Abstract This paper presents a construction of a minimum cycle basis for the direct product of two complete graphs on three or more vertices. With the exception of two special cases, such bases consist entirely of triangles

Richard Hammack

S E Hambrusch

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Information Processing Letters 102 (2007) 214–218
www.elsevier.com/locate/ipl
Minimum cycle bases of direct products of complete graphs
Richard Hammack
Department of Mathematics and Applied Mathematics, Virginia Commonwealth University, Richmond, VA 23284-2014, USA
Received 17 May 2006; received in revised form 17 October 2006; accepted 28 December 2006
Available online 16 January 2007
Communicated by S.E. Hambrusch
Abstract
This paper presents a construction of a minimum cycle basis for the direct product of two complete graphs on three or more
vertices. With the exception of two special cases, such bases consist entirely of triangles.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Combinatorial problems; Minimum cycle basis; Graph direct product1. Introduction
Cycle bases and minimum cycle bases for graphs
have a long history in applied discrete mathematics, go-
ing back at least as far as G. Kirchhoff’s 1847 treatise on
electrical networks [10]. More recently, minimal cycle
bases have been employed in structural flexibility analy-
sis [9], and in search-and-retrieval of molecular struc-
tures in chemical information systems [1,2]. Spurred by
these and other applications, much attention has focused
on the problem of constructing minimum cycle bases of
graphs. Although Horton [5] presents a polynomial al-
gorithm that finds a minimum cycle basis in any graph,
the algorithmic approach can lead us to miss deeper
connections between the structures of graphs and their
cycle bases. Consequently, some authors have recently
examined the problem of direct (non-algorithmic) con-
structions of minimum cycle bases for certain classes of
graphs [8,11,12].
E-mail address: rhammack@vcu.edu.0020-0190/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ipl.2006.12.012In particular, several papers have addressed the prob-
lem of finding minimum cycle bases for the various
graph-theoretical products. In the article Minimum Cy-
cle Bases of Product Graphs [7], W. Imrich and P. Stad-
ler construct minimum cycle bases for Cartesian and
strong products of graphs. F. Berger solves the same
problem for the lexicographical product [1]. The cor-
responding construction for the direct product appears
to be intriguingly subtle, for there seems to be no clear
connection between the minimum cycle bases of the
product and of its factors. Currently only partial results
have been established. (See, for instance, [4], which
constructs an MCB for the direct product of two bipar-
tite graphs.) The present contribution addresses the case
of the direct product Kp × Kq of two complete graphs
with p,q  3. The situation is fairly complex even in
this simple setting, but this work could possibly provide
insights into the general case.
Recall that if G = (V (G),E(G)) is a simple graph,
its cycle space, denoted C(G), is the vector space (over
the two-element field F2 = {0,1}) whose elements are
the subsets X ⊆ E(G) for which each vertex of G is
incident with an even number of the elements of X. Ad-
R. Hammack / Information Processing Letters 102 (2007) 214–218 215dition in C(G) is symmetric difference of sets, and zero
is the empty set. Elements of C(G) are called gener-
alized cycles, or just cycles. The dimension of C(G) is
ν(G) = |E(G)| − |V (G)| + c, where c is the number
of components of G. (See [3], Theorem 1.9.6.) A basis
B of C(G) is called a cycle basis of G, and its length
is l(B) = ∑C∈B |C|. Among all cycle bases of G, one
with smallest possible length is called a minimum cycle
basis, or MCB.
The direct product of graphs G and H is the graph
G × H whose vertex set is the Cartesian product
V (G) × V (H) and whose edges are (w,x)(y, z) where
wy ∈ E(G) and xz ∈ E(H). Observe G × H has
2|E(G)||E(H)| edges and |V (G)||V (H)| vertices, so
ν(G × H) = 2|E(G)||E(H)| − |V (G)||V (H)| + 1 if
G×H is connected. The vertices of the complete graph
Kp are denoted as {1,2,3, . . . , p}. Thus, the edges of
Kp × Kq are exactly the pairs (i, j)(k, ) with 1  i,
k  p, and 1  j ,   q , and i = k and j = . Note that
for p,q  3, Kp × Kq is connected, so
ν(Kp × Kq) = 2
(
p
2
)(
q
2
)
− pq + 1.
Any triangle (a, d)(b, e)(c, f )(a, d) in Kp × Kq
projects uniquely to two triangles abca and def d in Kp
and Kq , respectively. Conversely, given such triangles
abca and def d , the graph Kp × Kq has six triangles
that project onto them, namely (π(a), d)(π(b), e)(π(c),
f )(π(a), d), where π is any permutation of {a, b, c}.
Consequently, Kp × Kq has exactly 6
(
p
3
)(
q
3
)
triangles.
The following pages show that whenever this number is
at least as large as ν(Kp × Kq), the triangles will span
C(Kp × Kq), and an MCB can be extracted from them.
2. Two special cases
Here we describe MCBs for K3 × K3 and K3 × K4.
These graphs are special in that their MCBs contain
both triangles and squares. By contrast, the subsequent
section will show that if p,q  4, any MCB for Kp ×
Kq consists solely of triangles. The arguments in this
section use ideas from homology theory, but they do not
employ the machinery of this theory, for the benefit of
readers who are not conversant with algebraic topology.
Topologists will see how the arguments can be short-
ened.
Consider Γ = K3 × K3. Fig. 1 shows an embedding
of Γ on the torus, with vertices (i, j) abbreviated as
ij . (The torus is presented as an identification space,
with opposite sides of the large outer-most square iden-
tified. Note that these four sides are not edges of Γ .)
This embedding has six triangular regions, two squareFig. 1. K3 × K3 on the torus.
Fig. 2. K3 × K4 on the torus.
regions and one 10-gon region. Denote by T the set
of six triangular subgraphs of Γ that bound of the tri-
angular regions of the embedding. Elements of T are
pairwise edge-disjoint, so T is a linearly independent
set in C(Γ ). Note that T contains every triangle in Γ ,
so if it is enlarged to a cycle basis of Γ then longer
cycles must be appended to it; if all such larger cy-
cles are squares, we will have an MCB. Let S denote
the (independent) set of the two square subgraphs that
bound the square regions of the embedding. Set R =
{(3,2)(2,3)(1,2)(2,1)(3,2), (2,2)(3,3)(1,2)(3,1)(2,
2)}. Observe that T ∪ S ∪ R is linearly independent:
Suppose T + S + R = 0, where T ,S and R are in the
spans of T , S and R, respectively. Note T = 0 because
each nonzero element of Span(T ) has edges that do
not belong to elements of S ∪ R, and therefore can-
not be canceled by S and R. Next, S = 0 because each
nonzero element of Span(S) has edges belonging to no
element of R. Thus T = S = R = 0, proving indepen-
dence. Since |T ∪S∪R| = 10 = ν(K3 ×K3), it follows
T ∪ S ∪ R is an MCB of K3 × K3 consisting of six tri-
angles and four squares.
Next, consider Λ = K3 × K4. Fig. 2 shows an em-
bedding of Λ on the torus, again with vertices (i, j)
216 R. Hammack / Information Processing Letters 102 (2007) 214–218abbreviated as ij . The boundaries of the 24 regions
of this embedding are the 24 triangles in Λ. Now, if
all these triangles are added together, their edges will
cancel pair-by-pair and give 0, so they are not an inde-
pendent set. However, we claim that a set of any 23
of them is independent. Let T be a subset of C(Λ)
consisting of 23 of the 24 triangles in Λ. Then T is
linearly independent because any sum of its elements
that equals 0 cannot include the three triangles in T
with edges belonging to the missing triangle (for such
edges could not then be canceled); nor can the sum
include triangles with edges belonging to these three
triangles, etc., and it follows from connectivity of the
torus that only the trivial linear combination of ele-
ments of T can equal 0. Since ν(Λ) = 25, just two
more cycles are needed to enlarge T to an MCB,
and since all the triangles have been used up, the ex-
tra cycles must have length greater than 3. Set R =
{(1,1)(3,3)(1,4)(3,2)(1,1), (1,1)(2,4)(1,3)(2,2)(1,
1)}. (These squares are representatives of the two stan-
dard generators of the first homology group of the
torus.) It is straightforward—if tedious—to check that
T ∪ R is linearly independent and is therefore an MCB
of K3 × K4.
These two examples have proved
Proposition 1. The product K3 × K3 has an MCB con-
sisting of six triangles and four squares. The product
K3 × K4 has an MCB consisting of 23 triangles and
two squares.
3. A basis of triangles
Suppose p,q  4, or p = 3 and q  5. Here is a con-
struction of an MCB of Kp × Kq that consists entirely
of triangles.
For each integer m ∈ {1,2,3, . . . , q}, let Gm be the
subgraph of Kp × Kq induced on the vertices {(i, j) |
2  i  p, 1  j  q, j = m}. Thus Gm ∼= Kp−1 ×
Kq−1, and is connected because its factors are con-
nected, and at least one has an odd cycle (Theorem 5.29
of [6]). For each 2  m  q , let Tm be a spanning tree
of Gm. Further, T2 and T3 are required to have a special
form. Each vertex of form (i,1) is required to have de-
gree 1 in T2. Also, T3 is chosen to have no edges of form
(i,1)(j,2). (One easily checks that such trees exist for
the stated values of p and q .)
Define the following edge sets in Kp × Kq .
S1 = E(G1),
S2 = E(T2) ∪
[
E(G2) − E(G1)
]
,
S3 = E(T3) ∪
[
E(G3) −
(
E(G1) ∪ E(G2)
)]
,
S4 = E(T4),
S5 = E(T5),
...
Sq = E(Tq).
Observe that these sets have been constructed so that the
subgraph of Kp ×Kq induced on edges (S1 ∪S2 ∪ · · · ∪
Sm−1) ∩ Sm is acyclic for 2  m  q . This is obvious
for 4  m  q . For m = 2, note that
S1 ∩ S2 = E(G1) ∩
[
E(T2) ∪
[
E(G2) − E(G1)
]]
= [E(G1) ∩ E(T2)]
∪ [E(G1) ∩ [E(G2) − E(G1)]]
= [E(G1) ∩ E(T2)] ∪ ∅
is acyclic. Also
(S1 ∪ S2) ∩ S3 = (S1 ∪ S2) ∩
[
E(T3)
∪ [E(G3) − (E(G1) ∪ E(G2))]]
= [(S1 ∪ S2) ∩ E(T3)] ∪ [(S1 ∪ S2)
∩ [E(G3) − (E(G1) ∪ E(G2))]]
= [(S1 ∪ S2) ∩ E(T3)] ∪ ∅
is acyclic.
In what follows, the cardinalities of the Sm will be
needed, so these are now computed. First, since G1 ∼=
Kp−1 × Kq−1 we have:
|S1| = 2
(
p − 1
2
)(
q − 1
2
)
. (1)
For S2, note E(G2) − E(G1) = {(i,1)(k, ): 2  i, k
 p, i = k,3    q} contains exactly (p − 1)(p −
2)(q − 2) edges. By choice of T2, each of its p − 1
vertices of form (i,1) has degree 1 in T2, so |E(T2) ∩
[E(G2) − E(G1)]| = p − 1. Hence |S2| = |T2| +
|E(G2) − E(G1)| − (p − 1), so
|S2| = (p − 1)(q − 1) − 1 + (p − 1)(p − 2)(q − 2)
− (p − 1). (2)
Now consider S3. Note E(G3) − (E(G1) ∪ E(G2))
= {(i,1)(k,2): 2  i, k  p, i = k} has cardinality
(p − 1)(p − 2). Recall that T3 was chosen to contain
none of these edges, so
|S3| = (p − 1)(q − 1) − 1 + (p − 1)(p − 2). (3)
Lastly, note that
|S4| = |S5| = · · · = |Sq | = (p − 1)(q − 1) − 1. (4)
Now form the following sets of triangles in Kp ×Kq .
R. Hammack / Information Processing Letters 102 (2007) 214–218 217Fig. 3.
B1 =
{
(1,1)(i, j)(k, )(1,1) | (i, j)(k, ) ∈ E(S1)
}
,
B2 =
{
(1,2)(i, j)(k, )(1,2) | (i, j)(k, ) ∈ E(S2)
}
,
B3 =
{
(1,3)(i, j)(k, )(1,3) | (i, j)(k, ) ∈ E(S3)
}
,
...
Bq =
{
(1, q)(i, j)(k, )(1, q) | (i, j)(k, ) ∈ E(Sq)
}
.
Fig. 3 illustrates these sets. Any triangle in Bm is in-
cident with vertex (1,m) while its opposite edge is
in Sm. Notice that the sets Bm are pairwise disjoint
and |Bm| = |Sm| for 1  m  q . Moreover, observe that
each Bm is linearly independent in C(Kp × Kq): Each
edge in Sm is on exactly one triangle in Bm, and each
triangle in Bm contains exactly one edge of Sm. Conse-
quently no nonempty subset of Bm can sum to 0, since
such a sum will always contain edges in Sm.
Next, we argue that B1 ∪ B2 ∪ · · · ∪ Bq is a linearly
independent set by showing for each 2  m  q that
Span(B1 ∪ B2 ∪ · · · ∪ Bm−1) ∩ Span(Bm) = 0. Sup-
pose C is in this intersection. Since C ∈ Span(Bm),
any vertex of form (1, j) incident with an edge of C
must be of form (1,m). But as no cycle in Span(B1 ∪
B2 ∪ · · · ∪ Bm−1) has an edge incident with such a
vertex, no edge of C is incident with a vertex of form
(1, j). Then, from the definition of the sets Bj , the
fact that C ∈ Span(B1 ∪ B2 ∪ · · · ∪ Bm−1) means C ⊆
S1 ∪ S2 ∪ · · · ∪ Sm−1. The fact that C ∈ Span(Bm)
means C ⊆ Sm. Thus C is in the acyclic subgraph
(S1 ∪ S2 ∪ · · · ∪ Sm−1) ∩ Sm, so C = 0.
Finally, to show B = B1 ∪ B2 ∪ · · · ∪ Bq is a basis,
we confirm |B| = ν(Kp × Kq). Using Eqs. (1)–(4), as
well as the substitutions r = p − 1 and s = q − 1 (to
streamline the calculation), note|B| =
q∑
m=1
|Bm|
=
q∑
m=1
|Sm| = |S1| + |S2| + |S3| +
q∑
m=4
|Sm|
= 2
(
r
2
)(
s
2
)
+ [rs − 1 + r(r − 1)(s − 1) − r]
+ [rs − 1 + r(r − 1)] + (s − 2)(rs − 1)
= 2
(
r
2
)(
s
2
)
+ r(r − 1)s + rs(s − 1)
+ 2rs − rs − s − r
= 2
(
r
2
)(
s
2
)
+ 2
(
r
2
)
s + 2r
(
s
2
)
+ 2rs − (r + 1)(s + 1) + 1
= 2
[(
r
2
)
+ r
][(
s
2
)
+ s
]
− (r + 1)(s + 1) + 1
= 2
[(
r
2
)
+
(
r
1
)][(
s
2
)
+
(
s
1
)]
− (r + 1)(s + 1) + 1
= 2
(
r + 1
2
)(
s + 1
2
)
− (r + 1)(s + 1) + 1
= 2
(
p
2
)(
q
2
)
− pq + 1
= ν(Kp × Kq).
It follows that B = B1 ∪ B2 ∪ · · · ∪ Bq is a basis of
triangles for C(Kp × Kq), and is therefore an MCB.
This completes the construction, and proves the follow-
ing proposition.
Proposition 2. If p,q  4, or if p = 3 and q  5, then
any MCB of Kp × Kq consists of 2
(
p
2
)(
q
2
) − pq + 1 tri-
angles.
We do not address the problem of finding an MCB
for K2 × Kq . Indeed, to the author’s knowledge, this
problem still open, though it probably would not be
difficult to resolve. However, since such products are
bipartite, the approach would likely differ from the one
used here. Moreover, it may be more fruitful to exam-
ine the more general problem of graphs of form G × H
where G is bipartite, for, as [4] suggests, bipartiteness
of a factor tends to simplify the cycle structure of the
product.
218 R. Hammack / Information Processing Letters 102 (2007) 214–218Acknowledgements
The author thanks the referees for their helpful sug-
gestions and comments.
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Minimum cycle bases of direct products of complete graphs

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Minimum cycle bases of direct products of complete graphs

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