OFFSET
1,2
LINKS
R. H. Hardin, Table of n, a(n) for n = 1..210
FORMULA
Empirical: a(n) = 21*a(n-1) - 108*a(n-2) + 140*a(n-3).
Conjectures from Colin Barker, Jan 11 2019: (Start)
G.f.: x*(1 - 13*x + 26*x^2) / ((1 - 2*x)*(1 - 5*x)*(1 - 14*x)).
a(n) = (105*2^n + 196*5^n + 25*2^(1+n)*7^n) / 1890.
(End)
EXAMPLE
Some solutions for n=4:
..0..0..0..0....0..0..0..0....0..0..0..0....0..0..0..0....0..0..0..0
..0..1..0..1....0..1..1..0....0..0..1..1....0..1..1..1....0..1..0..1
..0..0..1..1....0..1..2..1....0..0..0..0....0..1..1..2....0..0..1..2
..0..1..0..0....0..1..0..1....0..1..1..0....0..1..0..1....0..1..1..1
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Jan 25 2016
STATUS
approved