Richard A. Brualdi Stephen J. Kirkland Aztec diamonds and digraphs, and Hankel determinants of Schröder numbers. 334-351 2005 94 J. Comb. Theory, Ser. B 2 http://dx.doi.org/10.1016/j.jctb.2005.02.001 db/journals/jct/jctb94.html#BrualdiK05