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Root Polytopes, Parking Functions, and the HOMFLY Polynomial

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Root Polytopes, Parking Functions, and the HOMFLY Polynomial Quantum Topol. x (201x), xxx{xxx Quantum Topology c European Mathematical Society Root polytopes, parking functions, and the HOMFLY polynomial Tam´asK´alm´anand Hitoshi Murakami Abstract. We show that for a special alternating link diagram, the following three poly- nomials are essentially the same: a) the part of the HOMFLY polynomial that corresponds to the leading term in the Alexander polynomial; b) the h-vector for a triangulation of the root polytope of the Seifert graph and c) the enumerator of parking functions for the planar dual of the Seifert graph. These observations yield formulas for the maximal z-degree part of the HOMFLY polynomial of an arbitrary homogeneous link as well. Our result is part of a program aimed at reading HOMFLY coefficients out of Floer homology. Mathematics Subject Classification (2010). 57M27, 57M25, 57M50. Keywords. Alternating link, HOMFLY polynomial, Seifert graph, root polytope, h- vector, parking function. 1. Introduction In this paper we report on a new kind of combinatorial phenomenon in knot theory. Our research was motivated by a desire to understand what the HOMFLY poly- nomial of an oriented link `measures,' i.e., to find a natural (for example, diagram- independent) definition for it. While that problem remains wide open (with the most promising approach being the Gopakumar{Ooguri{Vafa conjecture [3], [13]), we feel we did carry out an interesting case study with some surprising results. The HOMFLY polynomial P (v; z) [6] is an invariant of oriented links that specializes to the Conway polynomial r(z) via the substitution r(z) = P (1; z). The latter is equivalent to the Alexander polynomial ∆(t) through ∆(t) = r(t1=2 − t−1=2). Note that r and ∆ share the same leading coefficient. In P , on the other hand, one finds several terms that contribute to the leading monomial of r when we set v = 1. We will collectively refer to these as the top of the HOMFLY polynomial. For homogeneous links [2] (which include all alternating and positive 2 Tam´asK´alm´anand Hitoshi Murakami links), the top can also be described1 as the sum of those terms that realize the z-degree of P . A third, perhaps most useful description first associates the coefficient T (v) of zn−s+1 in P to an arbitrary link diagram, where n is the number of crossings and s is the number of Seifert circles, respectively. (Here we suppress the common z- power and write only the v-powers.) In general, T (v) may be the empty sum, i.e., 0 (cf. Theorem 3.2), but not for homogeneous diagrams. For those, T (v) provides an equivalent definition of the top of the HOMFLY polynomial of the link. (In particular, n−s is an invariant of the homogeneous link [2].) We recall that Seifert circles are the simple closed curves that result when we smooth every crossing of a link diagram in the orientation-preserving way. Seifert circles are the vertices of the Seifert graph, in which there is an edge between two of them for every crossing where they meet. A Seifert graph is always bipartite, which is the reason why it can be used in the standard construction of an oriented spanning surface for a link. Let us briefly recall some more terminology. A block sum of two connected graphs is a one-point union and under this, each connected graph has a unique decomposition into irreducible (a.k.a. 2-connected) ones, called its blocks. The median construction on a plane graph2 gives a link (diagram) as the boundary of a surface, which in turn is obtained by taking a small neighborhood of the embedding and twisting it (using one of two choices) once over each edge. A priori the link is unoriented. A special alternating link is an oriented link that has an alternating diagram obtained by first applying the median construction to a connected plane bipartite graph, and then orienting the link by letting the two color classes spin in opposite directions (cf. [7, Figure 3]). See Figure 2 for an example. Note that alternatingness and connectedness imply that one choice of twisting direction determines all others. Furthermore, the orientation is such that all crossings share the same sign, i.e., special alternating links are either positive or negative. (Here we use the standard convention of being a positive crossing and a negative one.) Moreover, the bipartite graph is recovered as the Seifert graph of the diagram. A special alternating link is a Murasugi atom if the graph in the description above is 2-connected. An oriented link is called homogeneous if it has a diagram D that decomposes as a star product (a.k.a. Murasugi sum) of Murasugi atoms [2]. On the level of Seifert graphs, a star product is a block sum. When constructing the star product, first we draw the circle C that corresponds to the merging vertices and then reproduce the two diagrams either on the same side of C (in which case the result is a connected sum) or on opposite sides (when we may or may not get a connected sum). Note the additional choices that are involved with the latter case. Figure 1 shows two of several possible star products of the same factors. When we decompose special alternating links, all star products follow the first 1This second definition is probably the most appropriate for the top of P of an arbitrary link. We will remain vague on this because in this paper we only consider homogeneous links. 2A plane graph is an isotopy class of embeddings of a graph into the plane R2 or the sphere S2. A planar graph is an abstract graph that admits such an embedding. Root polytopes, parking functions, and the HOMFLY polynomial 3 Figure 1. Two alternating links with the same Seifert graph. The signs mean the signs of the crossings in the Murasugi atoms. (`same side') pattern. This is because in the other pattern, being alternating forces opposite signs for crossings on opposing sides of C. I.e., special alternating links are connected sums of Murasugi atoms of uniform sign. (For example, the granny knot is the connected sum of two positive trefoils). Let D be a homogeneous link diagram. The aim of this paper is to describe the top of the associated HOMFLY polynomial PD in terms of the Seifert graph of D. A theorem of Murasugi and Przytycki [12] says that the top of the HOMFLY polynomial (more precisely, the polynomial T (v) of our third description above) behaves multiplicatively under star product. Thus it suffices for us to describe the top of P for Murasugi atoms; we will in fact do so for special alternating links. As ∗ −1 for any link L and its mirror image L we have PL∗ (v; z) = PL(−v ; z), we may, without loss of generality, concentrate on positive special alternating links only. r0 κ∗ κ Figure 2. A plane bipartite graph G, its planar dual G∗, and associated special alternating link LG. The last panel shows the three objects together. Hence, much of our discussion focuses on a connected plane bipartite graph G which gives rise (via the median construction) to the positive special alternating link LG. Alexander Postnikov [14] has recently developed a beautiful theory of (not necessarily planar) bipartite graphs. The gist of this paper is the realization that some of his ideas are closely related to knot theory. Postnikov associates a root polytope QG to any bipartite graph G, constructed 4 Tam´asK´alm´anand Hitoshi Murakami as follows. Denote the color classes of G by E and3 V and take the convex hull, in RE ⊕ RV , of the vectors e + v for all edges ev of G. Here e 2 RE and v 2 RV are the standard generators associated to e 2 E and v 2 V , respectively. If G is connected then the result is an (jEj + jV j − 2)-dimensional polytope so that, of course, edges of G translate to vertices of QG. It is also not hard to show that a set of vertices of QG is affinely independent if and only if the corresponding edges form a cycle-free subgraph of G. In particular, there is a one-to-one correspondence between maximal simplices formed by vertices of QG and spanning trees of G. A triangulation of QG is a collection of maximal simplices so that their union is the entire root polytope and any two of them intersect in a common face. (Note how it is not allowed to introduce new vertices when we triangulate a polytope.) Our first result describes a specific triangulation of QG in the case when G is a plane graph. Let us orient the dual graph G∗ so that each of its edges has an element of E to the right and an element of V to the left. After fixing a root r0 (a ∗ vertex of G ) arbitrarily, we consider spanning arborescences rooted at r0. These ∗ are those spanning trees of G in which each edge points away from r0. Each spanning arborescence has a dual spanning tree in G and we claim the following. Theorem 1.1. Let G be a connected plane bipartite graph. Fix a root r0 and ∗ consider all spanning arborescences of G rooted at r0, as well as the spanning trees of G dual to them. Then, the collection of those simplices in the root polytope QG that correspond to the latter forms a triangulation of QG. A concrete example of this phenomenon is shown in Figure 5.
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