A258235 - OEIS

A258235

Irregular triangle (or "upper Wythoff tree", or Beatty tree for r = (golden ratio)^2), T, of all nonnegative integers, each exactly once, as determined from the upper Wythoff sequence as described in Comments.

0, 2, 1, 7, 3, 5, 20, 10, 15, 8, 54, 4, 6, 23, 28, 41, 21, 143, 9, 11, 13, 16, 18, 57, 62, 75, 109, 55, 376, 31, 36, 44, 49, 22, 24, 26, 29, 42, 146, 151, 164, 198, 287, 144, 986, 12, 14, 17, 19, 65, 70, 78, 83, 96, 112, 117, 130, 56, 58, 60, 63, 76, 110

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

Suppose that r is an irrational number > 1, and let s = r/(r-1), so that the sequences u and v defined by u(n) = floor(r*n) and v(n) = floor(s*n), for n >=1 are the Beatty sequences of r and s, and u and v partition the positive integers.

The tree T has root 0 with an edge to 2, and all other edges are determined as follows: if x is in u(v), then there is an edge from x to floor(r + r*x) and an edge from x to ceiling(x/r); otherwise there is an edge from x to floor(r + r*x). (Thus, the only branchpoints are the numbers in u(v).)

Another way to form T is by "backtracking" to the root 0. Let b(x) = floor[x/r] if x is in (u(n)), and b(x) = floor[r*x] if x is in (v(n)). Starting at any vertex x, repeated applications of b eventually reach 0. The number of steps to reach 0 is the number of the generation of T that contains x. (See Example for x = 29).

See A258212 for a guide to Beatty trees for various choices of r.

LINKS

Table of n, a(n) for n=1..63.

EXAMPLE

Rows (or generations, or levels) of T:

1 7

3 5 20

10 15 8 54

4 6 23 28 41 21 143

9 11 13 16 18 57 62 75 109 55 376

Generations 0 to 9 of the tree are drawn by the Mathematica program. In T, the path from 0 to 29 is (0,2,7,3,10,28,75,29). The path obtained by backtracking (i.e., successive applications of the mapping b in Comments) is (29,75,28,10,3,7,2,0).

MATHEMATICA

r = (3 + Sqrt[5])/2; k = 1000; w = Map[Floor[r #] &, Range[k]];

f[x_] := f[x] = If[MemberQ[w, x], Floor[x/r], Floor[r*x]];

b := NestWhileList[f, #, ! # == 0 &] &;

bs = Map[Reverse, Table[b[n], {n, 0, k}]];

generations = Table[DeleteDuplicates[Map[#[[n]] &, Select[bs, Length[#] > n - 1 &]]], {n, 10}]

paths = Sort[Map[Reverse[b[#]] &, Last[generations]]]

graph = DeleteDuplicates[Flatten[Map[Thread[Most[#] -> Rest[#]] &, paths]]]

TreePlot[graph, Top, 0, VertexLabeling -> True, ImageSize -> 1400]

Map[DeleteDuplicates, Transpose[paths]] (* Peter J. C. Moses, May 21 2015 *)

CROSSREFS

Cf. A000201, A001950, A258236 (path-length from 0 to n).