Discrete Mathematics and Combinatorics Commons^™

Edge Colorings Of K(M,N) With M+N-1 Colors Which Forbid Rainbow Cycles, Peter Johnson, Claire Zhang

Theory & Applications of Graphs

For positive integers m, n, the greatest number of colors that can appear in an edge coloring of K(m,n) which avoids rainbow cycles is m + n - 1. Here these colorings are constructively characterized. It turns out that these colorings can be encoded by certain vertex labelings of full binary trees with m + n leafs.

An Eternal Domination Problem In Grids, William Klostermeyer, Margaret-Ellen Messinger, Alejandro Angeli Ayello

Theory & Applications of Graphs

A dynamic domination problem in graphs is considered in which an infinite sequence of attacks occur at vertices with mobile guards; 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, the vertices containing guards must form a dominating set of the graph. The minimum number of guards that can defend the graph against such an arbitrary sequence of attacks is called the m-eviction number of the graph. In this paper, the m-eviction …

Two Short Proofs Of The Perfect Forest Theorem, Yair Caro, Josef Lauri, Christina Zarb

Theory & Applications of Graphs

A perfect forest is a spanning forest of a connected graph G, all of whose components are induced subgraphs of G and such that all vertices have odd degree in the forest. A perfect forest can be thought of as a generalization of a perfect matching since, in a matching, all components are trees on one edge. Scott first proved the Perfect Forest Theorem, namely, that every connected graph of even order has a perfect forest. Gutin then gave another proof using linear algebra.

We give here two very short proofs of the Perfect Forest Theorem which use only …

Application Of An Extremal Result Of Erdős And Gallai To The (N,K,T) Problem, Matt Noble, Peter Johnson, Dean Hoffman, Jessica Mcdonald

Theory & Applications of Graphs

An extremal result about vertex covers, attributed by Hajnal to Erdős and Gallai, is applied to prove the following: If n, k, and t are integers satisfying n ≥ k ≥ t ≥ 3 and k ≤ 2t - 2, and G is a graph with the minimum number of edges among graphs on n vertices with the property that every induced subgraph on k vertices contains a complete subgraph on t vertices, then every component of G is complete.

Path-Factors Involving Paths Of Order Seven And Nine, Yoshimi Egawa, Michitaka Furuya

Theory & Applications of Graphs

In this paper, we show the following two theorems (here c_i(G-Χ) is the number of components C of G-Χ with |V(C)|=i): (i) If a graph G satisfies c₁(G-Χ)+⅓c₃(G-Χ)+⅓c₅(G-Χ) ≤ ⅔ |Χ| for all Χ ≤ V(G), then G has a {Ρ₂},Ρ₇}-factor. (ii)If a graph G satisfies c₁}(G-Χ)+c₃}(G-Χ)+⅔ c₅}(G-Χ)+⅓c₇}(G-Χ) ≤ ⅔|Χ| for all Χ⊆ V(G), then G has a {P₂,P₉}-factor.

Classifying Resolving Lists By Distances Between Members, Paul Feit

Theory & Applications of Graphs

Let G be a connected graph and let w₁,…w_r be a list of vertices. We refer the choice of a triple (r;G;w₁,…w_r), as a metric selection. Let ρ be the shortest path metric of G. We say that w₁,…w_r resolves G (metricly) if the function c:V(G) → ℤ^r given by

x → (ρ (w1,x),…,ρ(wr,x))

is injective. We refer to c as the code map and to its image as the codes of the triple (r;G;w₁,…,w_r).

This paper proves basic results …

Induced Subgraph Saturated Graphs, Craig M. Tennenhouse

Theory & Applications of Graphs

A graph G is said to be H-saturated if G contains no subgraph isomorphic to H but the addition of any edge between non-adjacent vertices in G creates one. While induced subgraphs are often studied in the extremal case with regard to the removal of edges, we extend saturation to induced subgraphs. We say that G is induced H-saturated if G contains no induced subgraph isomorphic to H and the addition of any edge to G results in an induced copy of H. We demonstrate constructively that there are non-trivial examples of saturated graphs for …

On Minimum Spanning Subgraphs Of Graphs With Proper Connection Number 2, Zhenming Bi, Gary Chartrand, Garry L. Johns, Ping Zhang

Theory & Applications of Graphs

An edge coloring of a connected graph G is a proper-path coloring if every two vertices of G are connected by a properly colored path. The minimum number of colors required of a proper-path coloring of G is called the proper connection number pc(G) of G. For a connected graph G with proper connection number 2, the minimum size of a connected spanning subgraph H of G with pc(H) = 2 is denoted by μ(G). It is shown that if s and t are integers such that t ≥ s + 2 ≥ 5 …

Eternal Independent Sets In Graphs, Yair Caro, William F. Klostermeyer

Theory & Applications of Graphs

The use of mobile guards to protect a graph has received much attention in the literature of late in the form of eternal dominating sets, eternal vertex covers and other models of graph protection. In this paper, eternal independent sets are introduced. These are independent sets such that the following can be iterated forever: a vertex in the independent set can be replaced with a neighboring vertex and the resulting set is independent.

Group-Antimagic Labelings Of Multi-Cyclic Graphs, Dan Roberts, Richard M. Low

Theory & Applications of Graphs

Let Α be a non-trivial abelian group. A connected simple graph G = (V, E) is Α-antimagic if there exists an edge labeling f: Ε(G) → A \{0} such that the induced vertex labeling f+: V(G) → Α, defined by f+(v) = Σ{f(u,v): (u, v) ∈ E(G)}, is a one-to-one map. The integer-antimagic spectrum of a graph G is the set IAM(G) = {κ: G is ℤ_k - antimagic and κ ≥ 2}. In this paper, we analyze the integer-antimagic spectra for various classes of multi-cyclic graphs.

An Edge-Swap Heuristic For Finding Dense Spanning Trees, Mustafa Ozen, Hua Wang, Kai Wang, Demet Yalman

Theory & Applications of Graphs

Finding spanning trees under various restrictions has been an interesting question to researchers. A "dense" tree, from a graph theoretical point of view, has small total distances between vertices and large number of substructures. In this note, the "density" of a spanning tree is conveniently measured by the weight of a tree (defined as the sum of products of adjacent vertex degrees). By utilizing established conditions and relations between trees with the minimum total distance or maximum number of sub-trees, an edge-swap heuristic for generating "dense" spanning trees is presented. Computational results are presented for randomly generated graphs and specific …

An Extremal Problem For Finite Lattices, John Goldwasser, Brendan Nagle, Andres Saez

Theory & Applications of Graphs

For a fixed M x N integer lattice L(M,N), we consider the maximum size of a subset A of L(M,N) which contains no squares of prescribed side lengths k(1),...,k(t). We denote this size by ex(L(M,N), {k(1),...,k(t)}), and when t = 1, we abbreviate this parameter to ex(L(M,N), k), where k = k(1).

Our first result gives an exact formula for ex(L( …

A Class Of Graphs Approaching Vizing's Conjecture, Aziz Contractor, Elliot Krop

Theory & Applications of Graphs

For any graph G=(V,E), a subset S ⊆ V dominates G if all vertices are contained in the closed neighborhood of S, that is N[S]=V. The minimum cardinality over all such S is called the domination number, written γ(G). In 1963, V.G. Vizing conjectured that γ(G □ H) ≥ γ(G)γ(H) where □ stands for the Cartesian product of graphs. In this note, we define classes of graphs Α_n, for n≥0, so that every graph belongs to some such class, and Α₀ corresponds to class A of Bartsalkin and German. We prove that …

An Isomorphism Problem In Z2, Matt Noble

Theory & Applications of Graphs

We consider Euclidean distance graphs with vertex set ℚ² or ℤ² and address the possibility or impossibility of finding isomorphisms between such graphs. It is observed that for any distances d₁, d₂ the non-trivial distance graphs G(ℚ², d₁) and G(ℚ², d₂) are isomorphic. Ultimately it is shown that for distinct primes _p1, _p2 the non-trivial distance graphs G(ℤ², √p1) and G(ℤ², √p2) are not isomorphic. We conclude with a few additional questions related to this work.

Properly Colored Notions Of Connectivity - A Dynamic Survey, Xueliang Li, Colton Magnant

Theory & Applications of Graphs

A path in an edge-colored graph is properly colored if no two consecutive edges receive the same color. In this survey, we gather results concerning notions of graph connectivity involving properly colored paths.

Dynamic Approach To K-Forcing, Yair Caro, Ryan Pepper

Theory & Applications of Graphs

The k-forcing number of a graph is a generalization of the zero forcing number. In this note, we give a greedy algorithm to approximate the k-forcing number of a graph. Using this dynamic approach, we give corollaries which improve upon two theorems from a recent paper of Amos, Caro, Davila and Pepper [2], while also answering an open problem posed by Meyer [9].

Second Hamiltonian Cycles In Claw-Free Graphs, Hossein Esfandiari, Colton Magnant, Pouria Salehi Nowbandegani, Shirdareh Haghighi

Theory & Applications of Graphs

Sheehan conjectured in 1975 that every Hamiltonian regular simple graph of even degree at least four contains a second Hamiltonian cycle. We prove that most claw-free Hamiltonian graphs with minimum degree at least 3 have a second Hamiltonian cycle and describe the structure of those graphs not covered by our result. By this result, we show that Sheehan’s conjecture holds for claw-free graphs whose order is not divisible by 6. In addition, we believe that the structure that we introduce can be useful for further studies on claw-free graphs.

Bounds For The Zero Forcing Number Of Graphs With Large Girth, Randy Davila, Franklin Kenter

Theory & Applications of Graphs

The zero-forcing number, Ζ(G) is an upper bound for the maximum nullity of all symmetric matrices with a sparsity pattern described by the graph. A simple lower bound is δ ≤ Ζ(G) where δ is the minimum degree. An improvement of this bound is provided in the case that G has girth of at least 5. In particular, it is shown that 2δ − 2 ≤ Ζ(G) for graphs with girth of at least 5; this can be further improved when G has a small cut set. Lastly, a conjecture is made regarding a lower bound for Ζ(G) as a …

Scheduling N Burgers For A K-Burger Grill: Chromatic Numbers With Restrictions, Peter Johnson, Xiaoya Zha

Theory & Applications of Graphs

The chromatic number has a well-known interpretation in the area of scheduling. If the vertices of a finite, simple graph are committees, and adjacency of two committees indicates that they must never be in session simultaneously, then the chromatic number of the graph is the smallest number of hours during which the committees/vertices of the graph may all have properly scheduled meetings of one continuous hour each. Slivnik [3] showed that the fractional chromatic number can be similarly characterized. In that characterization, the meetings are allowed to be broken into a finite number of disjoint intervals. Here we consider chromatic …

Path-Tables Of Trees: A Survey And Some New Results, Kevin Asciak

Theory & Applications of Graphs

The (vertex) path-table of a tree Τ contains quantitative information about the paths in Τ. The entry (i,j) of this table gives the number of paths of length j passing through vertex ^vi. The path-table is a slight variation of the notion of path layer matrix. In this survey we review some work done on the vertex path-table of a tree and also introduce the edge path-table. We show that in general, any type of path-table of a tree Τ does not determine Τ uniquely. We shall show that in trees, the number of paths passing through …

Connection And Separation In Hypergraphs, Mohammad A. Bahmanian, Mateja Sajna

Theory & Applications of Graphs

In this paper we study various fundamental connectivity properties of hypergraphs from a graph-theoretic perspective, with the emphasis on cut edges, cut vertices, and blocks. We prove a number of new results involving these concepts. In particular, we describe the exact relationship between the block decomposition of a hypergraph and the block decomposition of its incidence graph.

Rainbow Generalizations Of Ramsey Theory - A Dynamic Survey, Shinya Fujita, Colton Magnant, Yaping Mao, Kenta Ozeki

Theory & Applications of Graphs

In this work, we collect Ramsey-type results concerning rainbow edge colorings of graphs.

Note On Rainbow Connection In Oriented Graphs With Diameter 2, Rebecca Holliday, Colton Magnant, Pouria Salehi Nowbandegani

Theory & Applications of Graphs

In this note, we provide a sharp upper bound on the rainbow connection number of tournaments of diameter 2. For a tournament Τ of diameter 2, we show 2 ≤ → rc (Τ) ≤ 3. Furthermore, we provide a general upper bound on the rainbow κ-connection number of tournaments as a simple example of the probabilistic method. Finally, we show that an edge-colored tournament of κ^thdiameter 2 has rainbow κ-connection number at most approximately κ².

Hamiltonicity And Sigma-Hypergraphs, Christina Zarb

Theory & Applications of Graphs

We define and study a special type of hypergraph. A σ-hypergraph Η= Η(n,r,q |σ), where σ is a partition of r, is an r-uniform hypergraph having nq vertices partitioned into n classes of q vertices each. If the classes are denoted by V1, V2,...,Vn, then a subset Κ of V(Η) of size r is an edge if the partition of r formed by the non-zero cardinalities |Κ ∩ Vi |, 1 ≤ i ≤ n, is σ. The non-empty intersections Κ ∩ Vi are called the parts of Κ, and s(σ) …