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A216985
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Number of city-block distance 1, pressure limit 2 movements in an n X 2 array with each element moving exactly one horizontally or vertically, no element acquiring more than two neighbors, and without 2-loops.
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1
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0, 2, 14, 80, 436, 2346, 12578, 67368, 360720, 1931306, 10340030, 55359176, 296385268, 1586804322, 8495522018, 45483800496, 243513709632, 1303737284978, 6980021408654, 37370028005408, 200073740644948, 1071165953874618
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OFFSET
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1,2
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COMMENTS
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LINKS
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FORMULA
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Empirical: a(n) = 7*a(n-1) - 9*a(n-2) + a(n-3).
Empirical g.f.: 2*x^2 / (1 - 7*x + 9*x^2 - x^3). - Colin Barker, Jul 23 2018
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EXAMPLE
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Some solutions for n=3 (movement: 0=n 1=e 2=s 3=w):
..2..2....2..2....1..2....2..3....2..3....1..2....1..2....1..2....2..3....2..3
..1..2....2..3....1..2....1..0....1..0....0..3....0..2....0..3....1..2....2..0
..0..3....1..0....0..3....1..0....0..3....0..0....0..3....1..0....0..3....1..0
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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