OFFSET
0,2
COMMENTS
For each n we define an undirected labeled graph (with self loops), where the vertices are labeled with strings from {0,1}^n and there is an edge between two vertices exactly when we can form a 2 X n rectangle whose rows are the two labels and the 2 X n rectangle has no monochromatic 2 X 2 subsquares. a(n) is the number of walks of length n in this graph. Thus it is the sum of all of the entries of A^n, where A is the adjacency matrix of the graph.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..15
EXAMPLE
a(2) = 14 because 2 of the 16 unrestricted colorings are monochromatic.
CROSSREFS
KEYWORD
nonn
AUTHOR
Victor S. Miller, Sep 19 2007
EXTENSIONS
a(0)-a(1), a(11)-a(13) from Alois P. Heinz, Feb 18 2015
STATUS
approved