# Schedule of the Special Seminar "Universality of moduli spaces and geometry"

## Monday, November 4

 10:30 - 11:00 Coffee break 11:00 - 11:05 Opening of seminar 11:05 - 12:05 B. Sturmfels: Universality of Nash Equilibri 14:00 - 15:00 M. Kapovich: Introduction to geometric universality 15:00 - 16:00 N. Mnev: Unpublished and unwritten universality theorems 16:00 - 16:30 Tea and cake

## Tuesday, November 5

 10:30 - 11:00 Coffee break 11:00 - 12:00 M. Kapovich: Universality for character schemes for 3-manifold groups. 12:05 - 13:05 N. Mnev: Positive side of universality theorems 14:30 - 15:30 D. Grigoriev: Universal stratifications 16:00 - 16:30 Tea and cake

## Wednesday, November 6

 10:30 - 11:00 Coffee break 11:00 - 12:00 G. Panina: Moduli space of planar polygonal linkage: a combinatorial description 12:05 - 13:05 U. Brehm: A Universality Theorem for Realization Spaces of Polyhedral Maps 15:00 - 15:30 A. Padrol: Faces of projectively unique polytopes (and a conjecture of Shephard) 15:30 - 16:00 K. Adiprasito: Projectively unique polytopes 16:00 - 16:30 Tea and cake

## Thursday, November 7

 10:30 - 11:00 Coffee break 11:00 - 12:30 Yu. Manin: Kolmogorov complexity: fractality and probability contexts 14:00 - 15:00 K.-T. Sturm: Geometric Analysis on the Space of Metric Measure Spaces 15:30 - 16:00 Tea and cake 16:00 - 17:00 I. Streinu: Configuration spaces of Lang's origami Universal Molecules

## Friday, November 8

 10:00 - 11:00 Khimshiashvili: Configurations of points with quadratic constraints 11:00 - 11:30 Coffee break 11:30 - 12:00 L. Makar-Limanov: A free associative algebra and an algebraically closed skew ﬁeld 12:05 - 12:35 L. Theran: An algebraic-combinatorial viewpoint on low-rank matrix completion 14:00 - 15:00 G. Koshevoi: Affine base spaces and fans in the cone of semi-standard Young tableaux 16:00 - 16:30 Tea and cake

# Abstracts

(Underlined titles can be clicked for the video recording)

#### K. Adiprasito: Projectively unique polytopes

The polytopes with the smallest possible realization space are the projectively unique polytopes; they are, by definition, the polytopes for which the projective linear group acts transitively on the realization space.
We present two new methods for the construction of polytopes, that in particular lead to the resolution of old conjectures concerning projectively unique polytopes:

• A construction based on solving Cauchy problems for discrete differential equations, and
• A construction that extends on the important Lawrence lifting technique.
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#### U. Brehm: A Universality Theorem for Realization Spaces of Polyhedral Maps

We give an outline of the proof of the following universality theorem for realization spaces of polyhedral maps. Let $P\subseteq\mathbb{R}^{3n}$ be a semialgebraic set (real algebraic coefficients). Then there exists a map $M$ with $5+n+k$ vertices (on some orientable manifold) which contains only triangles and  quadrangles such that the image of the realization space of $M$ under the canonical projection $p:\mathbb{R}^{3(n+k)}\rightarrow \mathbb{R}^{3n}$ is equal to $P\backslash\Delta$, where $\Delta=\{(x_1,\dots,x_n)\in\mathbb{R}^{3n}|x_i=x_j \mbox{ for some }i\neq j\}$. As a corollary we get that for each strict subfield $L$ of the field of real algebraic numbers there is a map $M$ which can be realized as a polyhedron in $\mathbb{R}^3$ but not in $L^3$.

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#### D. Grigoriev (MPIM): Universal stratifications

A stratification of the set of critical points of a map is universal in the class of stratifications satisfying the classical Thom and Whitney-a conditions if it is the coarsest among all such stratifications. We show that a universal stratification exists if and only if the 'canonical subbundle' of the cotangent bundle of the source of the map (constructed via operations introduced by Glaeser) is Lagrangian. The proof relies on a new Bertini-type theorem for singular varieties proved via an intriguing use of resolution of singularities. Many examples are provided, including those of maps without universal stratifications. (joint work with P. Milman)

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#### M. Kapovich (UCDavis): Introduction to geometric universality

I will review several universality theorems on configuration/moduli spaces of geometric objects, going back to the 19th century. The main idea of  universality in this context is to encode arithmetic into some basic geometric and algebraic objects, like mechanical linkages, rod/gear mechanisms, projective arrangements, convex polytopes, Coxeter groups, etc.

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#### M. Kapovich (UCDavis): Universality for character schemes for 3-manifold groups

I will talk about a recent universality theorem for moduli spaces of representations of 3-manifold groups to SL(2) that I proved with John Millson. This is an application of our old universality theorem for character schemes of Coxeter groups and a recent theorem of Panov and Petrunin on fundamental groups of 3-dimensional orbifolds.

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#### L. Makar-Limanov: A free associative algebra and an algebraically closed skew ﬁeld

What is more universal than a free object? Of course, it may be not universal enough but often it is the best we can do. In research of free commutative associative algebras (polynomials) algebraically closed ﬁelds play very important role. Historically idea of an algebraically closed ﬁeld started with complex numbers and it takes some effort to prove that the complex numbers indeed form an algebraically closed ﬁeld. On the other hand if just an existence of an algebraically closed ﬁeld is of interest to us and we are ready to accept the transﬁnite induction, construction is easy and even not very tedious. If we replace free commutative associative algebras with free associative algebras and try to built an algebraically closed skew ﬁeld situation is com- pletely different: transﬁnite induction will not deliver. In my talk I'll outline a construction of a non-commutative “complex numbers” and discuss difficulties inherent in such a construction. (For example, it is still not know whether such a ﬁeld exists in the ﬁnite characteristic case.)

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#### A.Padrol: Faces of projectively unique polytopes (and a conjecture of Shephard)

I will present the following universality theorem for projectively unique polytopes: every polytope described by algebraic coordinates is the face of a projectively unique polytope. This result can be used to construct a combinatorial type of polytope that is not realizable as a subpolytope of any stacked polytope. This disproves a classical conjecture in polytope theory, first formulated by Shephard in the seventies.

This is joint work with Karim Adiprasito.

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#### I. Streinu (Smith College): Configuration spaces of Lang's origami Universal Molecules

In a beautiful paper from 1996, R. Lang, a well known origamist, described an algorithm for designing folded origami shapes with an underlying metric tree structure. We show that, very often, Lang's crease patterns are rigid, and his flat-folded shapes (although highly flexible) belong to isolate components in the origami configuration space from which they cannot "unfold". This is joint work with my PhD student John Bowers, who also implemented a very nice program
which visually illustrates Lang's method.

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#### K.-T. Sturm (Uni Bonn): Geometric Analysis on the Space of Metric Measure Spaces

The space of all metric measure spaces (X,d,m) plays an important role in image analysis, in the investigation of limits of Riemannnian manifolds and metric graphs as well as in the study of geometric flows that develop singularities. We show that this "space of spaces" -- equipped with the L^2-distortion distance -- is a challenging object of geometric interest in its own. In particular, we show that it has nonnegative curvature in the sense of Alexandrov. Geodesics and tangent spaces are characterized in detail. Moreover, classes of semiconvex functionals and their gradient flows on the space of spaces are presented.

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#### L. Theran (Berlin): An algebraic-combinatorial viewpoint on low-rank matrix completion

Low-rank matrix completion is the task to reconstruct (or “complete”) a large m x n matrix A of known rank r << n from a subset of its entries. The most successful algorithms with theoretical guarantees are based on convex relaxations and require assumptions on: (a) the sampling of the “true matrix” A; (b) the sampling of the observed entries. I’ll talk about how to formulate matrix completion in algebraic-combinatorial terms. This will lead to new algorithms and give an indication about which sampling models are computationally tractable.

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