Nonintersecting Paths, Pfaffians, and Plane Partitions

Gessel and Viennot have developed a powerful technique for enumerating various classes of plane partitions [GVl, 21. There are two fundamental ideas behind this technique. The first is the observation that most classes of plane partitions that are of interest--either by association with the representation theory of the classical groups, or for purely combinatorial reasons+an be interpreted as configurations of nonintersecting paths in a digraph (usually the lattice Z’). The second is the observation that the number of r-tuples of nonintersecting paths between two sets of r vertices can (often) be expressed as a determinant. The purpose of this article is to show by similar means that one may use pfaflians to enumerate configurations of nonintersecting paths in which the initial and/or terminal vertices of the paths are allowed to vary over specified regions of the digraph. This leads to the possibility of enumerating classes of plane partitions in which the shape is allowed to vary, whereas the previous applications of Gessel and Viennot were largely confined to plane partitions of a given shape. We have made no attempt to catalogue all possible classes of plane partitions that one could enumerate by these techniques; rather, we have confined ourselves to providing new, simple, unified proofs of a diverse collection of known results, including identities of Gansner, Jozetiak and Pragacz, Gordon, Gordon and Houten, Goulden, Lascoux and Pragacz, and Okada. In one instance, we give a new result; namely, a pfaffian for the number of totally symmetric, self-complementary plane partitions. It seems likely that the number of plane partitions belonging to the other symmetry classes for which there are only conjectured formulas (see [St3]) could also be expressed as pfafhans. We will not pursue this further here, except to note that Okada has already done this for the totally symmetric case [O]. A more detailed summary follows.