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Number of numbers in row n of the array at A243855.
4

%I #10 Aug 13 2014 18:21:34

%S 1,2,2,3,4,7,11,17,27,44,70,111,176,281,447,712,1130,1797,2856,4549,

%T 7233,11517,18317,29163,46389,73838,117503,187047,297690,473909,

%U 754298,1200808

%N Number of numbers in row n of the array at A243855.

%C Decree that (row 1) = (1) and (row 2) = (3,2). For n >= 4, row n consists of numbers in decreasing order generated as follows: x+1 for each x in row n-1 together with 3/x for each x in row n-1, and duplicates are rejected as they occur. Then a(n) = (number of numbers in row n); it appears that this sequence is not linearly recurrent.

%e First 6 rows of the array of rationals:

%e 1/1

%e 4/1 ... 2/1

%e 5/1 ... 3/1

%e 6/1 ... 4/3 ... 4/5

%e 7/1 ... 7/3 ... 9/5 ... 2/3

%e 8/1 ... 10/3 ... 14/5 .. 20/9 .. 12/7 .. 5/3 .. 4/7, so that A243856 begins with 1,2,2,3,4,7.

%t z = 12; g[1] = {1}; f1[x_] := x + 1; f2[x_] := 4/x; 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[Reverse[g[n]], {n, 1, z}]; v = Flatten[u];

%t Denominator[v] (* A243854 *)

%t Numerator[v] (* A243855 *)

%t Table[Length[g[n]], {n, 1, z}] (* A243856 *)

%Y Cf. A243850, A243853.

%K nonn,easy

%O 1,2

%A _Clark Kimberling_, Jun 12 2014