Two Greedy Consequences for Maximum Induced Matchings

We prove that, for every integer $d$ with $d\geq 3$, there is an approximation algorithm for the maximum induced matching problem restricted to $\{ C_3,C_5\}$-free $d$-regular graphs with performance ratio $0.708\bar{3}d+0.425$, which answers a question posed by Dabrowski et al. (Theor. Comput. Sci. 478 (2013) 33-40). Furthermore, we show that every graph with $m$ edges that is $k$-degenerate and of maximum degree at most $d$ with $k<d$, has an induced matching with at least $m/((3k-1)d-k(k+1)+1)$ edges

Maximal determinants of combinatorial matrices

We prove that $\det A\leq 6^\frac{n}{6}$ whenever $A\in\{0,1\}^{n\times n}$ contains at most $2n$ ones. We also prove an upper bound on the determinant of matrices with the $k$-consecutive ones property, a generalisation of the consecutive ones property, where each row is allowed to have up to $k$ blocks of ones. Finally, we prove an upper bound on the determinant of a path-edge incidence matrix in a tree and use that to bound the leaf rank of a graph in terms of its order.Comment: 17 page

Exponential Independence

For a set $S$ of vertices of a graph $G$, a vertex $u$ in $V(G)\setminus S$, and a vertex $v$ in $S$, let ${\rm dist}_{(G,S)}(u,v)$ be the distance of $u$ and $v$ in the graph $G-(S\setminus \{ v\})$. Dankelmann et al. (Domination with exponential decay, Discrete Math. 309 (2009) 5877-5883) define $S$ to be an exponential dominating set of $G$ if $w_{(G,S)}(u)\geq 1$ for every vertex $u$ in $V(G)\setminus S$, where $w_{(G,S)}(u)=\sum\limits_{v\in S}\left(\frac{1}{2}\right)^{{\rm dist}_{(G,S)}(u,v)-1}$. Inspired by this notion, we define $S$ to be an exponential independent set of $G$ if $w_{(G,S\setminus \{ u\})}(u)<1$ for every vertex $u$ in $S$, and the exponential independence number $\alpha_e(G)$ of $G$ as the maximum order of an exponential independent set of $G$. Similarly as for exponential domination, the non-local nature of exponential independence leads to many interesting effects and challenges. Our results comprise exact values for special graphs as well as tight bounds and the corresponding extremal graphs. Furthermore, we characterize all graphs $G$ for which $\alpha_e(H)$ equals the independence number $\alpha(H)$ for every induced subgraph $H$ of $G$, and we give an explicit characterization of all trees $T$ with $\alpha_e(T)=\alpha(T)$

Uniquely restricted matchings in subcubic graphs without short cycles

A matching $M$ in a graph $G$ is uniquely restricted if no other matching in $G$ covers the same set of vertices. We prove that any connected subcubic graph with $n$ vertices and girth at least $5$ contains a uniquely restricted matching of size at least $(n-1) / 3$ except for two exceptional cubic graphs of order $14$ and $20$

Equality of Distance Packing Numbers

We characterize the graphs for which the independence number equals the packing number. As a consequence we obtain simple structural descriptions of the graphs for which (i) the distance-$k$-packing number equals the distance-$2k$-packing number, and (ii) the distance-$k$-matching number equals the distance-$2k$-matching number. This last result considerably simplifies and extends previous results of Cameron and Walker (The graphs with maximum induced matching and maximum matching the same size, Discrete Math. 299 (2005) 49-55). For positive integers $k_1$ and $k_2$ with $k_1<k_2$ and $\lceil(3k_2+1)/2\rceil\leq 2k_1+1$, we prove that it is NP-hard to determine for a given graph whether its distance-$k_1$-packing number equals its distance-$k_2$-packing number.Comment: 8 page

Reconfiguring dominating sets in minor-closed graph classes

For a graph $G$, two dominating sets $D$ and $D'$ in $G$, and a non-negative integer $k$, the set $D$ is said to $k$-transform to $D'$ if there is a sequence $D_0,\ldots,D_\ell$ of dominating sets in $G$ such that $D=D_0$, $D'=D_\ell$, $|D_i|\leq k$ for every $i\in \{ 0,1,\ldots,\ell\}$, and $D_i$ arises from $D_{i-1}$ by adding or removing one vertex for every $i\in \{ 1,\ldots,\ell\}$. We prove that there is some positive constant $c$ and there are toroidal graphs $G$ of arbitrarily large order $n$, and two minimum dominating sets $D$ and $D'$ in $G$ such that $D$ $k$-transforms to $D'$ only if $k\geq \max\{ |D|,|D'|\}+c\sqrt{n}$. Conversely, for every hereditary class ${\cal G}$ that has balanced separators of order $n\mapsto n^\alpha$ for some $\alpha<1$, we prove that there is some positive constant $C$ such that, if $G$ is a graph in ${\cal G}$ of order $n$, and $D$ and $D'$ are two dominating sets in $G$, then $D$ $k$-transforms to $D'$ for $k=\max\{ |D|,|D'|\}+\lfloor Cn^\alpha\rfloor$

Dynamic Monopolies for Degree Proportional Thresholds in Connected Graphs of Girth at least Five and Trees

Let $G$ be a graph, and let $\rho\in (0,1)$. For a set $D$ of vertices of $G$, let the set $H_{\rho}(D)$ arise by starting with the set $D$, and iteratively adding further vertices $u$ to the current set if they have at least $\lceil \rho d_G(u)\rceil$ neighbors in it. If $H_{\rho}(D)$ contains all vertices of $G$, then $D$ is known as an irreversible dynamic monopoly or a perfect target set associated with the threshold function $u\mapsto \lceil \rho d_G(u)\rceil$. Let $h_{\rho}(G)$ be the minimum cardinality of such an irreversible dynamic monopoly. For a connected graph $G$ of maximum degree at least $\frac{1}{\rho}$, Chang (Triggering cascades on undirected connected graphs, Information Processing Letters 111 (2011) 973-978) showed $h_{\rho}(G)\leq 5.83\rho n(G)$, which was improved by Chang and Lyuu (Triggering cascades on strongly connected directed graphs, Theoretical Computer Science 593 (2015) 62-69) to $h_{\rho}(G)\leq 4.92\rho n(G)$. We show that for every $\epsilon>0$, there is some $\rho(\epsilon)>0$ such that $h_{\rho}(G) \leq(2+\epsilon)\rho n(G)$ for every $\rho$ in $(0,\rho(\epsilon))$, and every connected graph $G$ that has maximum degree at least $\frac{1}{\rho}$ and girth at least $5$. Furthermore, we show that $h_{\rho}(T) \leq \rho n(T)$ for every $\rho$ in $(0,1]$, and every tree $T$ that has order at least $\frac{1}{\rho}$

Relating broadcast independence and independence

An independent broadcast on a connected graph $G$ is a function $f:V(G)\to \mathbb{N}_0$ such that, for every vertex $x$ of $G$, the value $f(x)$ is at most the eccentricity of $x$ in $G$, and $f(x)>0$ implies that $f(y)=0$ for every vertex $y$ of $G$ within distance at most $f(x)$ from $x$. The broadcast independence number $\alpha_b(G)$ of $G$ is the largest weight $\sum\limits_{x\in V(G)}f(x)$ of an independent broadcast $f$ on $G$. Clearly, $\alpha_b(G)$ is at least the independence number $\alpha(G)$ for every connected graph $G$. Our main result implies $\alpha_b(G)\leq 4\alpha(G)$. We prove a tight inequality and characterize all extremal graphs

Approximating Connected Safe Sets in Weighted Trees

For a graph $G$ and a non-negative integral weight function $w$ on the vertex set of $G$, a set $S$ of vertices of $G$ is $w$-safe if $w(C)\geq w(D)$ for every component $C$ of the subgraph of $G$ induced by $S$ and every component $D$ of the subgraph of $G$ induced by the complement of $S$ such that some vertex in $C$ is adjacent to some vertex of $D$. The minimum weight $w(S)$ of a $w$-safe set $S$ is the safe number $s(G,w)$ of the weighted graph $(G,w)$, and the minimum weight of a $w$-safe set that induces a connected subgraph of $G$ is its connected safe number $cs(G,w)$. Bapat et al. showed that computing $cs(G,w)$ is NP-hard even when $G$ is a star. For a given weighted tree $(T,w)$, they described an efficient $2$-approximation algorithm for $cs(T,w)$ as well as an efficient $4$-approximation algorithm for $s(T,w)$. Addressing a problem they posed, we present a PTAS for the connected safe number of a weighted tree. Our PTAS partly relies on an exact pseudopolynomial time algorithm, which also allows to derive an asymptotic FPTAS for restricted instances. Finally, we extend a bound due to Fujita et al. from trees to block graphs

On some tractable and hard instances for partial incentives and target set selection

A widely studied model for influence diffusion in social networks are {\it target sets}. For a graph $G$ and an integer-valued threshold function $\tau$ on its vertex set, a {\it target set} or {\it dynamic monopoly} is a set of vertices of $G$ such that iteratively adding to it vertices $u$ of $G$ that have at least $\tau(u)$ neighbors in it eventually yields the entire vertex set of $G$. This notion is limited to the binary choice of including a vertex in the target set or not, and Cordasco et al.~proposed {\it partial incentives} as a variant allowing for intermediate choices. We show that finding optimal partial incentives is hard for chordal graphs and planar graphs but tractable for graphs of bounded treewidth and for interval graphs with bounded thresholds. We also contribute some new results about target set seletion on planar graphs by showing the hardness of this problem, and by describing an efficient $O(\sqrt{n})$-approximation algorithm as well as a PTAS for the dual problem of finding a maximum degenerate set