Series-Parallel Operations with Alpha-Graphs

Among difference vertex labelings of graphs, $\alpha$-labelings are the most restrictive one. A graph is an $\alpha$-graph if it admits an $\alpha$-labeling. In this work, we study a new alternative to construct $\alpha$-graphs using, the well-known, series-parallel operations on smaller $\alpha$-graphs. As an application of the series operation, we show that all members of a subfamily of all trees with maximum degree 4, obtained using vertex amalgamation of copies of the path $P_{11}$, are $\alpha$-graphs. We also show that the one-point union of up to four copies of $K_{n,n}$ is an $\alpha$-graph. In addition we prove that any $\alpha$-graph of order $m$ and size $n$ is an induced subgraph of a graph of order $m+2$ and size $m+n$. Furthermore, we prove that the Cartesian product of the bipartite graph $K_{2,n}$ and the path $P_m$ is an $\alpha$-graph

On the Graceful Cartesian Product of Alpha-Trees

A \emph{graceful labeling} of a graph $G$ of size $n$ is an injective assignment of integers from the set $\{0,1,\dots,n\}$ to the vertices of $G$ such that when each edge has assigned a \emph{weight}, given by the absolute value of the difference of the labels of its end vertices, all the weights are distinct. A graceful labeling is called an $\alpha$-labeling when the graph $G$ is bipartite, with stable sets $A$ and $B$, and the labels assigned to the vertices in $A$ are smaller than the labels assigned to the vertices in $B$. In this work we study graceful and $\alpha$-labelings of graphs. We prove that the Cartesian product of two $\alpha$-trees results in an $\alpha$-tree when both trees admit $\alpha$-labelings and their stable sets are balanced. In addition, we present a tree that has the property that when any number of pendant vertices are attached to the vertices of any subset of its smaller stable set, the resulting graph is an $\alpha$-tree. We also prove the existence of an $\alpha$-labeling of three types of graphs obtained by connecting, sequentially, any number of paths of equal size

Alpha Labeling of Amalgamated Cycles

A graceful labeling of a bipartite graph is an \a-labeling if it has the property that the labels assigned to the vertices of one stable set of the graph are smaller than the labels assigned to the vertices of the other stable set. A concatenation of cycles is a connected graph formed by a collection of cycles, where each cycle shares at most either two vertices or two edges with other cycles in the collection. In this work we investigate the existence of \a-labelings for this kind of graphs, exploring the concepts of vertex amalgamation to produce a family of Eulerian graphs, and edge amalgamation to generate a family of outerplanar graphs. In addition, we determine the number of graphs obtained with $k$ copies of the cycle $C_n$, for both types of amalgamations

New bounds on even cycle creating Hamiltonian paths using expander graphs

We say that two graphs on the same vertex set are $G$-creating if their union (the union of their edges) contains $G$ as a subgraph. Let $H_n(G)$ be the maximum number of pairwise $G$-creating Hamiltonian paths of $K_n$. Cohen, Fachini and K\"orner proved $$n^{\frac{1}{2}n-o(n)}\leq H_n(C_4) \leq n^{\frac{3}{4}n+o(n)}.$$ In this paper we close the superexponential gap between their lower and upper bounds by proving $$n^{\frac{1}{2}n-\frac{1}{2}\frac{n}{\log{n}}-O(1)}\leq H_n(C_4) \leq n^{\frac{1}{2}n+o\left(\frac{n}{\log{n}} \right)}.$$ We also improve the previously established upper bounds on $H_n(C_{2k})$ for $k>3$, and we present a small improvement on the lower bound of F\"uredi, Kantor, Monti and Sinaimeri on the maximum number of so-called pairwise reversing permutations. One of our main tools is a theorem of Krivelevich, which roughly states that (certain kinds of) good expanders contain many Hamiltonian paths.Comment: 14 pages, LaTeX2e; v2: updated Footnote 1 on Page 5; v3: revised version incorporating suggestions by the referees (the changes are mainly in Section 5); v4: final version to appear in Combinatoric

New bounds on even cycle creating Hamiltonian paths using expander graphs

We say that two graphs on the same vertex set are $G$-creating if their union (the union of their edges) contains $G$ as a subgraph. Let $H_n(G)$ be the maximum number of pairwise $G$-creating Hamiltonian paths of $K_n$. Cohen, Fachini and K\"orner proved $$n^{\frac{1}{2}n-o(n)}\leq H_n(C_4) \leq n^{\frac{3}{4}n+o(n)}.$$ In this paper we close the superexponential gap between their lower and upper bounds by proving $$n^{\frac{1}{2}n-\frac{1}{2}\frac{n}{\log{n}}-O(1)}\leq H_n(C_4) \leq n^{\frac{1}{2}n+o\left(\frac{n}{\log{n}} \right)}.$$ We also improve the previously established upper bounds on $H_n(C_{2k})$ for $k>3$, and we present a small improvement on the lower bound of F\"uredi, Kantor, Monti and Sinaimeri on the maximum number of so-called pairwise reversing permutations. One of our main tools is a theorem of Krivelevich, which roughly states that (certain kinds of) good expanders contain many Hamiltonian paths.Comment: 14 pages, LaTeX2e; v2: updated Footnote 1 on Page 5; v3: revised version incorporating suggestions by the referees (the changes are mainly in Section 5); v4: final version to appear in Combinatoric

NCUWM Poster Abstracts 2010

Twelfth Annual Nebraska Conference for Undergraduate Women in Mathematics Poster Abstracts January 29-31, 2010 Holly Arrowood, Furman University Kristen Bretney, Loyola Marymount University Suzanne Carter, University of Iowa Nicole Casella, Ithaca College Morgan Chatham, University of Montevallo Lilith Ciccarelli, Bellarmine University Amber Clinton, Clarkson University Jalonda Coats, Tougaloo College Natalie Coston, Northern Arizona University Belinda Cruz, University of Texas Pan American Anita Doerfler, Northern Arizona University Clarice Dziak, Clarkson University Terra Fox, Hope College Samantha Fuller, Penn State University April Harry, Xavier University of Louisiana Anne Ho, Regis University Rachel Keyser, Bellarmine University Hannah Kolb, Illinois Institute of Technology Lauren Kraus, Wheaton College Amanda Kriesel, Minnesota State University - Mankato Florida Levidiotis, University of Mississippi Emese Lipcsey-Magyar, Skidmore College Melissa Martinez, University of Puerto Rico at Cayey Laura McCormick, Louisiana St. University - Shreveport Brittney Miller, University of Southern California Krista Newell, University of Wisconsin - Oshkosh Catie Patterson, Furman University Katherine Poulsen, Columbia University Stephanie Reed, University of South Dakota Lauren Schmidt, Murray State University Emily Sergel, Rutgers University Ngoc Thai, Truman State University Jasmin Uribe, University of Arizona Kan Wu, Purdue University - Calumet Guangtao Zhang, Clarkson University Jingyu Zhao, Stony Brook Universit

A Classical Sequential Growth Dynamics for Causal Sets

Starting from certain causality conditions and a discrete form of general covariance, we derive a very general family of classically stochastic, sequential growth dynamics for causal sets. The resulting theories provide a relatively accessible ``half way house'' to full quantum gravity that possibly contains the latter's classical limit (general relativity). Because they can be expressed in terms of state models for an assembly of Ising spins living on the relations of the causal set, these theories also illustrate how non-gravitational matter can arise dynamically from the causal set without having to be built in at the fundamental level. Additionally, our results bring into focus some interpretive issues of importance for causal set dynamics, and for quantum gravity more generally.Comment: 28 pages, 9 figures, LaTeX, added references and a footnote, minor correction