OFFSET
1,1
COMMENTS
Column 2 of A200871.
LINKS
R. H. Hardin, Table of n, a(n) for n = 1..210
FORMULA
Empirical: a(n) = 2*a(n-1) +a(n-4).
Empirical g.f.: x*(17 + 3*x + 3*x^2 + 9*x^3) / (1 - 2*x - x^4). - Colin Barker, Oct 15 2017
EXAMPLE
Some solutions for n=3
..0....2....0....2....2....0....0....0....0....1....2....2....0....0....0....2
..0....2....0....2....1....1....0....1....1....0....2....0....2....0....2....1
..1....2....2....1....0....2....0....1....1....0....1....0....2....0....2....1
..2....2....2....1....0....2....0....2....0....1....1....1....2....1....2....1
..2....2....0....2....0....2....2....2....0....1....1....2....2....2....0....1
MATHEMATICA
a[0, x_, y_] := 1; a[n_, x_, y_] := a[n, x, y] = Sum[If[z <=x<= y || y <=x<= z, a[n-1, z, x], 0], {z, 3}]; a[n_] := Sum[a[n, x, y], {x, 3}, {y, 3}]; Array[a, 25] (* Giovanni Resta, Mar 05 2014 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Nov 23 2011
STATUS
approved