Activities This Week

A family of sets F is said to satisfy the (p,q)-property if among any p sets in F, some q have a non-empty intersection. Hadwiger and Debrunner (1957) conjectured that for any p > q > d there exists a constant c = c_d(p,q), such that any family of compact convex sets in R^d that satisfies the (p,q)-property, can be pierced by at most c points. Helly’s Theorem is equivalent to the fact that c_d(p,p)=1 (p > d).

In a celebrated result from 1992, Alon and Kleitman proved the conjecture. However, obtaining sharp bounds on the minimal such c_d(p,q), called `the Hadwiger-Debrunner numbers’, is still a major open problem in combinatorial geometry.

In this talk we present improved upper and lower bounds on the Hadwiger-Debrunner numbers, the latter using the hypergraph container method. Based on joint works with Shakhar Smorodinsky and Gabor Tardos.

קבוצה יוצרת $X$ של חבורה $G$ היא קבוצת איברים כך שכל איבר ב $G$ שווה למכפלה של איברים מ $X$ והופכיים של איברים מ $X$. בעיית היצירה עבור חבורה $G$ עוסקת בשאלה האם ניתן לקבוע, בהינתן תת קבוצה סופית $X$ של $G$, אם $X$ יוצרת את $G$. בעיית היצירה בחבורה $G$ פתירה, אם יש אלגוריתם שבהינתן תת קבוצה סופית $X$ של $G$ קובע אם $X$ יוצרת את $G$.

אנו נדון בבעיית היצירה במספר חבורות אינסופיות, ביניהן החבורה $F$ של תומפסון שניתנת להגדרה כחבורה של פונקציות על קטע היחידה $[0,1]$.

The monochromatic layers of the chromatic filtration on spectra, that is the K(n)-local (stable 00-)categories Sp_{K(n)} enjoy many remarkable properties. One example is the vanishing of the Tate construction due to Hovey-Greenlees-Sadofsky. The vanishing of the Tate construction can be considered as a natural equivalence between the colimits and limits in Sp_{K(n)} parametrized by finite groupoids. Hopkins and Lurie proved a generalization of this result where finite groupoids are replaced by arbitrary \pi-finite 00-groupoids.

There is another possible sequence of (stable 00-)categories who can be considered as “monochromatic layers”, those are the T(n)-local 00-categories Sp_{T(n)}. For the Sp_{T(n)} the vanishing of the Tate construction was proved by Kuhn. We shall prove that the analog of Hopkins and Lurie’s result in for Sp_{T(n)}. Our proof will also give an alternative proof for the K(n)-local case.

This is a joint work with Shachar Carmeli and Lior Yanovski

Consider a polygon-shaped billiard table on which a ball can roll along straight lines and reflect off of edges infinitely. In work joint with Moon Duchin, Viveka Erlandsson and Chris Leininger, we have characterized the relationship between the shape of a polygonal billiard table and the set of possible infinite edge itineraries of balls travelling on it. In this talk, we will explore this relationship and the tools used in our characterization (notably a new rigidity result for flat cone metrics).

The Toeplitz algebra of a directed graph is the C*-algebra generated by concatenation operators on square summable sequences over finite paths of the graph. The canonical quotient by its compact operators yields the celebrated Cuntz-Krieger algebra, which is deeply connected to the subshift of finite type and automata associated with the directed graph.

When the graph has a single vertex, representations of Toeplitz algebras were studied by Davidson, Katsoulis and Pitts, originating from work of Popescu on his non-commutative disk algebra. This is accomplished by working with the WOT closed algebra generated by operators corresponding to vertices and edges in the representation. These algebras are called free semigroup algebras, and provide a non-self-adjoint perspective for studying representations of Cuntz algebras.

The classification of representations of directed graphs up to unitary equivalence was used in producing wavelet on Cantor sets by Marcolli and Paolucci and in the study of semi-branching function systems by Bezuglyi and Jorgensen. Hence, extending the theory of free semigroup algebras to arbitrary directed graphs becomes an important endeavor, and new connections with graph theory emerge.

In this talk I will survey work I have done over the span of three years as part of a combined effort to understand representations of Toeplitz algebras of directed graphs via non-self-adjoint techniques. We will conclude with a characterization of those finite directed graphs that admit representations with self-adjoint WOT closed algebras generated by vertices and edges operators. This will make full circle with the theory of automata, as we will use a periodic version of the Road Coloring theorem due to Béal and Perrin, originally proved by Trahtman in the aperiodic case. This settles a question posed in a previous paper by Davidson, B. Li and myself, and is based on joint work with Christopher Linden.