OFFSET
1,1
LINKS
R. H. Hardin, Table of n, a(n) for n = 1..210
FORMULA
Empirical: a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4).
Conjectures from Colin Barker, Aug 14 2018: (Start)
G.f.: x*(6 - 12*x + 10*x^2 - 3*x^3) / (1 - x)^4.
a(n) = 2 + (1 + n)*(2 + n)*(3 + n) / 6.
(End)
EXAMPLE
Some solutions for n=3:
..1..3..2....3..1..2....2..3..1....2..1..3....2..3..1....3..1..2....1..2..3
..1..3..2....3..1..2....2..3..1....2..1..3....1..3..2....2..3..1....1..2..3
..1..3..2....3..1..2....2..1..3....2..1..3....1..3..2....1..3..2....1..2..3
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Feb 03 2013
STATUS
approved