A218657 Hilltop maps: number of n X 2 binary arrays indicating the locations of corresponding elements not exceeded by any king-move neighbor in a random 0..1 n X 2 array.

3, 15, 57, 225, 891, 3519, 13905, 54945, 217107, 857871, 3389769, 13394241, 52925643, 209128959, 826346529, 3265203393, 12902036643, 50980759695, 201443999193, 795980386593, 3145215436443, 12427919466687, 49107345869169

Offset: 1

Author

R. H. Hardin, Nov 04 2012

Keywords

• nonn

Comments

Column 2 of A218663.

Examples

			Some solutions for n=3:
..1..1....1..1....1..0....0..0....1..1....0..1....0..1....0..0....1..1....0..0
..1..1....1..0....1..1....0..1....0..1....0..0....1..0....1..0....0..1....0..1
..1..1....0..1....1..1....0..0....0..1....1..0....0..1....1..1....0..0....0..1
		

Links

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

Crossrefs

Cf. A218663.

Formula

Empirical: a(n) = 3*a(n-1) + 3*a(n-2) + 3*a(n-3).

Empirical g.f.: 3*x*(1 + x)^2 / (1 - 3*x - 3*x^2 - 3*x^3). - Colin Barker, Mar 10 2018