Prime Vertex Labelings of Graphs

This poster was presented by my undergraduate research students Hannah Prawzinsky and Emily White on January 25, 2015 at the Nebraska Conference for Undergraduate Women in Mathematics at the University of Nebraska-Lincoln.

This research was supported by the National Science Foundation grant #DMS-1148695 through the Center for Undergraduate Research (CURM).

Transcript

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   On Prime Vertex Labelings
   Hannah Prawzinsky & Emily White, Directed by Dana C. Ernst & Jeff Rushall
   Department of Mathematics & Statistics, Northern Arizona University
   Definition
   A graph G(V , E) is a set V of vertices and a set E of edges connecting some
   (possibly empty) subset of those vertices.
   Examples
   Definition
   A simple graph is a graph that contains neither “loops” nor multiple edges
   between vertices.
   Example
   Here is an example of a graph that is not simple.
   Definition
   1. The path Pn
   consists of n vertices and n − 1 edges such that 2 vertices
   have degree 1 and n − 2 vertices have degree 2. Here is P8
   .
   2. The cycle Cn
   consists of n vertices and n edges such that each vertex has
   degree 2. Here is C12
   .
   3. The star Sn
   consists of one vertex of degree n and n vertices of degree one.
   Here is S5
   .
   Definition
   G1 ⋆ G2
   is the graph that results from “selectively gluing” copies of G2
   to
   specified vertices of G1
   . Here is C4 ⋆ P2 ⋆ S4
   .
   Definition
   A graph labeling is an “assignment” of integers (possibly satisfying some con-
   ditions) to the vertices, edges, or both. Formal graph labelings are functions.
   Here is a vertex labeling, an edge labeling, and a vertex-edge labeling, respec-
   tively, for C4
   .
   2 3 2 3
   1 4 1 4
   1
   2
   3
   4
   1
   2 3
   4
   Definition
   A graph with n vertices has a prime vertex labeling if its vertices can be labeled
   with the integers 1, 2, 3, . . . , n such that every pair of adjacent vertices (i.e.,
   vertices that share an edge) have labels that are relatively prime.
   Theorem
   1. All paths have a prime vertex labeling.
   1 2 3 4 5 6 7 8
   2. All cycles have a prime vertex labeling.
   1
   2
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   5
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   7
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   11
   12
   3. All starts have a prime vertex labeling.
   1
   2
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   5 6
   Conjecture (Seoud)
   All unicyclic graphs (i.e., graphs containing exactly one cycle) have a prime vertex
   labeling.
   Theorem (Prawzinksky & White et al.)
   1. Every Cn ⋆ P2 ⋆ S3
   has a prime vertex labeling.
   2. Every Cn ⋆ P2 ⋆ S3 ⋆ S3
   has a prime vertex labeling.
   Labeling for Part 1 of Theorem
   Let c1, c2, . . . , cn
   denote the cycle labels, let p1, p2, . . . , pn
   denote the vertices
   adjacent to the corresponding cycle vertices and let the pendant vertices off of
   pi
   be denoted si,j, 1 ≤ j ≤ 3. The labeling function f : V → {1, 2, . . . , 5n} is
   given by:
   f (ci
   ) = 5i − 4, 1 ≤ i ≤ n
   f (pi
   ) =
   {
   5i − 2, if i is odd
   5i − 3, if i is even
   f (si,j
   ) =
   
   
   
   
   
   5i − 3 + j, i is even
   5i − 2 + j, j ̸= 3 and i is odd
   5i − 3, j = 3 and i is odd
   Example
   Here is an example of the labeling for part 1 of our theorem.
   1
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   2 5
   11
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   15
   6
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   Additional New Results
   We have also discovered prime vertex labelings for the following graphs:
   1. Double and triple-tailed tadpoles
   2. Cn ⋆ P2 ⋆ Sk
   for 3 ≤ k ≤ 8
   3. Cn ⋆ S3
   , Cn ⋆ S5
   , Cn ⋆ S7
   4. Cn ⋆ 3P2
   , Cn ⋆ 5P2
   , Cn ⋆ 7P2
   In addition, we have obtained results for a few families of non-unicyclic graphs.
   Email: Hannah [hpp3@nau.edu], Emily [ekw49@nau.edu] Typeset using L
   A
   TEX, TikZ, and beamerposter
   

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