#### Date of Award

2010

#### Document Type

Dissertation

#### Degree Name

Doctor of Philosophy in Mathematical Sciences (PhD)

#### College, School or Department Name

Department of Mathematical Sciences

#### Advisor

Donald L Kreher

#### Abstract

In 1969, Lovasz asked whether every connected, vertex-transitive graph has a Hamilton path. This question has generated a considerable amount of interest, yet remains vastly open. To date, there exist no known connected, vertex-transitive graph that does not possess a Hamilton path. For the Cayley graphs, a subclass of vertex-transitive graphs, the following conjecture was made:

**Weak Lovász Conjecture:** Every nontrivial, finite, connected Cayley graph is hamiltonian.

The Chen-Quimpo Theorem proves that Cayley graphs on abelian groups flourish with Hamilton cycles, thus prompting Alspach to make the following conjecture:

**Alspach Conjecture:** Every 2*k*-regular, connected Cayley graph on a finite abelian group has a Hamilton decomposition.

Alspach’s conjecture is true for *k* = 1 and 2, but even the case *k* = 3 is still open. It is this case that this thesis addresses.

Chapters 1–3 give introductory material and past work on the conjecture. Chapter 3 investigates the relationship between 6-regular Cayley graphs and associated quotient graphs. A proof of Alspach’s conjecture is given for the odd order case when *k* = 3. Chapter 4 provides a proof of the conjecture for even order graphs with 3-element connection sets that have an element generating a subgroup of index 2, and having a linear dependency among the other generators.

Chapter 5 shows that if Γ = Cay(A, {s_{1}, s_{2}, s_{3}}) is a connected, 6-regular, abelian Cayley graph of even order, and for some1 ≤ *i* ≤ 3, Δ_{i} = Cay(A/(s_{i}), {sj1 , sj2}) is 4-regular, and Δ_{i} ≄ Cay(ℤ_{3}, {1, 1}), then Γ has a Hamilton decomposition. Alternatively stated, if Γ = Cay(A, S) is a connected, 6-regular, abelian Cayley graph of even order, then Γ has a Hamilton decomposition if S has no involutions, and for some s ∈ S, Cay(A/(s), S) is 4-regular, and of order at least 4.

Finally, the Appendices give computational data resulting from C and MAGMA programs used to generate Hamilton decompositions of certain non-isomorphic Cayley graphs on low order abelian groups.

#### Recommended Citation

Westlund, Erik E., "Hamilton decompositions of 6-regular abelian Cayley graphs", Dissertation, Michigan Technological University, 2010.