Let $k$ be a positive integer, and let $G$ be a simple graph with vertex set $V\left(G\right)$. A $k$-dominating set of the graph $G$ is a subset $D$ of $V\left(G\right)$ such that every vertex of $V\left(G\right)-D$ is adjacent to at least $k$ vertices in $D$. A $k$-domatic partition of $G$ is a partition of $V\left(G\right)$ into $k$-dominating sets. The maximum number of dominating sets in a $k$-domatic partition of $G$ is called the $k$-domatic number$d_k\left(G\right)$. In this paper, we present upper and lower bounds for the $k$-domatic number, and we establish Nordhaus-Gaddum-type results. Some of...

For a nontrivial connected graph $G$ of order $n$, the detour distance $D(u,v)$ between two vertices $u$ and $v$ in $G$ is the length of a longest $u-v$ path in $G$. Detour distance is a metric on the vertex set of $G$. For each integer $k$ with $1\le k\le n-1$, a coloring $c:V\left(G\right)\to \mathbb{N}$ is a $k$-metric coloring of $G$ if $\left|c\right(u)-c(v\left)\right|+D(u,v)\ge k+1$ for every two distinct vertices $u$ and $v$ of $G$. The value ${\chi }_m^k\left(c\right)$ of a $k$-metric coloring $c$ is the maximum color assigned by $c$ to a vertex of $G$ and the $k$-metric chromatic number ${\chi }_m^k\left(G\right)$ of $G$ is the minimum value of a $k$-metric coloring of $G$. For every...

Let D = (V,A) be a finite and simple digraph. A k-rainbow dominating function (kRDF) of a digraph D is a function f from the vertex set V to the set of all subsets of the set {1, 2, . . . , k} such that for any vertex v ∈ V with f(v) = Ø the condition ∪u∈N−(v) f(u) = {1, 2, . . . , k} is fulfilled, where N−(v) is the set of in-neighbors of v. The weight of a kRDF f is the value w(f) = ∑v∈V |f(v)|. The k-rainbow domination number of a digraph D, denoted by γrk(D), is the minimum weight of a kRDF...

For a positive integer k, a k-rainbow dominating function of a graph G is a function f from the vertex set V(G) to the set of all subsets of the set 1,2, ...,k such that for any vertex v ∈ V(G) with f(v) = ∅ the condition ⋃u ∈ N(v)f(u) = 1,2, ...,k is fulfilled, where N(v) is the neighborhood of v. The 1-rainbow domination is the same as the ordinary domination. A set $f₁,f₂,...,f_d$ of k-rainbow dominating functions on G with the property that ${\sum }_{i=1}^d\left|f_i\left(v\right)\right|\le k$ for each v ∈ V(G), is called a k-rainbow dominating family (of...

We study the homotopy invariants of free cochain complexes and Hilbert complexes. These invariants are applied to calculation of exact values of Morse numbers of smooth manifolds.

The Laplacian spectral radius of a graph is the largest eigenvalue of the associated Laplacian matrix. In this paper, we improve Shi's upper bound for the Laplacian spectral radius of irregular graphs and present some new bounds for the Laplacian spectral radius of some classes of graphs.

The problem of distinguishing, in terms of graph topology, digraphs with real and partially non-real Laplacian spectra is important for applications. Motivated by the question posed in [R. Agaev, P. Chebotarev, Which digraphs with rings structure are essentially cyclic?, Adv. in Appl. Math. 45 (2010), 232-251], in this paper we completely list the Laplacian eigenvalues of some digraphs obtained from the wheel digraph by deleting some arcs.

The Laplacian spread of a graph is defined as the difference between the largest and second smallest eigenvalues of the Laplacian matrix of the graph. In this paper, bounds are obtained for the Laplacian spread of graphs. By the Laplacian spread, several upper bounds of the Nordhaus-Gaddum type of Laplacian eigenvalues are improved. Some operations on Laplacian spread are presented. Connected $c$-cyclic graphs with $n$ vertices and Laplacian spread $n-1$ are discussed.