OFFSET
1,3
COMMENTS
Row 3 of A220632.
LINKS
R. H. Hardin, Table of n, a(n) for n = 1..210
FORMULA
Empirical: a(n) = 4*a(n-1) - a(n-2) - 2*a(n-3) for n>5.
Empirical g.f.: x*(1 - x - x^2)*(1 - 2*x - x^2) / ((1 - x)*(1 - 3*x - 2*x^2)). - Colin Barker, Aug 02 2018
EXAMPLE
All solutions for n=3 0=self 3=ne 4=w 6=e 7=sw (reciprocal directions total 10):
.00.00.00...00.67.47...00.00.00
.00.67.47...36.34.00...00.00.00
.36.34.00...00.00.00...00.00.00
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Dec 17 2012
STATUS
approved