Search

Let $G$ be a simple, undirected, finite graph with vertex set $V(G)$ and edge set $E(G)$. A $k$-dimensional box is a Cartesian product of closed intervals $[a_1,b_1]\times [a_2,b_2]\times...\times [a_k,b_k]$. The {\it boxicity} of $G$, $\boxi(G)$ is the minimum integer $k$ such that $G$ can be represented as the intersection graph of $k$-dimensional boxes, i.e. each vertex is mapped to a $k$-dimensional box and two vertices are adjacent in $G$ if and only if their corresponding boxes intersect. Let $\poset=(S,P)$ be a poset where $S$ is the ground set and $P$ is a reflexive, anti-symmetric and transitive binary relation on $S$. The dimension of $\poset$, $\dim(\poset)$ is the minimum integer $t$ such that $P$ can be expressed as the intersection of $t$ total orders. Let $G_\poset$ be the \emph{underlying comparability graph} of $\poset$, i.e. $S$ is the vertex set and two vertices are adjacent if and only if they are comparable in $\poset$. It is a well-known fact that posets with the same underlying comparability graph have the same dimension. The first result of this paper links the dimension of a poset to the boxicity of its underlying comparability graph. In particular, we show that for any poset $\poset$, $\boxi(G_\poset)/(\chi(G_\poset)-1) \le \dim(\poset)\le 2\boxi(G_\poset)$, where $\chi(G_\poset)$ is the chromatic number of $G_\poset$ and $\chi(G_\poset)\ne1$. It immediately follows that if $\poset$ is a height-2 poset, then $\boxi(G_\poset)\le \dim(\poset)\le 2\boxi(G_\poset)$ since the underlying comparability graph of a height-2 poset is a bipartite graph. The second result of the paper relates the boxicity of a graph $G$ with a natural partial order associated with the \emph{extended double cover} of $G$, denoted as $G_c$: Note that $G_c$ is a bipartite graph with partite sets $A$ and $B$ which are copies of $V(G)$ such that corresponding to every $u\in V(G)$, there are two vertices $u_A\in A$ and $u_B\in B$ and $\{u_A,v_B\}$ is an edge in $G_c$ if and only if either $u=v$ or $u$ is adjacent to $v$ in $G$. Let $\poset_c$ be the natural height-2 poset associated with $G_c$ by making $A$ the set of minimal elements and $B$ the set of maximal elements. We show that $\frac{\boxi(G)}{2} \le \dim(\poset_c) \le 2\boxi(G)+4$. These results have some immediate and significant consequences. The upper bound $\dim(\poset)\le 2\boxi(G_\poset)$ allows us to derive hitherto unknown upper bounds for poset dimension such as $\dim(\poset)\le 2\tw(G_\poset)+4$, since boxicity of any graph is known to be at most its $\tw+2$. In the other direction, using the already known bounds for partial order dimension we get the following: (1) The boxicity of any graph with maximum degree $\Delta$ is $O(\Delta\log^2\Delta)$ which is an improvement over the best known upper bound of $\Delta^2+2$. (2) There exist graphs with boxicity $\Omega(\Delta\log\Delta)$. This disproves a conjecture that the boxicity of a graph is $O(\Delta)$. (3) There exists no polynomial-time algorithm to approximate the boxicity of a bipartite graph on $n$ vertices with a factor of $O(n^{0.5-\epsilon})$ for any $\epsilon>0$, unless $NP=ZPP$.

A graph $G(V,E)$ is a threshold graph if there exist non-negative reals $w_v, v \in V$ and $t$ such that for every $U \subseteq V$, $\sum_{v \in U} w_v\leq t$ if and only if $U$ is a stable set. The {\it threshold dimension} of a graph $G(V,E)$, denoted as $t(G)$, is the smallest integer $k$ such that $E$ can be covered by $k$ threshold spanning subgraphs of $G$. A permutation graph is a graph that can be represented as the intersection graph of a family of line segments that connect two parallel lines in the Euclidean plane. In this paper we will show that if $G$ is a permutation graph then $t(G) \leq \alpha(G)$ (where $\alpha(G)$ is the cardinality of maximum independent set in $G$) and this bound is tight. As a corollary we will show that $t(G) \leq \frac{n}{2}$ where $n$ is the number of vertices in the permutation graph $G$. This bound is also tight., Comment: 8 pages, 3 figures

A $k$-dimensional box is the Cartesian product $R_1 \times R_2 \times ... \times R_k$ where each $R_i$ is a closed interval on the real line. The {\it boxicity} of a graph $G$, denoted as $\boxi(G)$, is the minimum integer $k$ such that $G$ can be represented as the intersection graph of a collection of $k$-dimensional boxes. A unit cube in $k$-dimensional space or a $k$-cube is defined as the Cartesian product $R_1 \times R_2 \times ... \times R_k$ where each $R_i$ is a closed interval on the real line of the form $[a_i,a_i + 1]$. The {\it cubicity} of $G$, denoted as $\cub(G)$, is the minimum integer $k$ such that $G$ can be represented as the intersection graph of a collection of $k$-cubes. The {\it threshold dimension} of a graph $G(V,E)$ is the smallest integer $k$ such that $E$ can be covered by $k$ threshold spanning subgraphs of $G$. In this paper we will show that there exists no polynomial-time algorithm to approximate the threshold dimension of a graph on $n$ vertices with a factor of $O(n^{0.5-\epsilon})$ for any $\epsilon >0$, unless $NP=ZPP$. From this result we will show that there exists no polynomial-time algorithm to approximate the boxicity and the cubicity of a graph on $n$ vertices with factor $O(n^{0.5-\epsilon})$ for any $ \epsilon >0$, unless $NP=ZPP$. In fact all these hardness results hold even for a highly structured class of graphs namely the split graphs. We will also show that it is NP-complete to determine if a given split graph has boxicity at most 3., Comment: 13 pages

An axis parallel $d$-dimensional box is the Cartesian product $R_1 \times R_2 \times ... \times R_d$ where each $R_i$ is a closed interval on the real line. The {\it boxicity} of a graph $G$, denoted as $\boxi(G)$, is the minimum integer $d$ such that $G$ can be represented as the intersection graph of a collection of $d$-dimensional boxes. An axis parallel unit cube in $d$-dimensional space or a $d$-cube is defined as the Cartesian product $R_1 \times R_2 \times ... \times R_d$ where each $R_i$ is a closed interval on the real line of the form $[a_i,a_i + 1]$. The {\it cubicity} of $G$, denoted as $\cub(G)$, is the minimum integer $d$ such that $G$ can be represented as the intersection graph of a collection of $d$-cubes. Let $S(m)$ denote a star graph on $m+1$ nodes. We define {\it claw number} of a graph $G$ as the largest positive integer $k$ such that $S(k)$ is an induced subgraph of $G$ and denote it as $\claw$. Let $G$ be an AT-free graph with chromatic number $\chi(G)$ and claw number $\claw$. In this paper we will show that $\boxi(G) \leq \chi(G)$ and this bound is tight. We also show that $\cub(G) \leq \boxi(G)(\ceil{\log_2 \claw} +2)$ $\leq$ $\chi(G)(\ceil{\log_2 \claw} +2)$. If $G$ is an AT-free graph having girth at least 5 then $\boxi(G) \leq 2$ and therefore $\cub(G) \leq 2\ceil{\log_2 \claw} +4$., Comment: 15 pages: We are replacing our earlier paper regarding boxicity of permutation graphs with a superior result. Here we consider the boxicity of AT-free graphs, which is a super class of permutation graphs

A $k$-dimensional box is the cartesian product $R_1 \times R_2 \times ... \times R_k$ where each $R_i$ is a closed interval on the real line. The {\it boxicity} of a graph $G$, denoted as $box(G)$, is the minimum integer $k$ such that $G$ can be represented as the intersection graph of a collection of $k$-dimensional boxes: that is two vertices are adjacent if and only if their corresponding boxes intersect. A circular arc graph is a graph that can be represented as the intersection graph of arcs on a circle. Let $G$ be a circular arc graph with maximum degree $\Delta$. We show that if $\Delta <\lfloor \frac{n(\alpha-1)}{2\alpha}\rfloor$, $\alpha \in \mathbb{N}$, $\alpha \geq 2$ then $box(G) \leq \alpha$. We also demonstrate a graph with boxicity $> \alpha$ but with $\Delta=n\frac{(\alpha-1)}{2\alpha}+\frac{n}{2\alpha(\alpha+1)}+(\alpha+2)$. So the result cannot be improved substantially when $\alpha$ is large. Let $r_{inf}$ be minimum number of arcs passing through any point on the circle with respect to some circular arc representation of $G$. We also show that for any circular arc graph $G$, $box(G) \leq r_{inf} + 1$ and this bound is tight. Given a family of arcs $F$ on the circle, the circular cover number $L(F)$ is the cardinality of the smallest subset $F'$ of $F$ such that the arcs in $F'$ can cover the circle. Maximum circular cover number $L_{max}(G)$ is defined as the maximum value of $L(F)$ obtained over all possible family of arcs $F$ that can represent $G$. We will show that if $G$ is a circular arc graph with $L_{max}(G)> 4$ then $box(G) \leq 3$., Comment: 18 pages