login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A258245 Irregular triangle (Beatty tree for Pi) as determined in Comments; a permutation of the nonnegative integers. 2
0, 3, 1, 12, 6, 4, 40, 2, 15, 21, 13, 128, 5, 7, 9, 43, 50, 69, 41, 405, 25, 31, 14, 16, 18, 22, 131, 138, 160, 219, 129, 1275, 8, 10, 53, 59, 72, 81, 100, 42, 44, 47, 51, 70, 408, 414, 436, 505, 691, 406, 4008, 34, 17, 19, 23, 26, 28, 32, 141, 150, 163, 169 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The Beatty tree for an irrational number r > 1 (such as r = Pi), is formed as follows.  To begin, let s = r/(r-1), so that the sequences defined 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 3, 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 = 8).

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

LINKS

Table of n, a(n) for n=1..62.

EXAMPLE

Rows (or generations, or levels) of T:

0

3

1   12

6   4   40

2   21  15   13   128

9   7   69   5    50   43   42   405

31  25  22   219  18   16   160  14   138   131   129   1275

Generations 0 to 7 of the tree are drawn by the Mathematica program.  In T, the path from 0 to 8 is (0,3,1,6,21,7,25,8).  The path obtained by backtracking (i.e., successive applications of the mapping b in Comments) is (8,25,7,21,6,1,3,0).

MATHEMATICA

r = Pi; k = 2000; 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, 8}]

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

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

TreePlot[graph, Top, 0, VertexLabeling -> True, ImageSize -> 800]

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

CROSSREFS

Cf. A022844, A258244 (path-length, 0 to n), A258212.

Sequence in context: A078938 A135888 A329433 * A133366 A049458 A143492

Adjacent sequences:  A258242 A258243 A258244 * A258246 A258247 A258248

KEYWORD

nonn,tabf,easy

AUTHOR

Clark Kimberling, Jun 08 2015

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified October 16 10:19 EDT 2021. Contains 348041 sequences. (Running on oeis4.)