I recently gave three lectures at Yale University for the Hahn Lectures in Mathematics. The unifying theme of my talks was the notion of break divisor, a fascinating combinatorial concept related to the Riemann-Roch theorem for graphs. Some applications of break divisors to algebraic geometry will be discussed in a follow-up post.
Break divisors on graphs
Let be a connected finite graph of genus , where . Our central object of study will be the notion of a break divisor on . Recall that a divisor on a graph is an assignment of an integer to each vertex of . The divisor is called effective if for all ; in this case, we often visualize by placing “chips” at . And the degree of is the sum of over all vertices , i.e., the total number of chips. By analogy with algebraic geometry, we write divisors on as formal sums , i.e., as elements of the free abelian group on .
A break divisor on is an effective divisor of degree such that for every connected subgraph of , the degree of restricted to is at least . In other words, there are total chips and each connected subgraph contains at least genus-of- of these chips. One surprising fact, proved in this paper (referred to henceforth as [ABKS]), is that the number of break divisors on is equal to the number of spanning trees of . Continue reading
I plan to write several posts related to the Riemann-Roch Theorem for Graphs, which was published several years ago in this paper written jointly with Serguei Norine. In this post I want to explain the statement of the theorem, give some anecdotal background, and mention a few applications which have been discovered in recent years.
The Riemann-Roch Theorem
The (classical) Riemann-Roch Theorem is a very useful result about analytic functions on compact one-dimensional complex manifolds (also known as Riemann surfaces). Given a set of constraints on the orders of zeros and poles, the Riemann-Roch Theorem computes the dimension of the space of analytic functions satisfying those constraints. More precisely, if denotes the set of constraints and is the dimension of the space of analytic functions satisfying those constraints, then the Riemann-Roch theorem asserts that
where is the genus (“number of holes”) of the Riemann surface , is the total number of constraints, and is the “canonical divisor” on . See the Wikipedia page for much more information.
Before formulating the combinatorial analogue of this result which Norine and I discovered, I want to briefly reminisce about how this result came about. In the summer of 2006, my Georgia Tech REU (Research Experience for Undergraduates) student Dragos Ilas worked on a graph-theoretic conjecture which I had made some time earlier. Dragos spent eight weeks working on the problem and compiled a lot of experimental evidence toward my conjecture. He gave a talk about the problem one Friday toward the end of the summer in an REU Mini-Conference that I was organizing at Georgia Tech. Serguei Norine (then a postdoc working with my colleague Robin Thomas) was in the audience. On Monday morning, Serguei knocked on my office door and showed me an extremely clever proof of my conjecture. I told Serguei about my real goal, which was to prove a graph-theoretic analogue of the Riemann-Roch theorem. I outlined what I had in mind and within a week, we had exactly the kind of Riemann-Roch formula that I had hoped for… thanks in large part to Serguei’s amazing combinatorial mind! Continue reading
My new Georgia Tech colleague Joe Rabinoff and I recently posted this paper to the arXiv, entitled The skeleton of the Jacobian, the Jacobian of the skeleton, and lifting meromorphic functions from tropical to algebraic curves. In this post I will attempt to give some context and background for that paper.
Suppose is an algebraic variety over a complete non-Archimedean field . For simplicity of exposition, I will assume throughout that is algebraically closed and that the value group is nontrivial. The set of points possesses a natural (Hausdorff) analytic topology, but is totally disconnected and not even locally compact in this topology. This is bad for all sorts of reasons. Over the years there have been many attempts at “fixing” this problem, beginning with the Tate-Grothendieck theory of rigid analytic spaces. One of the most successful and elegant solutions to the problem is Berkovich’s theory of non-Archimedean analytic spaces. One can associate to , in a natural and functorial way, an analytic space which contains as a dense subspace but has much nicer topological properties. For example, is a locally compact Hausdorff space which is locally contractible, and is arcwise connected if is connected. As in the theory of complex analytic spaces, is compact if and only if is proper. For much more information about Berkovich analytic spaces, see for example these course notes of mine.
One of the nice features of Berkovich’s theory (established in full generality only very recently, by Hrushovski and Loeser) is that the space always admits a deformation retraction onto a finite polyhedral complex . In general, there is no canonical choice for . However, in certain special cases there is, e.g. when is a curve of positive genus or is an abelian variety. (More generally, the existence of canonical skeleta is closely tied in with the Minimal Model Program in birational algebraic geometry, see for example this recent paper of Mustata and Nicaise.) For the rest of this post, I will focus on the special cases of curves and abelian varieties. Continue reading