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A221652

Number of n X 2 arrays of occupancy after each element moves to some horizontal, diagonal or antidiagonal neighbor, without move-in move-out left turns.

1, 4, 12, 40, 136, 460, 1556, 5264, 17808, 60244, 203804, 689464, 2332440, 7890588, 26693668, 90304032, 305496352, 1033486756, 3496260652, 11827765064, 40013042600, 135363153516, 457930268212, 1549167000752, 5240794423984

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

OFFSET

1,2

COMMENTS

Column 2 of A221655.

LINKS

R. H. Hardin, Table of n, a(n) for n = 1..210

FORMULA

Empirical: a(n) = 3*a(n-1) + a(n-2) + a(n-3) for n>4.

Empirical g.f.: x*(1 - x)*(1 + x)^2 / (1 - 3*x - x^2 - x^3). - Colin Barker, Mar 14 2018

EXAMPLE

Some solutions for n=3: