Singer difference sets and the projective norm graph

Nilpotent Singer Groups

Let $N$ be a nilpotent group normal in a group $G$. Suppose that $G$ acts transitively upon the points of a finite non-Desarguesian projective plane $\mathcal{P}$. We prove that, if $\mathcal{P}$ has square order, then $N$ must act semi-regularly on $\mathcal{P}$. In addition we prove that if a finite non-Desarguesian projective plane $\mathcal{P}$ admits more than one nilpotent group which is regular on the points of $\mathcal{P}$ then $\mathcal{P}$ has non-square order and the automorphism group of $\mathcal{P}$ has odd order.

A Lyapunov type theorem from Kadison-Singer

Marcus, Spielman, and Srivastava recently solved the Kadison-Singer problem by showing that if u_1, ..., u_m are column vectors in C^d such that \sum u_iu_i^* = I, then a set of indices S \subseteq {1, ..., m} can be chosen so that \sum_{i \in S} u_iu_i^* is approximately (1/2)I, with the approximation good in operator norm to order ε^{1/2} where ε= \max \|u_i\|^2. We extend their result to show that every linear combination of the matrices u_iu_i^* with coefficients in [0,1] can be approximated in operator norm to order ε^{1/8} by a matrix of the form \sum_{i \in S} u_iu_i^*.

Kadison-Singer型结果的一个双曲推广

A Hyperbolic Extension of Kadison-Singer Type Results

该研究将Kadison-Singer型结果从行列式多项式推广至双曲多项式，分别放宽了向量范数假设与分布假设，并给出了对应的亚指数时间构造算法。

The Kadison-Singer Problem for Strongly Rayleigh Measures and Applications to Asymmetric TSP

Marcus, Spielman, and Srivastava in their seminal work \cite{MSS13} resolved the Kadison-Singer conjecture by proving that for any set of finitely supported independently distributed random vectors $v_1,\dots, v_n$ which have "small" expected squared norm and are in isotropic position (in expectation), there is a positive probability that the sum $\sum v_i v_i^\intercal$ has small spectral norm. Their proof crucially employs real stability of polynomials which is the natural generalization of real-rootedness to multivariate polynomials. Strongly Rayleigh distributions are families of probabili…

A note on projective norm graphs

The projective norm graphs P(q, 4) introduced by Alon, Rónyai and Szabó are explicit examples of extremal graphs not containing K_4,7. Ball and Pepe showed that P(q, 4) does not contain a copy of K_5,5 either for q >= 7, asymptotically improving the best lower bound for ex(n, K_5,5). We show that these results can not be improved, in the sense that P(q, 4) contains a copy of K_4,6 for infinitely many primes q.

Graph decompositions in projective geometries

Let PG$(\mathbb{F}_q^v)$ be the $(v-1)$-dimensional projective space over $\mathbb{F}_q$ and let $Γ$ be a simple graph of order ${q^k-1\over q-1}$ for some $k$. A 2$-(v,Γ,λ)$ design over $\mathbb{F}_q$ is a collection $\cal B$ of graphs (\textit{blocks}) isomorphic to $Γ$ with the following properties: the vertex set of every block is a subspace of PG$(\mathbb{F}_q^v)$; every two distinct points of PG$(\mathbb{F}_q^v)$ are adjacent in exactly $λ$ blocks. This new definition covers, in particular, the well known concept of a 2$-(v,k,λ)$ design over $\mathbb{F}_q$ corresponding to the case that …