Communications in Combinatorics and Optimization
Volume:6 Issue: 2, Summer-Autumn 2021

Let k ≥ 1 be an integer, and let G be a finite and simple graph with vertex set V (G). A signed total Italian k-dominating function (STIkDF) on a graph G is a functionf : V (G) → {−1, 1, 2} satisfying the conditions that $sum_{xin N(v)}f(x)ge k$ for each vertex v ∈ V (G), where N(v) is the neighborhood of $v$, and each vertex u with f(u)=-1 is adjacent to a vertex v with f(v)=2 or to two vertices w and z with f(w)=f(z)=1. The weight of an STIkDF f is$omega(f)=sum_{vin V(G)}f(v)$. The signed total Italian k-domination number $gamma_{stI}^k(G)$ of G is the minimum weight of an STIkDF on G. In this paper we initiate the study of the signed total Italian k-dominationnumber of graphs, and we present different bounds on $gamma_{stI}^k(G)$. In addition, we determine thesigned total Italian k-domination number of some classes of graphs. Some of our results are extensions ofwell-known properties of the signed total Roman $k$-domination number $gamma_{stR}^k(G)$,introduced and investigated by Volkmann [9,12].

Let G=(V,E) be a simple connected graph of order p and size q. A decomposition of a graph G is a collection π of edge-disjoint subgraphs G_1,G_2,…,G_n of G such that every edge of G belongs to exactly one G_i,(1≤i ≤n). The decomposition 〖π={G〗_1,G_2,…,G_n} of a connected graph G is said to be a distinct edge geodetic decomposition if g_1 (G_i )≠g_1 (G_j ),(1≤i≠j≤n). The maximum cardinality of π is called the distinct edge geodetic decomposition number of G and is denoted by π_dg1 (G), where g_1 (G) is the edge geodetic number of G. Some general properties satisfied by this concept are studied. Connected graphs which are edge geodetic decomposable are characterized. Connected distinct edge geodetic decomposable graphs of order p with π_dg1 (G)= p-2 are characterised.

A total Roman dominating function on a graph $G$ is a function $f: V(G) rightarrow {0,1,2}$ such that for every vertex $vin V(G)$ with $f(v)=0$ there exists a vertex $uin V(G)$ adjacent to $v$ with $f(u)=2$, and the subgraph induced by the set ${xin V(G): f(x)geq 1}$ has no isolated vertices. The total Roman domination number of $G$, denoted $gamma_{tR}(G)$, is the minimum weight $omega(f)=sum_{vin V(G)}f(v)$ among all total Roman dominating functions $f$ on $G$.It is known that $gamma_{tR}(G)geq gamma_{t2}(G)+gamma(G)$ for any graph $G$ with neither isolated vertex nor components isomorphic to $K_2$, where $gamma_{t2}(G)$ and $gamma(G)$ represent the semitotal domination number and the classical domination number, respectively. In this paper we give a constructive characterization of the trees that satisfy the equality above.

The concept of Lucky k-polynomials and in particular Lucky χ-polynomials was recently introduced. This paper introduces Stirling number of the fourth kind and Lucky partitions of a finite set in order to determine either the Lucky k- or Lucky χ-polynomial of a graph. The integer partitions influence Stirling partitions of the second kind.

A subset D of vertices of a graph G is a dominating set if for each u ∈ V (G) D, u is adjacent to somevertex v ∈ D. The domination number, γ(G) ofG, is the minimum cardinality of a dominating set of G. A setD ⊆ V (G) is a total dominating set if for eachu ∈ V (G), u is adjacent to some vertex v ∈ D. Thetotal domination number, γt (G) of G, is theminimum cardinality of a total dominating set of G. For an eveninteger $nge 2$ and $1le Delta le lfloorlog_2nrfloor$, aKnodel graph $W_{Delta,n}$ is a $Delta$-regularbipartite graph of even order n, with vertices (i,j), for$i=1,2$ and $0le jle n/2-1$, where for every $j$, $0le jlen/2-1$, there is an edge between vertex $(1, j)$ and every vertex$(2,(j+2^k-1)$ mod (n/2)), for $k=0,1,cdots,Delta-1$. In thispaper, we determine the total domination number in $3$-regularKnodel graphs $W_{3,n}$.

Let R be a commutative ring with non-zero identity. The annihilator-inclusion ideal graph of R , denoted by ξR, is a graph whose vertex set is the of allnon-zero proper ideals of $R$ and two distinct vertices $I$ and $J$ are adjacentif and only if either Ann(I) ⊆ J or Ann(J) ⊆ I. In this paper, we investigate the basicproperties of the graph ξR. In particular, we showthat ξR is a connected graph with diameter at most three, andhas girth 3 or ∞. Furthermore, we determine all isomorphic classes of non-local Artinian rings whose annihilator-inclusion ideal graphs have genus zero or one.

The variable sum exdeg index of a graph G is defined as $SEI_a(G)=sum_{uin V(G)}d_G(u)a^{d_G(u)}$, where $aneq 1$ is a positive real number,  du(u) is the degree of a vertex u ∈ V (G). In this paper, we characterize the graphs with the extremum variable sum exdeg index among all the graphs having a fixed number of vertices and cut edges, for every a>1.

Let G=(V,E), $V={v_1,v_2,ldots,v_n}$, be a simple connected graph with $%n$ vertices, $m$ edges and a sequence of vertex degrees $d_1geqd_2geqcdotsgeq d_n>0$, $d_i=d(v_i)$. Let ${A}=(a_{ij})_{ntimes n}$ and ${%D}=mathrm{diag }(d_1,d_2,ldots , d_n)$ be the adjacency and the diagonaldegree matrix of $G$, respectively. Denote by ${mathcal{L}^+}(G)={D}^{-1/2}(D+A) {D}^{-1/2}$ the normalized signless Laplacian matrix of graph $G$. Theeigenvalues of matrix $mathcal{L}^{+}(G)$, $2=gamma _{1}^{+}geq gamma_{2}^{+}geq cdots geq gamma _{n}^{+}geq 0$, are normalized signlessLaplacian eigenvalues of $G$. In this paper some bounds for the sum $%K^{+}(G)=sum_{i=1}^nfrac{1}{gamma _{i}^{+}}$ are considered.

‎A Roman dominating function (RDF) on a graph G=(V,E) is a function  f : V → {0, 1, 2}  such that every vertex u for which f(u)=0 is‎ ‎adjacent to at least one vertex v for which f(v)=2‎. ‎An RDF f is called‎‎an outer independent Roman dominating function (OIRDF) if the set of‎‎vertices assigned a 0 under f is an independent set‎. ‎The weight of an‎‎OIRDF is the sum of its function values over all vertices‎, ‎and the outer‎‎independent Roman domination number ΥoiR (G) is the minimum weight‎‎of an OIRDF on $G$‎. ‎In this paper‎, ‎we show that if T is a tree of order n ≥ 3 with s(T) support vertices‎, ‎then $gamma _{oiR}(T)leq min‎ {‎frac{5n}{6},frac{3n+s(T)}{4}}.$ Moreover‎, ‎we characterize the tress‎‎attaining each bound‎.

A set S of vertices in a graph G=(V,E) is a dominating set ofG if every vertex of V-S is adjacent to some vertex of S.For an integer k≥1, a set S of vertices is a k-step dominating set if any vertex of $G$ is at distance k from somevertex of S. In this paper, using membership values of vertices and edges in fuzzy graphs, we introduce the concepts of strength of strongestdominating set as well as strength of strongest$k$-step dominating set in fuzzy graphs. We determine various bounds forthese parameters in fuzzy graphs. We also determine the strengthof strongest dominating set in some families of fuzzy graphsincluding complete fuzzy graphs and complete bipartite fuzzygraphs.

The concept of super line graph was introduced in the year 1995 by Bagga, Beineke and Varma. Given a graph with at least r edges, the super line graph of index r, Lr(G), has as its vertices the sets of r-edges of G, with two adjacent if there is an edge in one set adjacent to an edge in the other set. The line completion number lc(G) of a graph G is the least positive integer r for which Lr(G) is a complete graph. In this paper, we find the line completion number of grid graph Pn × Pm for various cases of n and m.

‎Let G be a graph‎. ‎A 2-rainbow dominating function (or‎ 2-RDF) of G is a function f from V(G)‎ ‎to the set of all subsets of the set {1,2}‎ ‎such that for a vertex v ∈ V (G) with f(v) = ∅, ‎the‎‎condition $bigcup_{uin N_{G}(v)}f(u)={1,2}$ is fulfilled‎, wher NG(v)  is the open neighborhood‎‎of v‎. ‎The weight of 2-RDF f of G is the value‎‎$omega (f):=sum _{vin V(G)}|f(v)|$‎. ‎The 2-rainbow‎‎domination number of G‎, ‎denoted by Υr2 (G)‎, ‎is the‎‎minimum weight of a 2-RDF of G‎. ‎A 2-RDF f is called an outer independent 2-rainbow dominating function ‎(or OI2-RDF) of G if‎‎the set of all v ∈ V (G) with f(v) = ∅ is an‎ ‎independent set‎. ‎The outer independent 2-rainbow domination number Υoir2  (G) is‎‎the minimum weight of an OI2-RDF of G‎. ‎In this paper‎, ‎we obtain the‎‎outer independent 2-rainbow domination number of Pm□Pn‎ ‎and‎ Pm□Cn‎. ‎Also we determine the value of Υoir2  (Cm2Cn) when m or n is even‎.