N Numbers n such that phi(n) divides C(2n,n).

%H Vaclav Kotesovec, <a href="/A072993/b072993.txt">Table of n, a(n) for n = 1..10000</a>

%H Vaclav Kotesovec, <a href="/A072993/a072993.jpg">Plot of a(n)/n for n = 1..60000</a>

%F It seems that a(n) is asymptotic to C*n with C=2.3...

%t Select[Range[200], Divisible[Binomial[2*#, #], EulerPhi[#]] &] (* _Vaclav Kotesovec_, Feb 15 2019 *)

%o (PARI) isok(n) = ! (binomial(2*n, n) % eulerphi(n)); \\ _Michel Marcus_, Nov 28 2013

%K easy,nonn

%O 1,2

%A _Benoit Cloitre_, Aug 21 2002

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Last modified May 11 10:32 EDT 2021. Contains 343788 sequences. (Running on oeis4.)