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Giusy Monzillo

Publications and source records attributed to Giusy Monzillo.

17 recordsLinked to original sources
arXiv2026

On the combinatorial structure of graphs with a spectral idempotent of small dual diameter

Let $\Gamma$ be a connected regular graph with an eigenvalue $\lambda$ and corresponding idempotent $E_{\lambda}$. Let ${\cal E}_{\lambda}=\langle J,E_{\lambda}\rangle^\circ$ be the algebra generated by $J$ and $E_\lambda$ with respect to the entrywise-Hadamard product, where $J$ is the all-$1$ matrix. We study the combinatorial structure of a graph $\Gamma$ for which ${\cal E}_{\lambda}$ has dimension $2$, giving a combinatorial characterization of such graphs in terms of equitable partitions. We present many examples and classify the distance-regular graphs with this property, as well as graphs that generate a $3$-class association scheme. We also study the graphs that have two eigenvalues $\lambda$ for which ${\rm dim}({\cal E}_{\lambda})=2$ and determine all such graphs with four distinct eigenvalues.

math.CO↗
arXiv2025

On the $Q$-polynomial property of bipartite graphs admitting a uniform structure

Let $\Gamma$ denote a finite, connected graph with vertex set $X$. Fix $x \in X$ and let $\varepsilon \ge 3$ denote the eccentricity of $x$. For mutually distinct scalars $\{\theta^*_i\}_{i=0}^\varepsilon$ define a diagonal matrix $A^*=A^*(\theta^*_0, \theta^*_1, \ldots, \theta^*_{\varepsilon}) \in M_X(\mathbb{R})$ as follows: for $y \in X$ we let $(A^*)_{yy} = \theta^*_{\partial(x,y)}$, where $\partial$ denotes the shortest path length distance function of $\Gamma$. We say that $A^*$ is a dual adjacency matrix candidate of $\Gamma$ with respect to $x$ if the adjacency matrix $A \in M_X(\mathbb{R})$ of $\Gamma$ and $A^*$ satisfy $$ A^3 A^* - A^* A^3+(\beta+1)( A A^* A^2 - A^2 A^* A)= \gamma(A^2A^*-A^*A^2)+\rho( A A^* - A^* A) $$ for some scalars $\beta, \gamma, \rho\in \mathbb{R}$. Assume now that $\Gamma$ is uniform with respect to $x$ in the sense of Terwilliger [Coding theory and design theory, Part I, IMA Vol. Math. Appl., 20, 193-212 (1990)]. In this paper, we give sufficient conditions on the uniform structure of $\Gamma$, such that $\Gamma$ admits a dual adjacency matrix candidate with respect to $x$. As an application of our results, we show that the full bipartite graphs of dual polar graphs are $Q$-polynomial.

math.CO↗
arXiv2024

The association scheme on the set of flags of a finite generalized quadrangle

In this paper, the association scheme defined on the flags of a finite generalized quadrangle is considered. All possible fusions of this scheme are listed, and a full description for those of classes 2 and 3 is given. Furthermore, it is showed that an association scheme with appropriate parameters must arise from the flags of a generalized quadrangle. The same is done for one of its 4-class symmetric fusion.

math.CO↗
arXiv2024

On the $Q$-polynomial property of the full bipartite graph of a Hamming graph

The $Q$-polynomial property is an algebraic property of distance-regular graphs, that was introduced by Delsarte in his study of coding theory. Many distance-regular graphs admit the $Q$-polynomial property. Only recently the $Q$-polynomial property has been generalized to graphs that are not necessarily distance-regular. In [ J. Combin. Theory Ser. A, 205:105872, 2024 ], it was shown that graphs arising from the Hasse diagrams of the so-called attenuated space posets are $Q$-polynomial. These posets could be viewed as $q$-analogs of the Hamming posets, which were not studied in [ J. Combin. Theory Ser. A, 205:105872, 2024 ]. The main goal of this paper is to fill this gap by showing that the graphs arising from the Hasse diagrams of the Hamming posets are $Q$-polynomial.

math.CO↗
arXiv2024

On combinatorial structure and algebraic characterizations of distance-regular digraphs

Let $Γ=Γ(A)$ denote a simple strongly connected digraph with vertex set $X$, diameter $D$, and let $\{A_0,A:=A_1,A_2,\ldots,A_D\}$ denote the set of distance-$i$ matrices of $Γ$. Let $\{R_i\}_{i=0}^D$ denote a partition of $X\times X$, where $R_i=\{(x,y)\in X\times X\mid (A_i)_{xy}=1\}$ $(0\le i\le D)$. The digraph $Γ$ is distance-regular if and only if $(X,\{R_i\}_{i=0}^D)$ is a commutative association scheme. In this paper, we describe the combinatorial structure of $Γ$ in the sense of equitable partition, and from it we derive several new algebraic characterizations of such a graph, including the spectral excess theorem for distance-regular digraph. Along the way, we also rediscover all well-known algebraic characterizations of such graphs.

math.CO↗
arXiv2024

On Hoffman polynomials of $λ$-doubly stochastic irreducible matrices and commutative association schemes

Let $Γ$ denote a finite (strongly) connected regular (di)graph with adjacency matrix $A$. The {\em Hoffman polynomial} $h(t)$ of $Γ=Γ(A)$ is the unique polynomial of smallest degree satisfying $h(A)=J$, where $J$ denotes the all-ones matrix. Let $X$ denote a nonempty finite set. A nonnegative matrix $B\in{\mbox{Mat}}_X({\mathbb R})$ is called {\em $λ$-doubly stochastic} if $\sum_{z\in X} (B)_{yz}=\sum_{z\in X} (B)_{zy}=λ$ for each $y\in X$. In this paper we first show that there exists a polynomial $h(t)$ such that $h(B)=J$ if and only if $B$ is a $λ$-doubly stochastic irreducible matrix. This result allows us to define the Hoffman polynomial of a $λ$-doubly stochastic irreducible matrix. Now, let $B\in{\mbox{Mat}}_X({\mathbb R})$ denote a normal irreducible nonnegative matrix, and ${\cal B}=\{p(B)\mid p\in{\mathbb{C}}[t]\}$ denote the vector space over ${\mathbb{C}}$ of all polynomials in $B$. Let us define a $01$-matrix $\widehat{A}$ in the following way: $(\widehat{A})_{xy}=1$ if and only if $(B)_{xy}>0$ $(x,y\in X)$. Let $Γ=Γ(\widehat{A})$ denote a (di)graph with adjacency matrix $\widehat{A}$, diameter $D$, and let $A_D$ denote the distance-$D$ matrix of $Γ$. We show that ${\cal B}$ is the Bose--Mesner algebra of a commutative $D$-class association scheme if and only if $B$ is a normal $λ$-doubly stochastic matrix with $D+1$ distinct eigenvalues and $A_D$ is a polynomial in $B$.

math.CO↗
arXiv2024

Classifying pseudo-ovals, translation generalized quadrangles, and elation Laguerre planes of small order

We provide classification results for translation generalized quadrangles of order less or equal to $64$, and hence, for all incidence geometries related to them. The results consist of the classification of all pseudo-ovals in $PG(3n-1,2)$, for $n=3,4$, and that of the pseudo-ovals in $PG(3n-1,q)$, for $n=5,6$, such that one of the associated projective planes is Desarguesian.

math.CO↗
arXiv2023

On commutative association schemes and associated (directed) graphs

Let ${\cal M}$ denote the Bose--Mesner algebra of a commutative $d$-class association scheme ${\mathfrak X}$ (not necessarily symmetric), and $Γ$ denote a (strongly) connected (directed) graph with adjacency matrix $A$. Under the assumption that $A$ belongs to ${\cal M}$, we describe the combinatorial structure of $Γ$. Moreover, we provide an algebraic-combinatorial characterization of $Γ$ when $A$ generates ${\cal M}$. Among else, we show that, if ${\mathfrak X}$ is a commutative $3$-class association scheme that is not an amorphic symmetric scheme, then we can always find a (directed) graph $Γ$ such that the adjacency matrix $A$ of $Γ$ generates the Bose--Mesner algebra ${\cal M}$ of ${\mathfrak X}$.

math.CO↗
arXiv2023

Distance-regular graphs with classical parameters that support a uniform structure: case $q \ge 2$

Let $Γ=(X,\mathcal{R})$ denote a finite, simple, connected, and undirected non-bipartite graph with vertex set $X$ and edge set $\mathcal{R}$. Fix a vertex $x \in X$, and define $\mathcal{R}_f = \mathcal{R} \setminus \{yz \mid \partial(x,y) = \partial(x,z)\}$, where $\partial$ denotes the path-length distance in $Γ$. Observe that the graph $Γ_f=(X,\mathcal{R}_f)$ is bipartite. We say that $Γ$ supports a uniform structure with respect to $x$ whenever $Γ_f$ has a uniform structure with respect to $x$ in the sense of Miklavič and Terwilliger \cite{MikTer}. Assume that $Γ$ is a distance-regular graph with classical parameters $(D,q,α,β)$ and diameter $D\geq 4$. Recall that $q$ is an integer such that $q \not \in \{-1,0\}$. The purpose of this paper is to study when $Γ$ supports a uniform structure with respect to $x$. We studied the case $q \le 1$ in \cite{FMMM}, and so in this paper we assume $q \geq 2$. Let $T=T(x)$ denote the Terwilliger algebra of $Γ$ with respect to $x$. Under an additional assumption that every irreducible $T$-module with endpoint $1$ is thin, we show that if $Γ$ supports a uniform structure with respect to $x$, then either $α= 0$ or $α=q$, $β=q^2(q^D-1)/(q-1)$, and $D \equiv 0 \pmod{6}$.

math.CO↗
arXiv2023

Distance-regular graphs with classical parameters that support a uniform structure: case $q \le 1$

Let $Γ=(X,\mathcal{R})$ denote a finite, simple, connected, and undirected non-bipartite graph with vertex set $X$ and edge set $\mathcal{R}$. Fix a vertex $x \in X$, and define $\mathcal{R}_f = \mathcal{R} \setminus \{yz \mid \partial(x,y) = \partial(x,z)\}$, where $\partial$ denotes the path-length distance in $Γ$. Observe that the graph $Γ_f=(X,\mathcal{R}_f)$ is bipartite. We say that $Γ$ supports a uniform structure with respect to $x$ whenever $Γ_f$ has a uniform structure with respect to $x$. Assume that $Γ$ is a distance-regular graph with classical parameters $(D,q,α,β)$ with $q \le 1$. Recall that $q$ is an integer, which is not equal to $0$ or $-1$. The purpose of this paper is to study when $Γ$ supports a uniform structure with respect to $x$. The main result of the paper is a complete classification of graphs with classical parameters with $q\leq 1$ and $D \ge 4$ that support a uniform structure with respect to $x$.

math.CO↗
arXiv2023

Classification of spreads of Tits quadrangles of order 64

Brown et al. provide a representation of a spread of the Tits quadrangle $T_2(\mathcal O)$, $\mathcal O$ an oval of $\mathrm PG(2,q)$, $q$ even, in terms of a certain family of $q$ ovals of $\mathrm PG(2,q)$. By combining this representation with the Vandendriessche classification of hyperovals in $\mathrm PG(2,64)$ and the classification of flocks of the quadratic cone in $\mathrm PG(3,64)$, recently given by the authors, in this paper, we classify all the spreads of $T_2(\mathcal O)$, $\mathcal O$ an oval of $\mathrm PG(2,64)$, up to equivalence. These complete the classification of spreads of $T_2(\mathcal O)$ for $q\le 64$.

math.CO↗
arXiv2023

Eggs in finite projective spaces and unitals in translation planes

Inspired by the connection between ovoids and unitals arising from the Buekenhout construction in the André/Bruck-Bose representation of translation planes of dimension at most two over their kernel, and since eggs of PG(4m-1,q), m>=1, are a generalization of ovoids, we explore the relation between eggs and unitals in translation planes of higher dimension over their kernel. By investigating such a relationship, we construct a unital in the Dickson semifield plane of order 3^{10}, which is represented in PG(20,3) by a cone whose base is a set of points constructed from the dual of the Penttila-Williams egg in PG(19,3). This unital is not polar; so, up to the knowledge of the authors, it seems to be a new unital in such a plane.

math.CO↗
arXiv2022

Classification of flocks of the quadratic cone in PG(3,64)

Flocks are an important topic in the field of finite geometry, with many relations with other objects of interest. This paper is a contribution to the difficult problem of classifying flocks up to projective equivalence. We complete the classification of flocks of the quadratic cone in PG(3,q) for q <= 71, by showing by computer that there are exactly three flocks of the quadratic cone in PG(3,64), up to equivalence. The three flocks had previously been discovered, and they are the linear flock, the Subiaco flock and the Adelaide flock. The classification proceeds via the connection between flocks and herds of ovals in PG(2,q), q even, and uses the prior classification of hyperovals in PG(2,64).

math.CO↗
arXiv2021

Pseudo-ovals of elliptic quadrics as Delsarte designs of association schemes

A $pseudo$-$oval$ of a finite projective space over a finite field of odd order $q$ is a configuration of equidimensional subspaces that is essentially equivalent to a translation generalised quadrangle of order $(q^n,q^n)$ and a Laguerre plane of order $q^n$ (for some $n$). In setting out a programme to construct new generalised quadrangles, Shult and Thas asked whether there are pseudo-ovals consisting only of lines of an elliptic quadric ${Q}^-(5,q)$, non-equivalent to the $classical$ $example$, a so-called $pseudo$-$conic$. To date, every known pseudo-oval of lines of ${Q}^-(5,q)$ is projectively equivalent to a pseudo-conic. Thas characterised pseudo-conics as pseudo-ovals satisfying the $perspective$ property, and this paper is on characterisations of pseudo-conics from an algebraic combinatorial point of view. In particular, we show that pseudo-ovals in $Q^-(5,q)$ and pseudo-conics can be characterised as certain Delsarte designs of an interesting five-class association scheme. These association schemes are introduced and explored, and we provide a complete theory of how pseudo-ovals of lines of $Q^-(5,q)$ can be analysed from this viewpoint.

math.CO↗
arXiv2020

On the isomorphism of certain primitive $Q$-polynomial not $P$-polynomial association schemes

In 2011, Penttila and Williford constructed an infinite new family of primitive $Q$-polynomial 3-class association schemes, not arising from distance regular graphs, by exploring the geometry of the lines of the unitary polar space $H(3,q^2)$, $q$ even, with respect to a symplectic polar space $W(3,q)$ embedded in it. In a private communication to Penttila and Williford, H.~Tanaka pointed out that these schemes have the same parameters as the 3-class schemes found by Hollmann and Xiang in 2006 by considering the action of $\mathrm{PGL}(2,q^2)$, $q$ even, on a non-degenerate conic of $\mathrm{PG}(2,q^2)$ extended in $\mathrm{PG}(2,q^4)$. Therefore, the question arises whether the above association schemes are isomorphic. In this paper we provide the positive answer. As by product, we get an isomorphism of strongly regular graphs.

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