OFFSET
1,1
LINKS
R. H. Hardin, Table of n, a(n) for n = 1..210
N. H. Bong, C. Dalfó, and M. À. Fiol, and D. Závacká, Some inner metric parameters of a digraph: Iterated line digraphs and integer sequences, arXiv:2409.02125 [math.CO], 2024. See p. 19.
FORMULA
Empirical: a(n) = 2*a(n-1) + 2*a(n-2).
Conjectures from Colin Barker, Oct 25 2018: (Start)
G.f.: 6*x / (1 - 2*x - 2*x^2).
a(n) = sqrt(3)*((1+sqrt(3))^n-(1-sqrt(3))^n).
(End)
EXAMPLE
Some solutions for n=5:
..2..2....0..0....2..2....1..1....1..1....2..2....1..1....2..2....0..0....2..2
..0..0....2..2....0..0....2..2....0..0....0..0....0..0....1..1....2..2....0..0
..1..1....1..1....1..1....0..0....2..2....2..2....2..2....0..0....2..2....1..1
..0..0....1..1....1..1....2..2....1..1....0..0....0..0....2..2....1..1....1..1
..2..2....0..0....2..2....0..0....2..2....2..2....0..0....1..1....0..0....0..0
..1..1....2..2....0..0....1..1....0..0....0..0....2..2....0..0....1..1....1..1
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Mar 11 2014
STATUS
approved