A324797 Irregular triangle read by rows: row n gives denominators of fractions in the Farey subsequence B(m).

1, 2, 1, 1, 3, 2, 3, 1, 1, 4, 3, 5, 2, 5, 3, 4, 1, 1, 5, 4, 3, 5, 7, 2, 7, 5, 3, 4, 5, 1, 1, 6, 5, 4, 7, 3, 8, 5, 7, 9, 2, 9, 7, 5, 8, 3, 7, 4, 5, 6, 1, 1, 7, 6, 5, 4, 7, 3, 8, 5, 7, 9, 11, 2, 11, 9, 7, 5, 8, 3, 7, 4, 5, 6, 7, 1, 1, 8, 7, 6, 5, 9, 4, 7, 10, 3, 11, 8, 5, 12, 7, 9, 11, 13, 2, 13, 11, 9, 7, 12, 5, 8, 11, 3, 10, 7, 4, 9, 5, 6, 7, 8, 1

Offset: 1

Author

N. J. A. Sloane, Sep 10 2019

Keywords

• nonn
• frac
• tabf

Comments

B(n) is denoted by F(B(2n),n) in Matveev (2017) - see definition on page 1. B(n) consists of the terms h/k of the Farey series F_{2n} such that k-n <= h <= n.

A049691 gives the row lengths.

Examples

			The first few sequences B(1), B(2), B(3), B(4) are:
[0, 1/2, 1],
[0, 1/3, 1/2, 2/3, 1],
[0, 1/4, 1/3, 2/5, 1/2, 3/5, 2/3, 3/4, 1],
[0, 1/5, 1/4, 1/3, 2/5, 3/7, 1/2, 4/7, 3/5, 2/3, 3/4, 4/5, 1], [0, 1/6, 1/5, 1/4, 2/7, 1/3, 3/8, 2/5, 3/7, 4/9, 1/2, 5/9, 4/7, 3/5, 5/8, 2/3, 5/7, 3/4, 4/5, 5/6, 1],
...
		

References

• A. O. Matveev, Farey Sequences, De Gruyter, 2017.

Links

• A. O. Matveev, Farey Sequences: Errata + Haskell code

Crossrefs

Cf. A006842/A006843, A049691, A324796 (numerators).

Programs

Maple

Farey := proc(n) sort(convert(`union`({0}, {seq(seq(m/k, m=1..k), k=1..n)}), list)) end:
B := proc(m) local a,i,h,k; global Farey; a:=[];
for i in Farey(2*m) do
h:=numer(i); k:=denom(i);
if (h <= m) and (k-m <= h) then a:=[op(a),i]; fi; od: a; end;