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A221795
Number of n X 2 arrays of occupancy after each element moves to some horizontal or vertical neighbor, without move-in move-out straight through or left turns.
1
1, 5, 12, 39, 112, 338, 1001, 2985, 8881, 26440, 78707, 234288, 697446, 2076121, 6180257, 18397277, 54765156, 163024591, 485291776, 1444615370, 4300329993, 12801215625, 38106643849, 113435806112, 337675564859, 1005192182880
OFFSET
1,2
COMMENTS
Column 2 of A221800.
LINKS
FORMULA
Empirical: a(n) = a(n-1) +6*a(n-2) +a(n-3) -4*a(n-4).
Empirical g.f.: x*(1 + 4*x + x^2 - 4*x^3) / (1 - x - 6*x^2 - x^3 + 4*x^4). - Colin Barker, Aug 11 2018
EXAMPLE
Some solutions for n=3:
..1..0....1..1....2..1....0..2....1..1....0..1....1..1....0..1....1..1....2..1
..2..1....1..1....0..0....1..1....0..1....1..2....1..2....2..2....2..1....1..0
..1..1....1..1....1..2....2..0....1..2....1..1....0..1....1..0....1..0....1..1
CROSSREFS
Cf. A221800.
Sequence in context: A028322 A169588 A233007 * A092772 A164737 A120779
KEYWORD
nonn
AUTHOR
R. H. Hardin, Jan 26 2013
STATUS
approved