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The atom-bond sum-connectivity (ABS) index of a graph GG with edges e1,…,em{e}_{1},\ldots ,{e}_{m} is the sum of the numbers 1−2(dei+2)−1\sqrt{1-2{\left({d}_{{e}_{i}}+2)}^{-1}} over 1≤i≤m1\le i\le m, where dei{d}_{{e}_{i}} is the number of edges adjacent to ei{e}_{i}. In this article, we study the maximum values of the ABS index over graphs with given parameters. More specifically, we determine the maximum ABS index of connected graphs of a given order with a fixed (i) minimum degree, (ii) maximum degree, (iii) chromatic number, (iv) independence number, or (v) number of pendent vertices. We also characterize the graphs attaining the maximum ABS values in all of these classes.

AbstractA graph G is an integral sum graph (sum graph) if its vertices can be labeled with distinct integers (positive integers) so that e = uv is an edge of G if and only if the sum of the labels on vertices u and v is also a label in G. A graph G is perfect if the chromatic number of each of its induced subgraphs is equal to the clique number of the same. A simple graph G is of class 1 if its edge chromatic number and maximum degree are same. In this paper, we prove that integral sum graphs Gn, [Formula: see text] and [Formula: see text] over the label sets [Formula: see text] and [Formula: see text], respectively, are perfect graphs as well as of class 1 for [Formula: see text]. We also obtain a few structural properties of these graphs.

A prime labeling of a simple undirected graph G is to assign unique integer labels from the set {1,2,...,|V(G)|} to each vertex such that any two adjacent vertices in the graph have labels that are relatively prime. Studying prime labeling in graphs can help us understand the structure and properties of graphs and prime labeling has potential applications in cryptography and network security. In this paper we investigate when some graphs that are constructed from wheels are prime graphs.

A graph $G$ is called semi square stable if $\alpha (G^{2})=i(G)$ where $%\alpha (G^{2})$ is the independence number of $G^{2}$ and $i(G)$ is the independent dominating number of $G$. A subset $S$ of the vertex set of a graph $G$ is an efficient dominating set if $S$ is an independent set and every vertex of $G$ is either in $S$ or adjacent to exactly one vertex of $%S. $In this paper, we show that every square stable graph has an efficient dominating set and if a graph has an efficient dominating set, then it is semi square stable. We characterize when the join and the corona product of two disjoint graphs are semi square sable graphs and when they have efficient dominating sets.

Consider a simple graph $ \Gamma = (V(\Gamma), E(\Gamma)) $ with $ n $ vertices and $ m $ edges. Let $ P $ be a subset of $ V(\Gamma) $ and $ B(P) $ the set of neighbors of $ P $ in $ V(\Gamma)\backslash P $. In the study of graphs, the concept of differential refers to a measure of how much the number of edges leaving a set of vertices exceeds the size of that set. Specifically, given a subset $ P $ of vertices, the differential of $ P $, denoted by $ \partial(P) $, is defined as $ |B(P)|-|P| $. The differential of $ \Gamma $, denoted by $ \partial(\Gamma) $, is then defined as the maximum differential over all possible subsets of $ V(\Gamma) $. Additionally, the subdivision operator $ {{\mathcal{S}}({\Gamma})} $ is defined as the graph obtained from $ \Gamma $ by inserting a new vertex on each edge of $ \Gamma $. In this paper, we present results for the differential of graphs on the subdivision operator $ {{\mathcal{S}}({\Gamma})} $ where some of these show exact values of $ \partial({{\mathcal{S}}({\Gamma})}) $ if $ \Gamma $ belongs to a classical family of graphs. We obtain bounds for $ \partial({{\mathcal{S}}({\Gamma})}) $ involving invariants of a graph such as order $ n $, size $ m $ and maximum degree $ \Delta $, and we study the realizability of the graph $ \Gamma $ for any value of $ \partial({{\mathcal{S}}({\Gamma})}) $ in the interval $ \left[n-2, \frac{n(n-1)}{2}-n+2\right] $. Moreover, we give a characterization for $ \partial({{\mathcal{S}}({\Gamma})}) $ using the notion of edge star packing.

In this paper, we study the class of graphs Gm,n that have the same degree sequence as two disjoint cliques Km and Kn, as well as the class G¯m,n of the complements of such graphs. The problems of finding a maximum clique and a maximum independent set are NP-hard on Gm,n. Therefore, looking for upper and lower bounds for the clique and independence numbers of such graphs is a challenging task. In this article, we obtain such bounds, as well as other related results. In particular, we consider the class of regular graphs, which are degree-equivalent to arbitrarily many identical cliques, as well as such graphs of bounded degree.

It was conjectured by Jaeger et al. that all 5-edge-connected graphs are $ Z_3 $-connected. In this paper, we confirm this conjecture for all 5-edge-connected graphs with independence number at most 3.

Let R be a commutative ring with identity and Nil(R) be the set of nilpotent elements of R. The nil-graph of ideals of R is defined as the graph AG_N(R) whose vertex set is {I:(0)and there exists a non-trivial ideal such that and two distinct vertices and are adjacent if and only if . Here, we study conditions under which is complete or bipartite. Also, the independence number of is determined, where is a reduced ring. Finally, we classify Artinian rings whose nil-graphs of ideals have genus at most one.

The aim of the current research is to study and compare graphs authorship by Iranian researchers in Persian scientific research journals in the field of educational sciences by using algebraic parameters of mathematics. In this research, the data related to the scientific research documents of 336 Iranian researchers from 6 Persian scientific research journals in the field of educational sciences from 2017 to 2021 were extracted by using the issues printed in these journals and the graphs related to the authors of these journals have been drawn by mathematical methods and software, and then compared and analyzed by using algebraic parameters such as the average degrees of vertices, independence, color and matching numbers. In the main results, it was checked that the average grade of the Journal of the Educational Measurement and Evaluation Studies is 4.4 and higher than other journals. Also, the largest research group (cluster number) is 6 people, and the most research diversity (independence number) is 29 and it is related to of the Journal of the teaching research. Among the discussed journals in the field of educational sciences, according to the average grades, the amount of research cooperation of the Journal of Educational Measurement and Evaluation Studies is at a higher level than the other five journals.

In this paper we study relationships between the matching number, written µ(G), and the independence number, written α(G). Our first main result is to show

Given a graph G = (V, E), a set B ⊆ V (G) is a packing in G if the closed neighborhoods of every pair of distinct vertices in B are pairwise disjoint. The packing number ρ(G) of G is the maximum cardinality of a packing in G. Similarly, open packing sets and open packing number are defined for a graph G by using open neighborhoods instead of closed ones. We give several results concerning the (open) packing number of graphs in this paper. For instance, several bounds on these packing parameters along with some Nordhaus-Gaddum inequalities are given. We characterize all graphs with equal packing and independence numbers and give the characterization of all graphs for which the packing number is equal to the independence number minus one. In addition, due to the close connection between the open packing and total domination numbers, we prove a new upper bound on the total domination number γt(T) for a tree T of order n ≥ 2 improving the upper bound γt(T) ≤ (n + s)/2 given by Chellali and Haynes in 2004, in which s is the number of support vertices of T.

Let S be a Clifford semigroup and A a subset of S. We write Cay(S,A) for the Cayley digraph of a Clifford semigroup S relative to A. The (weak, path, weak path) independence number of a graph is the maximum cardinality of an (weakly, path, weakly path) independent set of vertices in the graph. In this paper, we characterize maximal connected subdigraphs of Cay(S,A) and apply these results to determine the (weak, path, weak path) independence number of Cay(S,A).

For a simple connected graph G of order n having distance signless Laplacian eigenvalues ρ1Q≥ρ2Q≥⋯≥ρnQ\rho _1^Q \ge \rho _2^Q \ge \cdots \ge \rho _n^Q, the distance signless Laplacian energy DSLE(G) is defined as DSLE(G)=∑i=1n|ρiQ-2W(G)n|DSLE\left( G \right) = \sum\nolimits_{i = 1}^n {\left| {\rho _i^Q - {{2W\left( G \right)} \over n}} \right|} where W(G) is the Weiner index of G. We show that the complete split graph has the minimum distance signless Laplacian energy among all connected graphs with given independence number. Further, we prove that the graph Kk ∨ ( Kt∪ Kn−k−t),1≤t≤⌊n-k2⌋1 \le t \le \left\lfloor {{{n - k} \over 2}} \right\rfloor has the minimum distance signless Laplacian energy among all connected graphs with vertex connectivity k.

For a finite dimensional vector space over a field the subspace sum graph of denoted by is defined to be a simple undirected graph with vertex set as the set of all non-trivial proper subspace of and, for any two distinct vertices V1 and V2 are adjacent if and only if In this paper, we establish some inter-relationship between as a graph and as a vector space by the study of genus of subspace sum graph of vector spaces. In particular, we characterize the collection of all the vector spaces for which the subspace sum graph of is either planar, toroidal or bi-toroidal. Furthermore, we determine the independence number of for a vector space of dimension at most 5.

‎There are many different graphs one can associate to a group‎. ‎Some examples are the well-known Cayley graph‎, ‎the zero divisor graph (of a ring)‎, ‎the power graph‎, ‎and the recently introduced coprime graph of a group‎. ‎The coprime graph of a group $G$‎, ‎denoted $\Gamma_G$‎, ‎is the graph whose vertices are the group elements with $g$ adjacent to $h$ if and only if $(o(g),o(h))=1$‎. ‎In this paper we calculate the independence number of the coprime graph of the dihedral groups‎. ‎Additionally‎, ‎we characterize the groups whose coprime graph is perfect‎.

Win proved a very famous conclusion that states the graph G with connectivity κ(G), independence number α(G) and α(G)≤κ(G)+k−1(k≥2) contains a spanning k-ended tree. This means that there exists a spanning tree with at most k leaves. In this paper, we strengthen the Win theorem to the following: Let G be a simple 2-connected graph such that |V(G)|≥2κ(G)+k, α(G)≤κ(G)+k(k≥2) and the number of maximum independent sets of cardinality κ+k is at most n−2κ−k+1. Then, either G contains a spanning k-ended tree or a subgraph of Kκ∨((k+κ−1)K1∪Kn−2κ−k+1).

For a positive integer k ⩾ 1, a graph G with vertex set V is said to be k-packing colorable if there exists a mapping f : V ↦ {1, 2, . . ., k} such that any two distinct vertices x and y with the same color f(x) = f(y) are at distance at least f(x) + 1. The packing chromatic number of a graph G, denoted by χρ(G), is the smallest integer k such that G is k-packing colorable.

Algebraic graph theory is the study of the interplay between algebraic structures (both abstract as well as linear structures) and graph theory. Many concepts of abstract algebra have facilitated through the construction of graphs which are used as tools in computer science. Conversely, graph theory has also helped to characterize certain algebraic properties of abstract algebraic structures. In this survey, we highlight the rich interplay between the two topics viz groups and power graphs from groups. In the last decade, extensive contribution has been made towards the investigation of power graphs. Our main motive is to provide a complete survey on the connectedness of power graphs and proper power graphs, the Laplacian and adjacency spectrum of power graph, isomorphism, and automorphism of power graphs, characterization of power graphs in terms of groups. Apart from the survey of results, this paper also contains some new material such as the contents of Section 2 (which describes the interesting case of the power graph of the Mathieu group M11) and Section 6.1 (where conditions are discussed for the reduced power graph to be not connected). We conclude this paper by presenting a set of open problems and conjectures on power graphs.

Adding a connected dominating set of vertices to a graph $G$ increases its number of Hadwiger $h(G)$. Based on this obvious property in [2] we introduced a new invariant $\eta(G)$ for which $\eta(G)\leq h(G)$. We continue to study its property. For a graph $G$ with independence number three without induced chordless cycles $C_7$ and with $n(G)$ vertices, $\eta(G)\geq n(G)/4$.

A network is an abstract structure that consists of nodes that are connected by links. A bipartite network is a type of networks where the set of nodes can be divided into two disjoint sets in a way that each link connects a node from one partition with a node from the other partition. In this paper, we first determine the maximum H-index of networks in the class of all n-node connected bipartite network with matching number t. We obtain that the maximum H-index of a bipartite network with a given matching number is Kt,n−t. Secondly, we characterize the network with the maximum H-index in the class of all the n-vertex connected bipartite network of given diameter. Based on our obtain results, we establish the unique bipartite network with maximum H-index among bipartite networks with a given independence number and cover of a network.

In mathematical chemistry, molecular descriptors serve an important role, primarily in quantitative structure–property relationship (QSPR) and quantitative structure–activity relationship (QSAR) studies. A topological index of a molecular graph is a real number that is invariant under graph isomorphism conditions and provides information about its size, symmetry, degree of branching, and cyclicity. For any graph N, the first and second Gourava indices are defined as GO1(N)=∑u′v′∈E(N)(d(u′)+d(v′)+d(u′)d(v′)) and GO2(N)=∑u′v′∈E(N)(d(u′)+d(v′))d(u′)d(v′), respectively.The independence number of a graph N, being the cardinality of its maximal independent set, plays a vital role in reading the energies of chemical trees. In this research paper, it is shown that among the family of trees of order δ and independence number ξ, the spur tree denoted as Υδ,ξ maximizes both 1st and 2nd Gourava indices, and with these characterizations this graph is unique.

A subset of vertices in a graph G is a total dominating set if every vertex in G is adjacent to at least one vertex in this subset. The total domination number of G is the minimum cardinality of any total dominating set in G and is denoted by γt(G). A total dominating set of G having nonempty intersection with all the independent sets of maximum cardinality in G is an independent transversal total dominating set. The minimum cardinality of any independent transversal total dominating set is denoted by γtt(G). Based on the fact that for any tree T, γt(T) ≤ γtt(T) ≤ γt(T) + 1, in this work we give several relationships between γtt(T) and γt(T) for trees T which are leading to classify the trees which are satisfying the equality in these bounds.

The independence number of circulant triangle-free graphs for 2-regular, 3-regular graphs are investigated. It is shown that the independence ratio of circulant triangle-free graphs for 3-regular graphs is at least 3/8. Some bounds for the number of vertices of r-regular circulant triangle-free graphs with independence number equal to r for odd degrees are determined. These bounds are close to Sidorenko’s bounds for even degrees.

Let be a graph and let be a positive integer. Let = and . The -token graph is the graph with vertex set and two vertices and are adjacent if and , where denotes the symmetric difference. In this paper we present several basic results on 2-token graphs.

A well-known theorem by Chvátal-Erdőos [A note on Hamilton circuits, Discrete Math. 2 (1972) 111–135] states that if the independence number of a graph G is at most its connectivity plus one, then G is traceable. In this article, we show that every 2-connected claw-free graph with independence number α(G) ≤ 6 is traceable or belongs to two exceptional families of well-defined graphs. As a corollary, we also show that every 2-connected claw-free graph with independence number α(G) ≤ 5 is traceable.

For a graph G with vertex set V (G) and independence number α(G), Selkow [A Probabilistic lower bound on the independence number of graphs, Discrete Math. 132 (1994) 363–365] established the famous lower bound ∑v∈V(G)1d(v)+1(1+max{d(v)d(v)+1-∑u∈N(v)1d(u)+1,0})$\sum {_{v \in V(G)}{1 \over {d(v) + 1}}} \left( {1 + \max \left\{ {{{d(v)} \over {d(v) + 1}} - \sum {_{u \in N(v)}{1 \over {d(u) + 1}},0} } \right\}} \right)$ on α (G), where N(v) and d(v) = |N(v)| denote the neighborhood and the degree of a vertex v ∈ V (G), respectively. However, Selkow’s original proof of this result is incorrect. We give a new probabilistic proof of Selkow’s bound here.

The k-independence number of a graph G, denoted as αk(G), is the order of a largest induced k-colorable subgraph of G. In [S. Špacapan, The k-independence number of direct products of graphs, European J. Combin. 32 (2011) 1377–1383] the author conjectured that the direct product G × H of graphs G and H obeys the following bound

A set S of vertices of a graph G is called a decycling set if G−S is acyclic. The minimum order of a decycling set is called the decycling number of G, and denoted by ∇(G). Our results include: (a) For any graph G,, where T is taken over all the spanning trees of G and α(G − E(T)) is the independence number of the co-tree G − E(T). This formula implies that computing the decycling number of a graph G is equivalent to finding a spanning tree in G such that its co-tree has the largest independence number. Applying the formula, the lower bounds for the decycling number of some (dense) graphs may be obtained. (b) For any decycling set S of a k-regular graph G, where β(G) = |E(G)|−|V (G)|+1 and m(S) = c+|E(S)|−1, c and |E(S)| are, respectively, the number of components of G − S and the number of edges in G[S]. Hence S is a ∇-set if and only if m(S) is minimum, where∇-set denotes a decycling set containing exactly ∇(G) vertices of G. This provides a new way to locate ∇(G) for k-regular graphs G. (c) 4-regular graphs G with the decycling number are determined.

Let G be a graph and a, b and k be nonnegative integers with 1 ≤ a ≤ b. A graph G is defined as all fractional (a, b, k)-critical if after deleting any k vertices of G, the remaining graph has all fractional [a, b]-factors. In this paper, we prove that if , then G is all fractional (a, b, k) -critical. If k = 0, we improve the result given in [Filomat 29 (2015) 757-761]. Moreover, we show that this result is best possible in some sense.

Let G = (VG, EG) be a simple and connected graph. The eccentric connectivity index of G is represented as ξc(G) = Σx∈VG degG(x)ecG(x), where degG(x) and ecG(x) represent the degree and the eccentricity of x, respectively. The eccentric adjacency index of G is represented as ξad(G) = Σx∈VG (SG(x)/ecG(x)), where SQ(x) is the sum of degrees of neighbors of x. In this paper, we determine the trees with the smallest eccentric connectivity index when bipartition size, independence number, and domination number are given. Furthermore, we discuss the trees with the largest eccentric adjacency index when bipartition size, matching number, and independence number are given.

A --critical graph is a graph with connected domination number and for any pair of non-adjacent vertices and of . Let and be respectively the clique number and the independence number of a graph. In this paper, we prove that every --critical graph satisfies for . We also characterize all --critical graphs achieving the upper bound. For , we show that there are infinitely many --critical graphs satisfying . Thus, we conclude this paper with an open problem that every --critical graph for satisfies .

Let G=(V,E) be a graph, and let β∈R. Motivated by a service coverage maximization problem with limited resources, we study the β-differential of G. The β-differential of G, denoted by ∂β(G), is defined as ∂β(G):=max{|B(S)|−β|S|suchthatS⊆V}. The case in which β=1 is known as the differential of G, and hence ∂β(G) can be considered as a generalization of the differential ∂(G) of G. In this paper, upper and lower bounds for ∂β(G) are given in terms of its order |G|, minimum degree δ(G), maximum degree Δ(G), among other invariants of G. Likewise, the β-differential for graphs with heavy vertices is studied, extending the set of applications that this concept can have.

For a given integer $k$, a graph $G$ of order $n$ is called $k$-step Hamiltonian if there is a labeling $v_1,v_2,...,v_n$ of vertices of $G$ such that $d(v_1,v_n)=d(v_i,v_{i+1})=k$ for $i=1,2,...,n-1$. The independence number of a graph is the maximum cardinality of a subset of pair-wise non-adjacent vertices. In this paper we study the independence number in $k$-step Hamiltonian graphs. We present sharp upper bounds as well as sharp lower bounds, and then present a construction that produces infinite families of $k$-step Hamiltonian graphs with arbitrarily large independence number.

The cardinality of a largest independent set of G, denoted by α(G), is called the independence number of G. The independent domination number i(G) of a graph G is the cardinality of a smallest independent dominating set of G. We introduce the concept of the common independence number of a graph G, denoted by αc(G), as the greatest integer r such that every vertex of G belongs to some independent subset X of VG with |X|≥r. The common independence number αc(G) of G is the limit of symmetry in G with respect to the fact that each vertex of G belongs to an independent set of cardinality αc(G) in G, and there are vertices in G that do not belong to any larger independent set in G. For any graph G, the relations between above parameters are given by the chain of inequalities i(G)≤αc(G)≤α(G). In this paper, we characterize the trees T for which i(T)=αc(T), and the block graphs G for which αc(G)=α(G).

The θ-commutators of elements of a group with respect to an automorphism are introduced and their properties are investigated. Also, corresponding to θ-commutators, we define the θ-non-commuting graphs of groups and study their correlations with other notions. Furthermore, we study independent sets in θ-non-commuting graphs, which enable us to evaluate the chromatic number of such graphs.

The size of the maximum independent set (MIS) in a graph G is called the independence number. The size of the minimum connected dominating set (MIN-CDS) in G is called the connected domination number. The aim of this paper is to determine two better upper bounds of the independence number; dependent on the connected domination number for a unit ball graph. Further, we improve the upper bound to obtain the best bound with respect to the upper bounds obtained thus far.

In this paper we obtain closed formulae for several parameters of generalized Sierpiński graphs S(G, t) in terms of parameters of the base graph G. In particular, we focus on the chromatic, vertex cover, clique and domination numbers.

For any graph G of order p, a bijection f:V(G)→{1,2,…,p} is called a numbering of G. The strength strf(G) of a numbering f of G is defined by strf(G)=max{f(u)+f(v)|uv∈E(G)}, and the strength str(G) of a graph G is str(G)=min{strf(G)|f is a numbering of G}. In this paper, many open problems are solved, and the strengths of new families of graphs are determined.

The concept of Sombor index (SO) was recently introduced by Gutman in the chemical graph theory. It is a vertex-degree-based topological index and is denoted by Sombor index SO: SO=SO(G)=∑vivj∈E(G)dG(vi)2+dG(vj)2, where dG(vi) is the degree of vertex vi in G. Here, we present novel lower and upper bounds on the Sombor index of graphs by using some graph parameters. Moreover, we obtain several relations on Sombor index with the first and second Zagreb indices of graphs. Finally, we give some conclusions and propose future work.

Let $R$ be a commutative ring with unity. The comaximal ideal graph of $R$, denoted by $mathcal{C}(R)$, is a graph whose vertices are the proper ideals of $R$ which are not contained in the Jacobson radical of $R$, and two vertices $I_1$ and $I_2$ are adjacent if and only if $I_1 +I_2 = R$. In this paper, we classify all comaximal ideal graphs with finite independence number and present a formula to calculate this number. Also, the domination number of $mathcal{C}(R)$ for a ring $R$ is determined. In the last section, we introduce all planar and toroidal comaximal ideal graphs. Moreover, the commutative rings with isomorphic comaximal ideal graphs are characterized. In particular we show that every finite comaximal ideal graph is isomorphic to some $mathcal{C}(mathbb{Z}_n)$.

Let R R be a commutative ring with unity. The comaximal ideal graph of R R, denoted by C(R) C(R), is a graph whose vertices are the proper ideals of R R which are not contained in the Jacobson radical of R R, and two vertices I 1 I1 and I 2 I2 are adjacent if and only if I 1 +I 2 =R I1+I2=R. In this paper, we classify all comaximal ideal graphs with finite independence number and present a formula to calculate this number. Also, the domination number of C(R) C(R) for a ring R R is determined. In the last section, we introduce all planar and toroidal comaximal ideal graphs. Moreover, the commutative rings with isomorphic comaximal ideal graphs are characterized. In particular we show that every finite comaximal ideal graph is isomorphic to some C(Z n ) C(Zn).

The looseness of a triangulation G on a closed surface F2, denoted by ξ (G), is defined as the minimum number k such that for any surjection c : V (G) → {1, 2, . . . , k + 3}, there is a face uvw of G with c(u), c(v) and c(w) all distinct. We shall bound ξ (G) for triangulations G on closed surfaces by the independence number of G denoted by α(G). In particular, for a triangulation G on the sphere, we have

Let G be a graph, for any real 0≤α≤1, Nikiforov defines the matrix Aα(G) as Aα(G)=αD(G)+(1−α)A(G), where A(G) and D(G) are the adjacency matrix and diagonal matrix of degrees of the vertices of G. This paper presents some extremal results about the spectral radius ρα(G) of the matrix Aα(G). In particular, we give a lower bound on the spectral radius ρα(G) in terms of order and independence number. In addition, we obtain an upper bound for the spectral radius ρα(G) in terms of order and minimal degree. Furthermore, for n>l>0 and 1≤p≤⌊n−l2⌋, let Gp≅Kl∨(Kp∪Kn−p−l) be the graph obtained from the graphs Kl and Kp∪Kn−p−l and edges connecting each vertex of Kl with every vertex of Kp∪Kn−p−l. We prove that ρα(Gp+1)<ρα(Gp) for 1≤p≤⌊n−l2⌋−1.

A graph property is a set of (countable) graphs. A homomorphism from a graph G to a graph H is an edge-preserving map from the vertex set of G into the vertex set of H; if such a map exists, we write G → H. Given any graph H, the hom-property →H is the set of H-colourable graphs, i.e., the set of all graphs G satisfying G → H. A graph property P is of finite character if, whenever we have that F ∈ P for every finite induced subgraph F of a graph G, then we have that G ∈ P too. We explore some of the relationships of the property attribute of being of finite character to other property attributes such as being finitely-induced-hereditary, being finitely determined, and being axiomatizable. We study the hom-properties of finite character, and prove some necessary and some sufficient conditions on H for →H to be of finite character. A notable (but known) sufficient condition is that H is a finite graph, and our new model-theoretic proof of this compactness result extends from hom-properties to all axiomatizable properties. In our quest to find an intrinsic characterization of those H for which →H is of finite character, we find an example of an infinite connected graph with no finite core and chromatic number 3 but with hom-property not of finite character.

We consider a dynamic domination problem for graphs in which an infinite sequence of attacks occur at vertices with guards and the guard at the attacked vertex is required to vacate the vertex by moving to a neighboring vertex with no guard. Other guards are allowed to move at the same time, and before and after each attack and the resulting guard movements, the vertices containing guards form a dominating set of the graph. The minimum number of guards that can successfully defend the graph against such an arbitrary sequence of attacks is the m-eviction number. This parameter lies between the domination and independence numbers of the graph. We characterize the classes of trees for which the m-eviction number equals the domination number and the independence number, respectively.

In the paper we give some theoretical and computational results on the third strong power of cycle-powers, for example, we have found the independence numbers α((C102)√3) = 30 and α((C144)√3) = 14. A number of optimizations have been introduced to improve the running time of our exhaustive algorithm used to establish the independence number of the third strong power of cycle-powers. Moreover, our results establish new exact values and/or lower bounds on the Shannon capacity of noisy channels.

A total dominating set D of a graph G is said to be a secure total dominating set if for every vertex u ∈ V ( G ) \ D , there exists a vertex v ∈ D , which is adjacent to u, such that ( D \ { v } ) ∪ { u } is a total dominating set as well. The secure total domination number of G is the minimum cardinality among all secure total dominating sets of G. In this article, we obtain new relationships between the secure total domination number and other graph parameters: namely the independence number, the matching number and other domination parameters. Some of our results are tight bounds that improve some well-known results.

For positive integers s and t, the Ramsey number R ( s , t ) is the smallest positive integer n such that every graph of order n contains either a clique of order s or an independent set of order t. The triangle-free process begins with an empty graph of order n, and iteratively adds edges chosen uniformly at random subject to the constraint that no triangle is formed. It has been an important tool in studying the asymptotic lower bound for R ( 3 , t ) . Cyclic graphs are vertex-transitive. The symmetry of cyclic graphs makes it easier to compute their independent numbers than related general graphs. In this paper, the cyclic triangle-free process is studied. The sizes of the parameter sets and the independence numbers of the graphs obtained by the cyclic triangle-free process are studied. Lower bounds on R ( 3 , t ) for small t’s are computed, and R ( 3 , 35 ) ≥ 237 , R ( 3 , 36 ) ≥ 244 , R ( 3 , 37 ) ≥ 255 , R ( 3 , 38 ) ≥ 267 , etc. are obtained based on the graphs obtained by the cyclic triangle-free process. Finally, some problems on the cyclic triangle-free process and R ( 3 , t ) are proposed.

The maximum independent set problem is an NP-hard problem. In this paper, we consider Algorithm MAX, which is a polynomial time algorithm for finding a maximal independent set in a graph G. We present a set of forbidden induced subgraphs such that Algorithm MAX always results in finding a maximum independent set of G. We also describe two modifications of Algorithm MAX and sets of forbidden induced subgraphs for the new algorithms.