A221440 Hilltop maps: number of n X 2 binary arrays indicating the locations of corresponding elements not exceeded by any horizontal or antidiagonal neighbor in a random 0..1 n X 2 array.

3, 9, 31, 105, 355, 1201, 4063, 13745, 46499, 157305, 532159, 1800281, 6090307, 20603361, 69700671, 235795681, 797691075, 2698569577, 9129195487, 30883847113, 104479306403, 353450961809, 1195716038943, 4045078385041, 13684402155875

Offset: 1

Author

R. H. Hardin, Jan 17 2013

Keywords

• nonn

Comments

Column 2 of A221446.

Examples

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

Links

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

Crossrefs

Cf. A221446.

Formula

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

Empirical g.f.: x*(3 + x^2) / (1 - 3*x - x^2 - x^3). - Colin Barker, Aug 05 2018