OFFSET
1,1
LINKS
R. H. Hardin, Table of n, a(n) for n = 1..210
Robert Israel, Maple-assisted derivation of recurrence
FORMULA
Empirical: a(n) = 2*a(n-1) + 6*a(n-2) - a(n-3).
Empirical g.f.: 8*x*(21 + 47*x - 8*x^2) / (1 - 2*x - 6*x^2 + x^3). - Colin Barker, Nov 04 2018
Empirical recurrence verified: see link. - Robert Israel, May 13 2020
EXAMPLE
Some solutions for n=6:
..4....6....7....0....7....3....6....4....0....2....6....1....6....5....0....2
..2....4....6....0....2....6....3....5....7....6....0....7....6....6....7....5
..3....3....1....7....0....1....4....2....6....1....1....0....1....2....6....6
..5....6....4....4....5....6....0....6....1....1....6....1....1....1....0....1
..2....1....3....0....2....5....7....5....0....6....7....6....6....6....7....7
..4....6....4....3....4....2....2....2....7....1....0....6....7....7....2....6
..5....3....3....4....3....5....5....0....5....1....7....1....0....0....5....1
..2....4....2....3....2....4....6....5....2....6....0....2....2....6....3....7
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Aug 04 2014
STATUS
approved