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Destroying symmetry by orienting edges: complete graphs and complete bigraphs

Frank Harary; Michael S. Jacobson — 2001

Discussiones Mathematicae Graph Theory

Our purpose is to introduce the concept of determining the smallest number of edges of a graph which can be oriented so that the resulting mixed graph has the trivial automorphism group. We find that this number for complete graphs is related to the number of identity oriented trees. For complete bipartite graphs $K_{s, t}$ , s ≤ t, this number does not always exist. We determine for s ≤ 4 the values of t for which this number does exist.

On non-z(mod k) dominating sets

Yair Caro; Michael S. Jacobson — 2003

Discussiones Mathematicae Graph Theory

For a graph G, a positive integer k, k ≥ 2, and a non-negative integer with z < k and z ≠ 1, a subset D of the vertex set V(G) is said to be a non-z (mod k) dominating set if D is a dominating set and for all x ∈ V(G), |N[x]∩D| ≢ z (mod k).For the case k = 2 and z = 0, it has been shown that these sets exist for all graphs. The problem for k ≥ 3 is unknown (the existence for even values of k and z = 0 follows from the k = 2 case.) It is the purpose of this paper to show that for k ≥ 3 and with...

Weak Saturation Numbers for Sparse Graphs

Ralph J. Faudree; Ronald J. Gould; Michael S. Jacobson — 2013

Discussiones Mathematicae Graph Theory

For a fixed graph F, a graph G is F-saturated if there is no copy of F in G, but for any edge e ∉ G, there is a copy of F in G + e. The minimum number of edges in an F-saturated graph of order n will be denoted by sat(n, F). A graph G is weakly F-saturated if there is an ordering of the missing edges of G so that if they are added one at a time, each edge added creates a new copy of F. The minimum size of a weakly F-saturated graph G of order n will be denoted by wsat(n, F). The graphs of order...

Generalized ramsey theory and decomposable properties of graphs

Stefan A. Burr; Michael S. Jacobson; Peter Mihók; Gabriel Semanišin — 1999

Discussiones Mathematicae Graph Theory

In this paper we translate Ramsey-type problems into the language of decomposable hereditary properties of graphs. We prove a distributive law for reducible and decomposable properties of graphs. Using it we establish some values of graph theoretical invariants of decomposable properties and show their correspondence to generalized Ramsey numbers.

Forbidden triples implying Hamiltonicity: for all graphs

Ralph J. Faudree; Ronald J. Gould; Michael S. Jacobson — 2004

Discussiones Mathematicae Graph Theory

In [2], Brousek characterizes all triples of graphs, G₁, G₂, G₃, with $G_{i} = K_{1, 3}$ for some i = 1, 2, or 3, such that all G₁G₂G₃-free graphs contain a hamiltonian cycle. In [6], Faudree, Gould, Jacobson and Lesniak consider the problem of finding triples of graphs G₁, G₂, G₃, none of which is a $K_{1, s}$ , s ≥ 3 such that G₁, G₂, G₃-free graphs of sufficiently large order contain a hamiltonian cycle. In this paper, a characterization will be given of all triples G₁, G₂, G₃ with none being $K_{1, 3}$ , such that all G₁G₂G₃-free...

Potential forbidden triples implying hamiltonicity: for sufficiently large graphs

Ralph J. Faudree; Ronald J. Gould; Michael S. Jacobson — 2005

Discussiones Mathematicae Graph Theory

In [1], Brousek characterizes all triples of connected graphs, G₁,G₂,G₃, with $G_{i} = K_{1, 3}$ for some i = 1,2, or 3, such that all G₁G₂ G₃-free graphs contain a hamiltonian cycle. In [8], Faudree, Gould, Jacobson and Lesniak consider the problem of finding triples of graphs G₁,G₂,G₃, none of which is a $K_{1, s}$ , s ≥ 3 such that G₁G₂G₃-free graphs of sufficiently large order contain a hamiltonian cycle. In [6], a characterization was given of all triples G₁,G₂,G₃ with none being $K_{1, 3}$ , such that all G₁G₂G₃-free graphs are...

On a conjecture of quintas and arc-traceability in upset tournaments

Arthur H. Busch; Michael S. Jacobson; K. Brooks Reid — 2005

Discussiones Mathematicae Graph Theory

A digraph D = (V,A) is arc-traceable if for each arc xy in A, xy lies on a directed path containing all the vertices of V, i.e., hamiltonian path. We prove a conjecture of Quintas [7]: if D is arc-traceable, then the condensation of D is a directed path. We show that the converse of this conjecture is false by providing an example of an upset tournament which is not arc-traceable. We then give a characterization for upset tournaments in terms of their score sequences, characterize which arcs of...

On isopart parameters of complete bipartite graphs and $n$ -cubes

Richard M. Davitt; John Frederick Fink; Michael S. Jacobson — 1985

Mathematica Slovaca

Chvátal-Erdös type theorems

Jill R. Faudree; Ralph J. Faudree; Ronald J. Gould; Michael S. Jacobson; Colton Magnant — 2010

Discussiones Mathematicae Graph Theory

The Chvátal-Erdös theorems imply that if G is a graph of order n ≥ 3 with κ(G) ≥ α(G), then G is hamiltonian, and if κ(G) > α(G), then G is hamiltonian-connected. We generalize these results by replacing the connectivity and independence number conditions with a weaker minimum degree and independence number condition in the presence of sufficient connectivity. More specifically, it is noted that if G is a graph of order n and k ≥ 2 is a positive integer such that κ(G) ≥ k, δ(G) > (n+k²-k)/(k+1),...

Saturation Spectrum of Paths and Stars

Jill Faudree; Ralph J. Faudree; Ronald J. Gould; Michael S. Jacobson; Brent J. Thomas — 2017

Discussiones Mathematicae Graph Theory

A graph G is H-saturated if H is not a subgraph of G but the addition of any edge from G̅ to G results in a copy of H. The minimum size of an H-saturated graph on n vertices is denoted sat(n,H), while the maximum size is the well studied extremal number, ex(n,H). The saturation spectrum for a graph H is the set of sizes of H saturated graphs between sat(n,H) and ex(n,H). In this paper we completely determine the saturation spectrum of stars and we show the saturation spectrum of paths is continuous...

Linear forests and ordered cycles

Guantao Chen; Ralph J. Faudree; Ronald J. Gould; Michael S. Jacobson; Linda Lesniak; Florian Pfender — 2004

Discussiones Mathematicae Graph Theory

A collection $L = P ¹ \cup P ² \cup . . . \cup P^{t}$ (1 ≤ t ≤ k) of t disjoint paths, s of them being singletons with |V(L)| = k is called a (k,t,s)-linear forest. A graph G is (k,t,s)-ordered if for every (k,t,s)-linear forest L in G there exists a cycle C in G that contains the paths of L in the designated order as subpaths. If the cycle is also a hamiltonian cycle, then G is said to be (k,t,s)-ordered hamiltonian. We give sharp sum of degree conditions for nonadjacent vertices that imply a graph is (k,t,s)-ordered hamiltonian.