Abstract. A graph is called a maximalnonhamiltonian graph if it is nonhamiltonian and will be hamiltonian by adding any new edge. This paper
presents a conjecture that if C is a longest cycle in a maximalnonhamiltonian graph G then G−C is a complete graph and shows that the conjecture
is true for some classes of graphs.

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JOURNAL OF SCIENCE OF HNUE
Natural Sci., 2008, Vol. 53, N
◦
. 5, pp. 49-53
LONGEST CYCLES AND RESTGRAPH
IN MAXIMALNONHAMITONIAN GRAPHS
Vu Dinh Hoa
Hanoi National University of Education
Abstract. A graph is called a maximalnonhamiltonian graph if it is non-
hamiltonian and will be hamiltonian by adding any new edge. This paper
presents a conjecture that if C is a longest cycle in a maximalnonhamilto-
nian graph G then G−C is a complete graph and shows that the conjecture
is true for some classes of graphs.
1. Terminology and notation
We consider only undirected graphs without loop or multiple edges. Our ter-
minology is standard except as indicated. We begin by introducing some definitions
and notation. Let ω(G) denote the number of components of a graph G. Following
Chv¡tal [3] a graph G is 1-tough if |S| ≥ ω(G − S) for any nonempty subset S of
the vertex set V (G) of G. G is called nontough if there exists a nonempty subset
S such that |S| < ω(G − S). We shall denote the number of vertices of G by n.
We often study special spanning graphs, for example the cubic graphs [8] or the
hamiltonian cycles. A graph G is called a hamiltonian graph if G contains a hamil-
tonian cycle, i.e.a cycle of length n. Otherwise, G is nonhamiltonian. Similarly, a
path is called a hamiltonian path if it contains all vertices in G. A graph G is called
a maximalnonhamiltonian graph if G is nonhamiltonian and will be hamiltonian by
adding any new edge. Clearly, for every nonadjacent vertices u and v in a maximal-
nonhamiltonian graph G there is a hamiltonian path joining u with v. A cycle C
of G is a dominating cycle if G− C is an edgeless graph and G is called dominated
by C or dominable. The length `(C) of a longest cycle C in a graph G, called the
circumference of G, will be denoted by c(G). For k ≤ α the independent number we
denote by σk the minimum value of the degree sum of any k pairwise nonadjacent
vertices. For k > α let σk =∞. Instead of σ1 we use the more common notation δ.
Following Skupi²n [14], a graph G is called homogeneously traceable if for each
vertex x of G there is a Hamiltonian path beginning at x. As Skupi²n defined in [12],
pi(G) denotes the path partition number of G, i.e. pi(G) = −1 if G is hamiltonian
connected; and pi(G) = 0 if G is hamiltonian but not hamiltonian-connected; and
49
Vu Dinh Hoa
pi(G) is the smallest number such that there exists a system of pairwise disjoint
paths P1, ... , Pt containing all vertices of G if G is not hamiltonian. This parameter
has been investigated by Goodman and Hedetniemi [5] and Boesch et al. [2]. In
[12,13] some relations between path partition number and hamiltonicity of graphs
are proved which allow us to get some upper bounds of pi(G). Similarly, we can ask
for an upper bound of pi(G) if G satisfies toughness conditions in the well-known
conjecture of Chv¡tal on 2-tough graphs. In fact, it is a special case of the question
posed by Skupi²n in the problem 1 in [12].
In this paper we also consider a special class of graphs with pi(G) ≤ 2 as
follows. Let G be a graph with a Hamilton cycle and let S be a non-empty subset of
V (G). Then we have pi(G−S) ≤ |S| and therefore pi(G−S) ≤ |S| for all non-empty
sets S ⊆ V (G) is a necessary condition for hamiltonicity. This condition appeared
already in Haggkvist [6] and Schiermeyer's definition of path-tough graphs [11]. In
order to preclude trivialities we put that definition as follows. A graph G is called
path-tough if G has at least three vertices and pi(G− S) ≤ |S| for all nonempty sets
S ≤ V (G). Clearly, every homogeneously traceable graph is a path-tough graph.
And, we have pi(G) ≤ 2 if G is a path-tough graph since for an arbitrary vertex v
of G the graph G− {v} contains a hamiltonian path.
Maximalnonhamiltonian tough graphs fall into 1-tough graphs with total ver-
tices (vertices joining to all other vertices) and path-tough graphs without total
vertices. In [15], conditions for the existence of hamiltonian cycles in 1-tough graphs
with total vertices are proved. But, the study for hamiltonian cycles in path-tough
graphs [6,11] seem very difficult.
2. Results and conjecture
We begin with a well-known theorem of Nash-Williams [7].
Theorem 2.1. Let G be a 2-connected graph on n vertices with δ ≥ (n+2)/3. Then
every longest cycle in G is a dominating cycle.
Bigalke and Jung [1] improved Theorem 2.1 under the assumption that G is
1-tough.
Theorem 2.2. Let G be a 1-tough graph on n vertices with δ ≥ n/3. Then every
longest cycle in G is a dominating cycle.
The class of graphs with δ ≥
n
3
was also studied by Enomoto et al. in [4].
Let C be a longest cycle in a nonhamiltonian graph G. In the proof techniques
for the existence of a hamiltonian cycle we often ask for the rest graph G − C.
Clearly, it is very useful if we know about G − C for any longest cycle C in a
50
Longest cycles and restgraph in maximalnonhamiltonian graphs
maximalnonhamiltonian graph G. For some special cases we can describe the rest
graph G− C. A such result is proved in [10].
Theorem 2.3. Let G be a 2-connected graph on n vertices with δ ≥ n/3 and C be
a longest cycle in it. Then G− C is either a complete graph or an edgeless graph.
It was also shown in [10] that if δ ≥ n/3 and G is 2-connected then G− C is
only dependent on G, namely: The definition of the class < shows that Theorems
2.4 is a generalization of Theorem 2.1 and Theorem 2.2. The class < is the union
of the classes of graphs <1, ... , <5 and it takes only a polynomial time to know
whether a given graph G ∈ < or not.
<1 is the class of graphs on 3r vertices and with minimal degree δ = r ≥ 3,
which are isomorphic to a graph G, where
2Kr−1 + K¯2 ⊆ G ⊆ (2Kr−1 ∪Kr) +K2;
<2 is the class of graphs on 15 vertices and with minimal degree δ = 5, which
are isomorphic to a graph G, where
4K3 + K¯3 ⊆ G ⊆ 4K3 +K3;
<3 is the class of graphs on 3r vertices and with minimal degree δ = r ≥ 3,
which are isomorphic to a graph G, where
(r − 1)K2 + K¯r−1 ⊆ G ⊆ ((r − 1)K2 ∪K3) +Kr−1;
<4 is the class of graphs on 3r+2 vertices and with minimal degree δ = r+1,
which are isomorphic to a graph G, where
3Kr + K¯2 ⊆ G ⊆ 3Kr +K2;
<5 is the class of graphs on 3r+2 vertices and with minimal degree δ = r+1 ≥
3, which are isomorphic to a graph G, where
(r + 1)K2 + K¯r ⊆ G ⊆ (r + 1)K2 +Kr.
Theorem 2.4. Let G be a 2-connected graph on n ≥ 3 vertices with δ ≥ n/3. Then
the following properties are equivalent:
(a) G is a dominable graph,
(b) Every longest cycle in G is a dominating cycle,
(c) G /∈ < for a class < of special graphs.
51
Vu Dinh Hoa
Note that all graphs of < are nontough graphs, we can easily see that the
graph G−C is a complete graph for any graph G ∈ < and every longest cycle C in
G. Thus, we believe that:
Conjeture 2.1. If G is a maximalnonhamiltonian graph and C is any longest cycle
in G, then G− C is a complete graph.
In the following, we show that the conjecture is true for nontough maximal-
nonhamiltonian graphs.
Theorem 2.5. Let C be a longest cycle in a nontough maximalnonhamiltonian
graph G, then G− C is a complete graph.
Proof. Since G is nontough, there exists a set S of vertices such that ω(G − S) ≥
|S| + 1. Let s = |S| and G1, G2, ... ,Gm be the components of G − S with m ≥
s + 1. Clearly, G1, G2, ... ,Gm are the complete graphs and m = s + 1 since G is a
maximalnonhamiltonian graph. Otherwise, we can add a new edge in some Gi or a
new edge joining two of the components of G− S to obtain a new nonhamiltonian
graph with more edges. Similarly, S is a complete graph and every vertex of S is
joining with any vertex of G−S. Now, we can easily see that for every longest cycle
C the graph G−C is one of the components G1, G2, ... ,Gm, so complete graph.
Now we will show that the same result holds for the class of graphs with δ ≥
n
3
.
We denote by p(G) the length of a longest path in G. In [9], Van den Heuvel proved
the following result.
Lemma 2.1. (Corollary 6.8 in [9]) Let G be a 1-tough graph on n ≥ 3 vertices such
that σ3 ≥ n. Then G satisfies c(G) ≥ p(G).
Thus, we conclude the following result.
Theorem 2.6. Let G be a maximalnonhamiltonian graph on n ≥ 3 vertices such
that δ ≥
n
3
and C be a longest cycle in G, then G− C is a complete graph.
Proof. If G is a nontough graph, then G− C is a complete one from Theorem 2.5.
Otherwise, G is a 1-tough graph and G satisfies the condition σ3 ≥ n. By Lemma 2.1,
we have c(G) ≥ p(G). But, since G is maximalnonhamiltonian graph, p(G) ≥ n− 1.
Thus G− C has at most one vertex and therefore G− C is a complete graph.
By the similar proof, we have the same result for graph G with σ3 ≥ n.
Theorem 2.7. Let G be a maximalnonhamiltonian graph on n ≥ 3 vertices such
that σ3 ≥ n and C be a longest cycle in a graph G. Then G−C is a complete graph.
52
Longest cycles and restgraph in maximalnonhamiltonian graphs
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