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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
OFFSET
1,2
LINKS
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
KEYWORD
easy,nonn
AUTHOR
Benoit Cloitre, Aug 21 2002
STATUS
approved