Pawan Kumar Aurora | Searcharxiv

arXiv2018

Spectral properties of the Cayley Graphs of split metacyclic groups

Let $Γ(G,S)$ denote the Cayley graph of a group $G$ with respect to a set $S \subset G$. In this paper, we analyze the spectral properties of the Cayley graphs $\mathcal{T}_{m,n,k} = Γ(\mathbb{Z}_m \ltimes_k \mathbb{Z}_n, \{(\pm 1,0),(0,\pm 1)\})$, where $m,n \geq 3$ and $k^m \equiv 1 \pmod{n}$. We show that the adjacency matrix of $\mathcal{T}_{m,n,k}$, upto relabeling, is a block circulant matrix, and we also obtain an explicit description of these blocks. By extending a result due to Walker-Mieghem to Hermitian matrices, we show that $\mathcal{T}_{m,n,k}$ is not Ramanujan, when either $m > 8$, or $n \geq 400$.

math.CO↗

arXiv2014

New Facets of the QAP-Polytope

The Birkhoff polytope is defined to be the convex hull of permutation matrices, $P_σ\ \forall σ\in S_n$. We define a second-order permutation matrix $P^{[2]}_σ$ in $\mathbb{R}^{n^2\times n^2}$ corresponding to a permutation $σ$ as $(P^{[2]}_σ)_{ij,kl} = (P_σ)_{ij}(P_σ)_{kl}$. We call the convex hull of the second-order permutation matrices, the {\em second-order Birkhoff polytope} and denote it by ${\cal B}^{[2]}$. It can be seen that ${\cal B}^{[2]}$ is isomorphic to the QAP-polytope, the domain of optimization in {\em quadratic assignment problem}. In this work we revisit the polyhedral combinatorics of the QAP-polytope viewing it as ${\cal B}^{[2]}$. Our main contribution is the identification of an exponentially large set of new facets of this polytope. Also we present a general inequality of which all the known facets of this polytope as well as the new ones, that we present in this paper, are special instances. We also establish the existence of more facets which are yet to be identified.

math.OC↗