A220564 Number of ways to reciprocally link elements of an 3 X n array either to themselves or to exactly one horizontal or antidiagonal neighbor.

1, 13, 64, 430, 2604, 16310, 101052, 628269, 3901815, 24240377, 150578968, 935414825, 5810847665, 36097422853, 224239670624, 1392992694942, 8653368270212, 53755331997318, 333931897912068, 2074408410086741

Offset: 1

Author

R. H. Hardin, Dec 16 2012

Keywords

• nonn

Comments

Row 3 of A220562.

Examples

			Some solutions for n=3 0=self 3=ne 4=w 6=e 7=sw (reciprocal directions total 10):
..0..6..4....6..4..0....0..7..0....0..7..7....0..6..4....6..4..7....0..0..0
..6..4..0....0..7..0....3..0..0....3..3..7....0..0..0....0..3..7....6..4..0
..0..6..4....3..0..0....6..4..0....0..3..0....0..0..0....0..3..0....0..6..4
		

Links

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

Crossrefs

Cf. A220562.

Formula

Empirical: a(n) = 4*a(n-1) + 15*a(n-2) - 5*a(n-3) - 19*a(n-4) + 9*a(n-5) + 2*a(n-6) - a(n-7).

Empirical g.f.: x*(1 - x)*(1 + 10*x + 7*x^2 - 9*x^3 - x^4 + x^5) / ((1 + x - x^2)*(1 - 5*x - 9*x^2 + 9*x^3 + x^4 - x^5)). - Colin Barker, Aug 01 2018