On **April 27–29, 2022**, the CMSA will host a workshop on Nonlinear Algebra and Combinatorics.

**Organizers**: Bernd Sturmfels (MPI Leipzig) and Lauren Williams (Harvard).

In recent years, ideas from integrable systems and scattering amplitudes have led to advances in nonlinear algebra and combinatorics. In this short workshop, aimed at younger participants in the field, we will explore some of the interactions between the above topics.

The workshop will be held in room G10 of the CMSA, located at 20 Garden Street, Cambridge, MA. For a list of lodging options convenient to the Center, please visit our recommended lodgings page.

All non-Harvard affiliated visitors to the CMSA building are required to complete this covid form prior to arrival: https://forms.gle/xKykcNcXq7ciZuvJ8

**Registration is required**: ** Register Online**

Speakers:

- Federico Ardila (San Francisco State)
- Nima Arkani-Hamed (IAS)
- Madeline Brandt (Brown)
- Nick Early (Max Planck Institute)
- Chris Eur (Harvard)
- Claudia Fevola (Max Planck Institute)
- Christian Gaetz (Harvard)
- Yuji Kodama (Ohio State University)
- Yelena Mandelshtam (Berkeley)
- Sebastian Mizera (IAS)
- Matteo Parisi (Harvard CMSA)
- Emma Previato (Boston University)
- Anna Seigal (Harvard)
- Melissa Sherman-Bennett (University of Michigan)
- Simon Telen (Max Planck Institute)
- Charles Wang (Harvard)

**Wednesday, April 27, 2022**

9:30 am–10:30 am | Federico Ardila | Title: Nonlinear spaces from linear spaces Abstract: Matroid theory provides a combinatorial model for linearity, but it plays useful roles beyond linearity. In the classical setup, a linear subspace V of an n-dimensional vector space gives rise to a matroid M(V) on {1,…,n}. However, the matroid M(V) also knows about some nonlinear geometric spaces related to V. Conversely, those nonlinear spaces teach us things we didn’t know about matroids. My talk will discuss some examples. |

10:30 am–11:00 am | COFFEE BREAK | |

11:00 am–11:45 am | Chris Eur | Title: Tautological classes of matroidsAbstract: Algebraic geometry has furnished fruitful tools for studying matroids, which are combinatorial abstractions of hyperplane arrangements. We first survey some recent developments, pointing out how these developments remained partially disjoint. We then introduce certain vector bundles (K-classes) on permutohedral varieties, which we call “tautological bundles (classes)” of matroids, as a new framework that unifies, recovers, and extends these recent developments. Our framework leads to new questions that further probe the boundary between combinatorics and geometry. Joint work with Andrew Berget, Hunter Spink, and Dennis Tseng. |

11:45 am–2:00 pm | LUNCH BREAK | |

2:00 pm–2:45 pm | Nick Early | Title: Biadjoint Scalars and Associahedra from Residues of Generalized AmplitudesAbstract: The associahedron is known to encapsulate physical properties such as the notion of tree-level factorization for one of the simplest Quantum Field Theories, the biadjoint scalar, which has only cubic interactions. I will discuss novel instances of the associahedron and the biadjoint scalar in a class of generalized amplitudes, discovered by Cachazo, Early, Guevara and Mizera, by taking certain limits thereof. This connection leads to a simple proof of a new realization of the associahedron involving a Minkowski sum of certain positroid polytopes in the second hypersimplex. |

2:45 pm–3:30 pm | Anna Seigal | Title: Invariant theory for maximum likelihood estimationAbstract: I will talk about work to uncover connections between invariant theory and maximum likelihood estimation. I will describe how norm minimization over a torus orbit is equivalent to maximum likelihood estimation in log-linear models. We will see the role played by polytopes and discuss connections to scaling algorithms. Based on joint work with Carlos Améndola, Kathlén Kohn, and Philipp Reichenbach. |

3:30 pm–4:00 pm | COFFEE BREAK | |

4:00 pm–4:45 pm | Matteo Parisi | Title: Amplituhedra, Scattering Amplitudes, and TriangulationsAbstract: In this talk I will discuss about Amplituhedra – generalizations of polytopes inside the Grassmannian – introduced by physicists to encode interactions of elementary particles in certain Quantum Field Theories. In particular, I will explain how the problem of finding triangulations of Amplituhedra is connected to computing scattering amplitudes of N=4 super Yang-Mills theory.Triangulations of polygons are encoded in the associahedron, studied by Stasheff in the sixties; in the case of polytopes, triangulations are captured by secondary polytopes, constructed by Gelfand et al. in the nineties. Whereas a “secondary” geometry describing triangulations of Amplituhedra is still not known, and we pave the way for such studies. I will discuss how the combinatorics of triangulations interplays with T-duality from String Theory, in connection with the Momentum Amplituhedron. A generalization of T-duality led us to discover a striking duality between Amplituhedra of “m=2” type and a seemingly unrelated object – the Hypersimplex. The latter is a polytope which appears in many contexts, from matroid theory to tropical geometry.Based on joint works with Lauren Williams, Melissa Sherman-Bennett, Tomasz Lukowski. |

4:45 pm–5:30 pm | Melissa Sherman-Bennett | Title: The hypersimplex and the m=2 amplituhedronAbstract: In this talk, I’ll continue where Matteo left off. I’ll give some more details about the curious correspondence between the m=2 amplituhedron, a 2k-dimensional subset of Gr(k, k+2), and the hypersimplex, an (n-1)-dimensional polytope in R^n. The amplituhedron and hypersimplex are both images of the totally nonnegative Grassmannian under some map (the amplituhedron map and the moment map, respectively), but are different dimensions and live in very different ambient spaces. I’ll talk about joint work with Matteo Parisi and Lauren Williams in which we give a bijection between decompositions of the amplituhedron and decompositions of the hypersimplex (originally conjectured by Lukowski–Parisi–Williams). The hypersimplex decompositions are closely related to matroidal subdivisions. Along the way, we prove a nice description of the m=2 amplituhedron conjectured by Arkani-Hamed–Thomas–Trnka and give a new decomposition of the m=2 amplituhedron into Eulerian-number-many chambers, inspired by an analogous triangulation of the hypersimplex into Eulerian-number-many simplices. |

**Thursday, April 28, 2022**

9:30 am–10:30 am | Claudia Fevola | Title: Nonlinear Algebra meets Feynman integrals Abstract: Feynman integrals play a central role in particle physics in the theory of scattering amplitudes. They form a finite-dimensional vector space and the elements of a basis are named “master integrals” in the physics literature. The number of master integrals has been interpreted in different ways: it equals the dimension of a twisted de Rham cohomology group, the Euler characteristic of a very affine variety, and the holonomic rank of a D-module. In this talk, we are interested in a more general family of integrals that contains Feynman integrals as a special case. We explore this setting using tools coming from nonlinear algebra. This is an ongoing project with Daniele Agostini, Anna-Laura Sattelberger, and Simon Telen. |

10:30 am–11:00 am | COFFEE BREAK | |

11:00 am–11:45 am | Simon Telen | Title: Landau discriminantsAbstract: The Landau discriminant is a projective variety containing kinematic parameters for which a Feynman integral can have singularities. We present a definition and geometric properties. We discuss how to compute Landau discriminants using symbolic and numerical methods. Our methods can be used, for instance, to compute the Landau discriminant of the pentabox diagram, which is a degree 12 hypersurface in 6-space. This is joint work with Sebastian Mizera. |

11:45 am–2:00 pm | LUNCH BREAK | |

2:00 pm–2:45 pm | Christian Gaetz | Title: 1-skeleton posets of Bruhat interval polytopesAbstract: Bruhat interval polytopes are a well-studied class of generalized permutohedra which arise as moment map images of various toric varieties and totally positive spaces in the flag variety. I will describe work in progress in which I study the 1-skeleta of these polytopes, viewed as posets interpolating between weak order and Bruhat order. In many cases these posets are lattices and the polytopes, despite not being simple, have interesting h-vectors. In a special case, work of Williams shows that Bruhat interval polytopes are isomorphic to bridge polytopes, so that chains in the 1-skeleton poset correspond to BCFW-bridge decompositions of plabic graphs. |

2:45 pm–3:30 pm | Madeleine Brandt | Title: Top Weight Cohomology of $A_g$Abstract: I will discuss a recent project in computing the top weight cohomology of the moduli space $A_g$ of principally polarized abelian varieties of dimension $g$ for small values of $g$. This piece of the cohomology is controlled by the combinatorics of the boundary strata of a compactification of $A_g$. Thus, it can be computed combinatorially. This is joint work with Juliette Bruce, Melody Chan, Margarida Melo, Gwyneth Moreland, and Corey Wolfe. |

3:30 pm–4:00 pm | COFFEE BREAK | |

4:00 pm–5:00 pm | Emma Previato | Title: Sigma function on curves with non-symmetric semigroupAbstract: We start with an overview of the correspondence between spectral curves and commutative rings of differential operators, integrable hierarchies of non-linear PDEs and Jacobian vector fields. The coefficients of the operators can be written explicitly in terms of the Kleinian sigma function: Weierstrass’ sigma function was generalized to genus greater than one by Klein, and is a ubiquitous tool in integrability. The most accessible case is the sigma function of telescopic curves. In joint work with J. Komeda and S. Matsutani, we construct a curve with non-symmetric Weierstrass semigroup (equivalently, Young tableau), consequently non-telescopic, and its sigma function. We conclude with possible applications to commutative rings of differential operators. |

6:00 pm | Dinner Banquet, Gran Gusto Trattoria |

**Friday, April 29, 2022**

9:00 am–10:00 am | Yuji Kodama | Title: KP solitons and algebraic curvesAbstract: It is well-known that soliton solutions of the KdV hierarchy are obtained by singular limits of hyper-elliptic curves. However, there is no general results for soliton solutions of the KP hierarchy, KP solitons. In this talk, I will show that some of the KP solitons are related to the singular space curves associated with certain class of numerical semigroups. |

10:00 am–10:30 am | COFFEE BREAK | |

10:30 am–11:15 am | Yelena Mandelshtam | Title: Curves, degenerations, and Hirota varietiesAbstract: The Kadomtsev-Petviashvili (KP) equation is a differential equation whose study yields interesting connections between integrable systems and algebraic geometry. In this talk I will discuss solutions to the KP equation whose underlying algebraic curves undergo tropical degenerations. In these cases, Riemann’s theta function becomes a finite exponential sum that is supported on a Delaunay polytope. I will introduce the Hirota variety which parametrizes all KP solutions arising from such a sum. I will then discuss a special case, studying the Hirota variety of a rational nodal curve. Of particular interest is an irreducible subvariety that is the image of a parameterization map. Proving that this is a component of the Hirota variety entails solving a weak Schottky problem for rational nodal curves. This talk is based on joint work with Daniele Agostini, Claudia Fevola, and Bernd Sturmfels. |

11:15 am–12:00 pm | Charles Wang | Title: Differential Algebra of Commuting OperatorsAbstract: In this talk, we will give an overview of the problem of finding the centralizer of a fixed differential operator in a ring of differential operators, along with connections to integrable hierarchies and soliton solutions to e.g. the KdV or KP equations. Given these interesting connections, it is important to be able to compute centralizers of differential operators, and we discuss how to use techniques from differential algebra to approach this question, as well as how having these computational tools can help in understanding the structure of soliton solutions to these equations. |

12:00 pm–2:00 pm | LUNCH BREAK | |

2:00 pm–3:00 pm | Sebastian Mizera | Title: Feynman PolytopesAbstract: I will give an introduction to a class of polytopes that recently emerged in the study of scattering amplitudes in quantum field theory. |

3:00 pm–3:30 pm | COFFEE BREAK | |

3:30 pm–4:30 pm | Nima Arkani-Hamed | Title: Spacetime, Quantum Mechanics and Combinatorial Geometries at Infinity |