Mihai Ciucu Theresia Eisenkölbl Christian Krattenthaler D. Zare Enumeration of Lozenge Tilings of Hexagons with a Central Triangular Hole. 251-334 2001 95 J. Comb. Theory, Ser. A 2 http://dx.doi.org/10.1006/jcta.2000.3165 db/journals/jct/jcta95.html#CiucuEKZ01