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A072993 Numbers n such that phi(n) divides C(2n,n). 1
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, 56, 60, 66, 67, 69, 70, 73, 74, 75, 76, 77, 78, 82, 86, 87, 88, 89, 92, 95, 99, 100, 105, 106, 110, 115, 116, 119, 122, 123, 124, 126, 127, 129, 133, 135, 138, 142, 143, 146 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Vaclav Kotesovec, Table of n, a(n) for n = 1..10000

Vaclav Kotesovec, Plot of a(n)/n for n = 1..60000

FORMULA

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

MATHEMATICA

Select[Range[200], Divisible[Binomial[2*#, #], EulerPhi[#]] &] (* Vaclav Kotesovec, Feb 15 2019 *)

PROG

(PARI) isok(n) = ! (binomial(2*n, n) % eulerphi(n)); \\ Michel Marcus, Nov 28 2013

CROSSREFS

Sequence in context: A307625 A165722 A082400 * A018444 A032378 A342726

Adjacent sequences:  A072990 A072991 A072992 * A072994 A072995 A072996

KEYWORD

easy,nonn

AUTHOR

Benoit Cloitre, Aug 21 2002

STATUS

approved

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Last modified April 13 18:38 EDT 2021. Contains 342939 sequences. (Running on oeis4.)