OFFSET
1,3
COMMENTS
Row 4 of A220708.
LINKS
R. H. Hardin, Table of n, a(n) for n = 1..210
FORMULA
Empirical: a(n) = a(n-1) + 6*a(n-2) - 6*a(n-4) - a(n-5) + a(n-6) for n>7.
Empirical g.f.: x*(1 - 2*x^2 + 2*x^3 - 3*x^5 + x^6) / ((1 - x)*(1 + x)*(1 - x - 5*x^2 - x^3 + x^4)). - 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...00.00.00...00.67.47...00.67.47...00.00.00
.00.67.47...00.00.00...36.34.00...36.34.00...00.00.00
.36.34.00...00.00.00...00.00.00...00.67.47...00.67.47
.00.00.00...00.00.00...00.00.00...36.34.00...36.34.00
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Dec 18 2012
STATUS
approved