A223212 3X3X3 triangular graph coloring a rectangular array: number of nX2 0..5 arrays where 0..5 label nodes of a graph with edges 0,1 0,2 1,2 1,3 1,4 2,4 3,4 2,5 4,5 and every array movement to a horizontal or vertical neighbor moves along an edge of this graph.

18, 126, 918, 6642, 48114, 348462, 2523798, 18278946, 132387858, 958837662, 6944516694, 50296639122, 364280484978, 2638352661966, 19108640336598, 138397015977282, 1002359858893074, 7259732297153982, 52579632512961558

Offset: 1

Author

R. H. Hardin Mar 18 2013

Keywords

• nonn

Comments

Column 2 of A223218

Examples

			Some solutions for n=3
..0..1....1..4....0..1....0..2....4..1....4..1....4..2....2..4....0..2....5..2
..2..4....2..1....1..0....1..4....1..2....2..4....2..4....4..1....2..0....2..5
..0..2....1..2....4..1....3..1....4..1....4..2....0..2....1..2....0..1....0..2
		

Links

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

Formula

Empirical: a(n) = 6*a(n-1) +9*a(n-2).

Empirical g.f.: -18*x*(1+x)/(-1+6*x+9*x^2) . a(n) = 18*(A189801(n)+A189801(n-1)). - R. J. Mathar, May 21 2018