OFFSET
1,1
LINKS
R. H. Hardin, Table of n, a(n) for n = 1..210
FORMULA
Empirical: a(n) = 4*a(n-1) - 2*a(n-2) - 4*a(n-3) + a(n-4).
Empirical g.f.: x*(3 - 3*x - 3*x^2 + x^3) / (1 - 4*x + 2*x^2 + 4*x^3 - x^4). - Colin Barker, Jan 24 2019
EXAMPLE
Some solutions for n=6:
..2. .2. .0. .0. .1. .0. .1. .1. .2. .2. .2. .2. .1. .1. .2. .0
..2. .1. .0. .0. .0. .0. .2. .1. .0. .1. .0. .1. .1. .0. .1. .1
..0. .2. .2. .1. .2. .1. .1. .1. .2. .1. .2. .1. .1. .0. .0. .0
..1. .0. .0. .1. .1. .2. .2. .0. .1. .1. .1. .1. .2. .1. .0. .0
..1. .2. .0. .0. .2. .1. .0. .1. .0. .0. .2. .2. .1. .0. .0. .0
..0. .2. .2. .0. .0. .1. .2. .0. .2. .0. .2. .2. .0. .2. .2. .2
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Mar 01 2016
STATUS
approved