Mihai Ciucu Christian Krattenthaler Enumeration of Lozenge Tilings of Hexagons with Cut-Off Corners. 201-231 2002 100 J. Comb. Theory, Ser. A 2 http://dx.doi.org/10.1006/jcta.2002.3288 db/journals/jct/jcta100.html#CiucuK02