OFFSET
1,1
COMMENTS
Column 2 of A220553.
LINKS
R. H. Hardin, Table of n, a(n) for n = 1..210
FORMULA
Empirical: a(n) = 3*a(n-1) + 3*a(n-2) - a(n-3) - a(n-4).
Empirical g.f.: x*(1 + x)*(2 - x^2) / (1 - 3*x - 3*x^2 + x^3 + x^4). - Colin Barker, Mar 13 2018
Empirical: a(n) = A000078(2*n + 3). - Greg Dresden, Jan 10 2021
EXAMPLE
Some solutions for n=3 0=self 2=n 3=ne 4=w 6=e 7=sw 8=s (reciprocal directions total 10):
..8..8....0..7....6..4....6..4....8..0....0..0....0..0....0..0....0..8....6..4
..2..2....3..8....0..0....6..4....2..8....8..8....0..8....0..0....8..2....8..8
..6..4....0..2....0..0....0..0....0..2....2..2....0..2....0..0....2..0....2..2
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Dec 15 2012
STATUS
approved