

A276029


Number of ways to transform a sequence of n ones and n twos to a single number by continually removing two numbers and replacing them with their sum modulo 3.


2



1, 4, 27, 228, 2226, 23778, 270693, 3229106, 39922172, 507680620, 6604676830, 87549425068, 1178880306174, 16086844260290, 222045139578443, 3095457073064120, 43529719213465854, 616853383573066504, 8801227720060618544, 126344910516550743232
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OFFSET

1,2


COMMENTS

Originally this entry had a reference to a paper on the arXiv by Caleb Ji, Enumerative Properties of Posets Corresponding to a Certain Class of No Strategy Games, arXiv:1608.06025 [math.CO], 2016. However, this article has since been removed from the arXiv.  N. J. A. Sloane, Sep 07 2018


LINKS



FORMULA

a(n) = b(0, n, n) where f(a, b, c) is the number of ways to reach one number beginning with a zeros, b ones, and c twos.
Then f(a, b, c) = f_1 + f_2 + f_3 + f_4 where f_1 = f(a1, b, c) if a>=2 or a, b >=1 or a,c >=1, f_2 = f(a, b2, c+1) if b >= 2, f_3 = f(a, b+1, c2) if c >= 2, and f_4 = f(a+1, b1, c1) if b, c >= 1, and each are 0 otherwise. Initial terms: f(a, b, c) = 1 for all 1 <= a+b+c <= 2, where a, b, c >= 0.


MAPLE

b:= proc(x, y, z) option remember;
`if`(x+y+z=1, 1, `if`(y>0 and z>0, b(x+1, y1, z1), 0)+
`if`(x>1 or x>0 and y>0 or x>0 and z>0, b(x1, y, z), 0)+
`if`(y>1, b(x, y2, z+1), 0)+`if`(z>1, b(x, y+1, z2), 0))
end:
a:= n> b(0, n, n):


MATHEMATICA

b[x_, y_, z_] := b[x, y, z] = If[x + y + z == 1, 1, If[y > 0 && z > 0, b[x + 1, y  1, z  1], 0] + If[x > 1  x > 0 && y > 0  x > 0 && z > 0, b[x  1, y, z], 0] + If[y > 1, b[x, y  2, z + 1], 0] + If[z > 1, b[x, y + 1, z  2], 0]];
a[n_] := b[0, n, n];


CROSSREFS



KEYWORD

nonn


AUTHOR



EXTENSIONS



STATUS

approved



