arXiv subjects
Publications and source records attributed to G. Araujo-Pardo.
Cages ($r$-regular graphs of girth $g$ and minimum order) and their variants have been studied for over seventy years. Here we propose a new variant, "weighted cages". We characterize their existence; for cases $g=3,4$ we determine their order; we give Moore-like bounds and present some computational results.
A bipartite graph $G=(V,E)$ with $V=V_1\cup V_2$ is biregular if all the vertices of each stable set, $V_1$ and $V_2$, have the same degree, $r$ and $s$, respectively. This paper studies difference sets derived from both Abelian and non-Abelian groups. From them, we propose some constructions of bipartite biregular graphs with diameter $d=3$ and asymptotically optimal order for given degrees $r$ and $s$. Moreover, we find some biMoore graphs, that is, bipartite biregular graphs that attain the Moore bound.
A linear system is a pair $(P,\mathcal{L})$ where $\mathcal{L}$ is a family of subsets on a ground finite set $P$, such that $|l\cap l^\prime|\leq 1$, for every $l,l^\prime \in \mathcal{L}$. The elements of $P$ and $\mathcal{L}$ are called points and lines, respectively, and the linear system is called intersecting if any pair of lines intersect in exactly one point. A subset $T$ of points of $P$ is a transversal of $(P,\mathcal{L})$ if $T$ intersects any line, and the transversal number, $τ(P,\mathcal{L})$, is the minimum order of a transversal. On the other hand, a 2-packing set of a linear system $(P,\mathcal{L})$ is a set $R$ of lines, such that any three of them have a common point, then the 2-packing number of $(P,\mathcal{L})$, $ν_2(P,\mathcal{L})$, is the size of a maximum 2-packing set. It is known that the transversal number $τ(P,\mathcal{L})$ is bounded above by a quadratic function of $ν_2(P,\mathcal{L})$. An open problem is to haracterize the families of linear systems which satisfies $τ(P,\mathcal{L})\leq λν_2(P,\mathcal{L})$, for some $λ\geq1$. In this paper, we give an infinite family of linear systems $(P,\mathcal{L})$ which satisfies $τ(P,\mathcal{L})=ν_2(P,\mathcal{L})$ with smallest possible cardinality of $\mathcal{L}$, as well as some properties of $r$-uniform intersecting linear systems $(P,\mathcal{L})$, such that $τ(P,\mathcal{L})=ν_2(P,\mathcal{L})=r$. Moreover, we state a characterization of $4$-uniform intersecting linear systems $(P,\mathcal{L})$ with $τ(P,\mathcal{L})=ν_2(P,\mathcal{L})=4$.
A complete $k$-coloring of a graph $G$ is a (not necessarily proper) $k$-coloring of the vertices of $G$, such that each pair of different colors appears in an edge. A complete $k$-coloring is also called connected, if each color class induces a connected subgraph of $G$. The pseudoachromatic index of a graph $G$, denoted by $ψ'(G)$, is the largest $k$ for which the line graph of $G$ has a complete $k$-coloring. Analogously the connected-pseudoachromatic index of $G$, denoted by $ψ_c'(G)$, is the largest $k$ for which the line graph of $G$ has a connected and complete $k$-coloring. In this paper we study these two parameters for the complete graph $K_n$. Our main contribution is to improve the linear lower bound for the connected pseudoachromatic index given by Abrams and Berman [Australas J Combin 60 (2014), 314--324] and provide an upper bound. These two bounds prove that for any integer $n\geq 8$ the order of $ψ_c'(K_n)$ is $n^{3/2}$. Related to the pseudoachromatic index we prove that for $q$ a power of $2$ and $n=q^2+q+1$, $ψ'(K_n)$ is at least $q^3+2q-3$ which improves the bound $q^3+q$ given by Araujo, Montellano and Strausz [J Graph Theory 66 (2011), 89--97].
We introduce the notion of a $[z, r; g]$-mixed cage. A $[z, r; g]$-mixed cage is a mixed graph $G$, $z$-regular by arcs, $r$-regular by edges, with girth $g$ and minimum order. In this paper we prove the existence of $[z, r ;g]$-mixed cages and exhibit families of mixed cages for some specific values. We also give lower and upper bounds for some choices of $z, r$ and $g$. In particular we present the first results on $[z,r;g]$- mixed cages for $z=1$ and any $r\geq 1$ and $g\geq 3$, and for any $z\geq 1$, $r=1$ and $g=4$.
In this paper, we generalize the concept of {\it{perfect graphs}} to other parameters related to graph vertex coloring. This idea was introduced by Christen and Selkow in 1979 and Yegnanarayanan in 2001. Let $ a,b \in \{ ω, χ, Γ, α, ψ\} $ where $ ω$ is the clique number, $ χ$ is the chromatic number, $ Γ$ is the Grundy number, $ α$ is the achromatic number and $ ψ$ is the pseudoachromatic number. A graph $ G $ is \emph{$ ab $-perfect}, if for every induced subgraph $ H $ of $G$, $ a(H)$ equals $b(H) $. In this paper, we characterize the $ab$-perfect graphs when $a=ω$ and $b=ψ$.
In this paper we are interested in the {\it{Cage Problem}} that consists in constructing regular graphs of given girth $g$ and minimum order. We focus on girth $g=5$, where cages are known only for degrees $k \le 7$. We construct regular graphs of girth $5$ using techniques exposed by Funk [Note di Matematica. 29 suppl.1, (2009) 91 - 114] and Abreu et al. [Discrete Math. 312 (2012), 2832 - 2842] to obtain the best upper bounds known hitherto. The tables given in the introduction show the improvements obtained with our results.
Let $ Π_q $ be the projective plane of order $ q $, let $ψ(m):=ψ(L(K_m))$ the pseudoachromatic number of the complete line graph of order $ m $, let $ a\in \{ 3,4,\dots,\tfrac{q}{2}+1 \} $ and $ m_a=(q+1)^2-a $. In this paper, we improve the upper bound of $ ψ(m) $ given by Araujo-Pardo et al. [J Graph Theory 66 (2011), 89--97] and Jamison [Discrete Math. 74 (1989), 99--115] in the following values: if $ x\geq 2 $ is an integer and $m\in \{4x^2-x,\dots,4x^2+3x-3\}$ then $ψ(m) \leq 2x(m-x-1)$. On the other hand, if $ q $ is even and there exists $ Π_q $ we give a complete edge-colouring of $ K_{m_a} $ with $(m_a-a)q$ colours. Moreover, using this colouring we extend the previous results for $a=\{-1,0,1,2\}$ given by Araujo-Pardo et al. in [J Graph Theory 66 (2011), 89--97] and [Bol. Soc. Mat. Mex. (2014) 20:17--28] proving that $ψ(m_a)=(m_a-a)q$ for $ a\in \{3,4,\dots,\left\lceil \frac{1+\sqrt{4q+9}}{2}\right\rceil -1 \} $.
Let $q\ge 2$ be a prime power. In this note we present a formulation for obtaining the known $(q+1,8)$-cages which has allowed us to construct small $(k,g)$--graphs for $k=q-1, q$ and $g=7,8$. Furthermore, we also obtain smaller $(q,8)$-graphs for even prime power $q$.
In this note we construct a new infinite family of $(q-1)$-regular graphs of girth $8$ and order $2q(q-1)^2$ for all prime powers $q\ge 16$, which are the smallest known so far whenever $q-1$ is not a prime power or a prime power plus one itself.
The pseudoachromatic index of a graph is the maximum number of colors that can be assigned to its edges, such that each pair of different colors is incident to a common vertex. If for each vertex its incident edges have different color, then this maximum is known as achromatic index. Both indices have been widely studied. A geometric graph is a graph drawn in the plane such that its vertices are points in general position, and its edges are straight-line segments. In this paper we extend the notion of pseudoachromatic and achromatic indices for geometric graphs, and present results for complete geometric graphs. In particular, we show that for $n$ points in convex position the achromatic index and the pseudoachromatic index of the complete geometric graph are $\lfloor \tfrac{n^2+n}{4} \rfloor$.
The first known families of cages arised from the incidence graphs of generalized polygons of order $q$, $q$ a prime power. In particular, $(q+1,6)$--cages have been obtained from the projective planes of order $q$. Morever, infinite families of small regular graphs of girth 5 have been constructed performing algebraic operations on $\mathbb{F}_q$. In this paper, we introduce some combinatorial operations to construct new infinite families of small regular graphs of girth 7 from the $(q+1,8)$--cages arising from the generalized quadrangles of order $q$, $q$ a prime power.
Let $2 \le r < m$ and $g$ be positive integers. An $({r,m};g)$--graph} (or biregular graph) is a graph with degree set ${r,m}$ and girth $g$, and an $({r,m};g)$-cage (or biregular cage) is an $({r,m};g)$-graph of minimum order $n({r,m};g)$. If $m=r+1$, an $({r,m};g)$-cage is said to be a semiregular cage. In this paper we generalize the reduction and graph amalgam operations from M. Abreu, G. Araujo-Pardo, C. Balbuena, D. Labbate (2011) on the incidence graphs of an affine and a biaffine plane obtaining two new infinite families of biregular cages and two new semiregular cages. The constructed new families are $({r,2r-3};5)$-cages for all $r=q+1$ with $q$ a prime power, and $({r,2r-5};5)$-cages for all $r=q+1$ with $q$ a prime. The new semiregular cages are constructed for r=5 and 6 with 31 and 43 vertices respectively.
Let $q$ be a prime power; $(q+1,8)$-cages have been constructed as incidence graphs of a non-degenerate quadric surface in projective 4-space $P(4, q)$. The first contribution of this paper is a construction of these graphs in an alternative way by means of an explicit formula using graphical terminology. Furthermore by removing some specific perfect dominating sets from a $(q+1,8)$-cage we derive $k$-regular graphs of girth 8 for $k= q-1$ and $k=q$, having the smallest number of vertices known so far.
In this paper we obtain $(q+3)$--regular graphs of girth 5 with fewer vertices than previously known ones for $q=13,17,19$ and for any prime $q \ge 23$ performing operations of reductions and amalgams on the Levi graph $B_q$ of an elliptic semiplane of type ${\cal C}$. We also obtain a 13-regular graph of girth 5 on 236 vertices from $B_{11}$ using the same technique.