A049809 a(n)=number of Farey fractions of order n that are <=1/5; cf. A049805.

1, 1, 1, 1, 2, 3, 4, 5, 6, 7, 9, 10, 12, 13, 15, 17, 20, 21, 24, 26, 29, 31, 35, 36, 40, 43, 47, 50, 55, 56, 62, 65, 69, 72, 77, 80, 87, 91, 96, 99, 107, 109, 117, 121, 126, 131, 140, 143, 151, 155, 162, 167, 177, 180, 188, 193, 201, 207

Offset: 1

Author

Clark Kimberling

Keywords

• nonn

Crossrefs

Cf. A005728, A049806, A049807, A049808, A049810, A049811, A049812, A049813, A049814.

Programs

Mathematica

Farey[n_] := Union[Flatten[Join[{0}, Table[a/b, {b, n}, {a, b}]]]]; f[n_] := Length@ Select[ Farey@ n, # <= 1/5 &]; Array[f, 58] (* Robert G. Wilson v, Nov 14 2012 *)