A268635 Number of n_X_4 0..2 arrays with some element plus some horizontally or vertically adjacent neighbor totalling two exactly once.

36, 504, 5178, 47640, 412740, 3440052, 27906474, 221913216, 1737860310, 13445785116, 103012659468, 782824552488, 5908380409134, 44334271544616, 330997648937706, 2460394469263680, 18218335277707956, 134439600276305244

Offset: 1

Author

R. H. Hardin, Feb 09 2016

Keywords

• nonn

Examples

			Some solutions for n=4:
..0..0..1..0. .0..0..1..0. .2..1..2..1. .1..2..2..2. .2..1..0..1
..0..0..0..1. .2..1..0..1. .2..2..1..0. .0..2..2..1. .2..2..1..0
..1..0..1..0. .2..2..1..0. .1..1..0..0. .1..2..1..0. .1..2..1..0
..1..0..0..1. .2..1..2..1. .0..0..1..0. .2..1..2..1. .2..1..0..0
		

Links

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

Crossrefs

Column 4 of A268639.

Formula

Empirical: a(n) = 18*a(n-1) - 111*a(n-2) + 282*a(n-3) - 333*a(n-4) + 180*a(n-5) - 36*a(n-6) for n>7.

Empirical g.f.: 6*x*(6 - 24*x + 17*x^2 + 38*x^3 - 27*x^4 - 12*x^5 + 8*x^6) / (1 - 9*x + 15*x^2 - 6*x^3)^2. - Colin Barker, Jan 14 2019