Discrete Mathematics 335 (2014) 25–34
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Discrete Mathematics
journal homepage: www.elsevier.com/locate/disc
On the girth of the bipartite graph D(k, q)
✩
Xiaoyan Cheng, Wenbing Chen, Yuansheng Tang
∗
School of Mathematical Sciences, Yangzhou University, Yangzhou, Jiangsu, 225002, PR China
a r t i c l e i n f o
Article history:
Received 28 May 2013
Received in revised form 28 June 2014
Accepted 3 July 2014
Available online 2 August 2014
Keywords:
Bipartite graph
Girth
Edge-transitive
Automorphism
Algebraic graph
a b s t r a c t
For integer k ≥ 2 and prime power q, an algebraic bipartite graph D(k, q) of girth at least
k + 4 was introduced by Lazebnik and Ustimenko (1995). Füredi et al. (1995) shown that
the girth of D(k, q) is equal to k + 5 if k is odd and q is a prime power of form 1 + n(k + 5)/2
and, conjectured further that D(k, q) has girth k+5 for all odd k and all q ≥ 4. In this paper,
we show that this conjecture is true when (k + 5)/2 is a power of the characteristic of F
q
.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
The graphs we consider in this paper are simple, i.e. undirected, without loops and multiple edges. For a graph G, its vertex
set and edge set are denoted by V (G) and E(G), respectively. The order of G is the number of vertices in V (G). The degree of a
vertex v ∈ G is the number of the vertices that are adjacent to it. A graph is said r-regular if the degree of every vertex is equal
to r. An automorphism of G means a bijection φ from V (G) to itself such that {φ(v), φ(v
′
)} is an edge iff {v, v
′
} is. G is said to
be edge-transitive if for any two edges {v
1
, v
′
1
}, {v
2
, v
′
2
} there is an automorphism φ of G such that {φ(v
1
), φ(v
′
1
)} = {v
2
, v
′
2
}.
A sequence v
1
v
2
· · · v
n
of vertices of G is called a path of length n if {v
i
, v
i+1
} ∈ E(G) for i = 1, 2, . . . , n − 1 and v
j
= v
j+2
for j = 1, 2, . . . , n − 2. A path v
1
v
2
· · · v
n
is called a cycle further if its length n is not smaller than 3 and v
3
v
4
· · · v
n
v
1
v
2
is still a path. If G contains at least one cycle, then the girth of G, denoted by g(G), is the length of a shortest cycle in G. In
the literature, graphs with large girth and a high degree of symmetry have been shown to be useful in different problems in
extremal graph theory, finite geometry, coding theory, cryptography, communication networks and quantum computations
(cf. [1–18]).
Let q be a prime power and F
q
the finite field of q elements. For k ≥ 2, in [7] Lazebnik and Ustimenko proposed a
bipartite graph, denoted by D(k, q), which is q-regular, edge-transitive and of large girth. The bipartite graph D(k, q) can
be equivalently described as follows (see [12]): The vertex sets L(k) and P(k) of D(k, q) are two copies of F
k
q
such that two
vertices (l
1
, l
2
, . . . , l
k
) ∈ L(k) and (p
1
, p
2
, . . . , p
k
) ∈ P(k) are adjacent in D(k, q) if and only if
l
2
+ p
2
= p
1
l
1
, (1)
l
3
+ p
3
= p
1
l
2
, (2)
✩
This work was supported by the Natural Science Foundation of China (No. 61379004), the Key Project of Chinese Ministry of Education (No. 208045)
and the open Foundation of NCRL of Southeast University (W200819).
∗
Corresponding author.
E-mail addresses: cxy-nt@hotmail.com (X. Cheng), chenwenbing007@163.com (W. Chen), ystang@yzu.edu.cn (Y. Tang).
http://dx.doi.org/10.1016/j.disc.2014.07.004
0012-365X/© 2014 Elsevier B.V. All rights reserved.