The Compact Tropical Grassmannian Daniel Corey Yale University Email: [email protected] The Tropical Grassmannian The following is joint work with Maria Angelica Cueto. Our main objects of study are initial degenerations of the Grassmannian Gr(k, n) of k planes in affine n-dimensional space, viewed as a projective variety via the Pl¨ucker embedding. Grassmannians are one of the first higher dimensional varieties studied from the viewpoint of tropical geometry. In particular, Speyer and Sturmfels investigated the tropicalization T Gr0(2, n) of the open dense subset Gr0(2, n) of Gr(2, n) defined by the nonvanishing of all Pl¨ucker coordinates. They identify T Gr0(2, n) with the space of phylogenetic trees on n-leaves. This description of T Gr0(2, n) was used to deduce faithful tropicalization of the compact Grassmannian of 2-planes in the following theorem: Theorem 1 (Cueto, H¨abich, Werner [1]). There exists a continuous section to the tropicalization map trop : Gr(2, n)an → T Gr(2, n). That is to say, the tropicalization of Gr(2,n) with respect to the Pl¨ucker embedding is faithful.

Theorem 2 (C, Cueto). For each w ∈ T Gr0(2, n), the coordinate ring of the initial degeneration inw Gr(2, n)aff is generated by a cluster of Pl¨ucker coordinate. Moreover, every cluster mutation equivalent to this initial cluster also forms a generating set. As a consequence of this result, we obtain a new proof of Theorem 1. As an illustration of how to construct this section map σ, let us see how to define σ(w) at a point w in the relative interior of a facet of T Gr0(2, n). We need to define a seminorm σ(w) on K[Gr0(2, n)aff]. To do this, take a planar embedding of the phylogenetic tree representing w and a triangulated n-gon dual to this tree. This triangulation will give us a cluster that generates the coordinate ring of inw Gr0(2, n)aff. We use this basis of Pl¨ucker coordinates to construct the seminorm σ(w). Then we show that this is a well-defined section map by expressing the remaining Pl¨ucker coordinates in terms of this initial cluster by mutations. This procedure is illustrated in Figure 2 2

2 1

Cluster algebras were introduce by Fomin and Zelevinsky in the early 2000’s in [2]. Roughly speaking, a cluster algebra is an integral domain together with distinguished transcendence bases for its fraction field. These bases, called clusters overlap in such a way that one can get from one cluster to another by swapping out one coordinate at a time through a process called mutation. More precisely, given a cluster x and x ∈ x, an adjacent cluster is obtained from replacing x with x0 where xx0 = M1 + M2 for relatively prime monomials M1 and M2 in the cluster variables in x \ {x}. In [2] Fomin and Zelevinsky showed that K[Gr(2, n)], the homogeneous coordinate ring of Gr(2, n), has a cluster algebra structure which can be described by triangulations of the n-gon. The cluster associated to a triangulation is given by the Pl¨ucker coordinates corresponding to the edges in the triangulation. Mutation is given by flipping the diagonal in a quadrilateral. Examples of clusters and a mutation for Gr(2, 5) are given in Figure 1. 2 2 3 3 p13p24 = p12p34 + p14p23 1 1 4 4 5 5 (p12, p23, p34, p45, p15, p14, p13)

1 5

(p12, p23, p34, p45, p15, p14, p24)

4

4

n where R = R ∪ {−∞} is the extended real line, and N = k −1. Points in T Gr(k, n) with coor-

dinates equal to −∞ correspond to vanishing of Pl¨ucker coordinates, so we are naturally lead to consider the matroid stratification of Gr(k, n). The strata are parameterized by rank k realizable matroids on [n]. If F is such a matroid and B(F ) is its set of bases, then the associated stratum ΓF is ΓF = {(pI )|pI = 0 iff I ∈ / B(F )}. In general, this stratification is not well behaved. In [7] Sturmfels produced an example of two rank 3 realizable matroids F and G on [7] with B(F ) ⊂ B(G), but ΓF ∩ ΓG = ∅. However, this does not happen in the Grassmannians Gr(2, n) and Gr(3, 6). Theorem 4 (C, Cueto). Let k = 2 or (k, n) = (3, 6). If F and G are realizable rank k matroids on [n] with B(F ) ⊂ B(G), then ΓF ⊂ ΓG.

5

Figure 2: Use the cluster (p12, p13, p14, p15, p23, p34, p45) to define the seminorm in Gr(2, 5)an.

Grassmannians with k ≥ 3 By work of Scott in [5], Grassmannians Gr(k, n) for 3 ≤ k ≤ n2 also admit cluster structures, but their combinatorics is substantially more complicated than the k = 2 case. For example, the only Grassmannians with finite type cluster algebra structures are Gr(2, n), Gr(3, 6), Gr(3, 7), and Gr(3, 8). Their corresponding root systems are An−3, D4, E6, and E8. For Gr(2, n), all cluster variables were Pl¨ucker coordinates, but this is no longer the case for higher Grassmannians, even the remaining finite type ones. For example, Gr(3, 6) has twenty-two cluster variables: the twenty Pl¨ucker coordinates together with two quadratic binomials in Pl¨ucker coordinates. Speyer and Sturmfels [6] give a classification of the facets of T Gr0(3, 6) into seven S6 symmetry classes. Using this description of T Gr0(3, 6) and the software package Macaulay2 we arrive at the analog of Theorem 2.

Thus for these Grassmannians we will like to investigate how the tropicalizations of the matroid strata intersect. For other Grassmannians, we wish to consider alternate stratifications that behave better. Recently, Knutson, Lam, and Speyer [4] have investigated the stratification of Gr(k, n) by open positroid varieties. This stratification lives between the Bruhat and matroid stratifications, but still retains many of the nice geometric properties of the former. With this in mind, we will like to investigate the tropicalizations of the positroid varieties and how they meet each other in the boundary of the tropical Grassmannian. All of our computations done using using Gfan, Macaulay2, Python, and Sage.

References [1] M. A. Cueto, M. H¨abich, and A. Werner. Faithful tropicalization of the Grassmannian of planes. Math. Ann., 360(1-2):391–437, 2014. [2] S. Fomin and A. Zelevinsky. Cluster algebras i: Foundations. J. Amer. Math. Soc., 15(2):497–529, 2002. [3] S. Fomin and A. Zelevinsky. Cluster algebras ii: Finite type classification. Invent. Math., 154(1):63–121, 2003. [4] A. Knutson, T. Lam, and D. E. Speyer. Positroid varieties: juggling and geometry. Compos. Math., 149(10):1710–1752, 2013. [5] J. Scott. Grassmannians and cluster algebras. Proc. London Math Soc., 92(2):345–380, 2006.

Theorem 3 (C, Cueto). If w ∈ T Gr0(3, 6) is not in the relative interior of a cone in the EEEE symmetry class as in [6], then K[inw Gr0(3, 6)af f ] is generated by a cluster. Moreover, if w is in the relative interior of a EEEE cone, then no cluster generates K[inw Gr0(3, 6)af f ].

Figure 1: An illustration of the cluster algebra structure on C[Gr(2, 5)]

A cluster algebra is said to be of finite type if there are only finitely many clusters. Fomin and Zelevinsky [3] give a classification of finite type cluster algebras in terms of root systems of simply-laced Dynkin diagrams. In particular, the homogeneous coordinate ring of Gr(2, n) admits a finite type cluster algebra structure corresponding to the root system An−3. Using this cluster algebra structure, we arrive at the following theorem.

T PN = (R \ (−∞, . . . , −∞))N +1/(1, . . . , 1) · R

3

3

Cluster Algebras

the tropicalization of this projective variety lives in the tropical projective space T PN :

[6] D. Speyer and B. Sturmfels. The tropical Grassmannian. Adv. Geom., 4:389–411, 2004. [7] B. Sturmfels. On the matroid stratification of Grassmann varieties, specialization of coordinates, and a problem of N. White. Adv. Math., 75(2):202–211, 1989.

Acknowledgements The Boundary of the Grassmannian We would like to extend these results to the boundary of Gr(k, n) inside of projective space, i.e. the part of the Grassmannian defined by the various vanishings of Pl¨ucker coordinates. Note that

This project started at the Graduate Student Bootcamp for the Summer Research Institute in Algebraic Geometry (Salt Lake City, Utah 2015). We want to thank the organizers of that event. We wish to thank Alex Fink, Rachel Karpman, Sam Payne, Felipe Rincon, Vivek Shende, David Speyer, Bernd Sturmfels, Daping Weng, and Lauren Williams for their fruitful conversations. Finally, we want to thank the Fields Institute for Research in Mathematical Sciences and the organizers of the Major Thematic Program on Combinatorial Algebraic Geometry (July-December 2016) for hosting us while we complete this work.

Daniel Corey Yale University

/(1,...,1) · R where R = R∪ {−∞} is the extended real line, and N = ... All of our computations done using using Gfan, Macaulay2, Python, and Sage. References.

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