פעילויות השבוע

Continuing with the topics of last week‘s talk, I will explain how one can use this to compute the “local contributions away from p” for Quadratic Chabauty, which are crucial for computations.

The size of the smallest $k$-regular graph of girth $g$ is denoted by the well studied function $n(k,g)$. We suggest generalizing this function to $n(H,g)$, defined as the smallest size girth $g$ graph covering the, possibly non-regular, graph $H$. We prove that the two main combinatorial bounds on $n(k,g)$, the Moore lower bound and the Erdos-Sachs upper bound, carry over to the new setting of lifts, even in their non-asymptotic form.

We also consider two other generalizations of $n(k,g)$: i) The smallest size girth $g$ graph sharing a universal cover with $H$. We prove that it is the same as $n(H,g)$ up to a multiplicative constant. ii) The smallest size girth $g$ graph with a prescribed degree distribution. We discuss this known generalization and argue that the new suggested definitions are superior.

We conclude with experimental results for a specific base graph and with some conjectures and open problems.

https://arxiv.org/abs/2401.01238

In this talk, I will discuss isometric dilations of completely contractive representations (in short c.c. representation) of product systems (of $W^∗$-correspondences) over the semigroup $\mathbb{Z}^n_{+}$. It is known that for $n = 1, 2$, c.c. representations of such product systems always have isometric dilations and the result fails for $n > 2$, in general. We will see that under certain positivity and pureness conditions c.c. representations of product systems over $\mathbb{Z}^n_{+}$ have isometric dilations, also we will see an explicit form of the dilations. If time permits, I will discuss some applications of it.

This talk is based on joint work with Monojit Bhattacharjee and Baruch Solel.