A242308 - OEIS

%I #4 Jun 08 2014 09:17:23

%S 1,1,2,2,1,3,3,2,5,3,5,3,1,3,4,8,5,4,8,5,2,5,3,7,13,7,8,4,7,13,7,8,4,

%T 1,3,4,8,5,5,11,11,21,12,7,13,7,5,11,11,21,12,7,13,7,2,5,3,7,13,7,8,4,

%U 9,18,10,19,34,18,19,9,12,21,11,11,5,9,18,10

%N Irregular triangular array of numerators of the positive rational numbers ordered as in Comments.

%C Decree that row 1 is (1) and row 2 is (1/2).  For n >=3, row n consists of numbers in increasing order generated  as follows:  1/(x + 1) for each x in row n-1 together with x + 1 for each x in row n-2.  It is easy to prove that row n consists of F(n) numbers, where F = A000045 (the Fibonacci numbers), and that every positive rational number occurs exactly once.

%H Clark Kimberling, <a href="/A242308/b242308.txt">Table of n, a(n) for n = 1..6000</a>

%e First 6 rows of the array of rationals:

%e 1/1

%e 1/2

%e 2/3 ... 2/1

%e 1/3 ... 3/5 ... 3/2

%e 2/5 ... 5/8 ... 3/4 ... 5/3 ... 3/1

%e 1/4 ... 3/8 ... 4/7 ... 8/13 .. 5/7 .. 4/3 .. 8/5 .. 5/2

%e The numerators, by rows:  1,1,2,2,1,3,3,2,5,3,5,3,1,3,4,8,5,4,8,5,...

%t z = 18; g[1] = {1}; f1[x_] := 1/x; f2[x_] := 1/(x + 1); h[1] = g[1];

%t b[n_] := b[n] = DeleteDuplicates[Union[f1[g[n - 1]], f2[g[n - 1]]]];

%t h[n_] := h[n] = Union[h[n - 1], g[n - 1]];

%t g[n_] := g[n] = Complement [b[n], Intersection[b[n], h[n]]]

%t u = Table[g[n], {n, 1, z}]; v = Flatten[u];

%t Denominator[v]; (* A243574 *)

%t Numerator[v];   (* A242308 *)

%Y Cf. A242574, A242360, A000045.

%K nonn,easy,tabf,frac

%O 1,3

%A _Clark Kimberling_, Jun 07 2014