# On one-factorizations of replacement products

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1 Filomat 27:1 (2013), DOI /FIL A Published by Faculty of Sciences and Mathematics, University of Niš, Serbia Available at: On one-factorizations of replacement products Alireza Abdollahi a, Amir Loghman b a Department of Mathematics, University of Isfahan, Isfahan , Iran and School of Mathematics, Institute for Research in Fundamental Sciences, P.O. box , Tehran, Iran. b Department of Mathematics, University of Isfahan, Isfahan , Iran. Abstract. Let be an (n, m)-graph (n vertices and m-regular) and H be an (m, d)-graph. Randomly number the edges around each vertex of by {1,..., m} and fix it. Then the replacement product RH of graphs and H (with respect to the numbering) has vertex set V(RH) = V() V(H) and there is an edge between (v, k) and (w, l) if v = w and kl E(H) or vw E() and kth edge incident on vertex v in is connected to the vertex w and this edge is the lth edge incident on w in, where the numberings k and l refers to the random numberings of edges adjacent to any vertex of. If the set of edges of a graph can be partitioned to a set of complete matchings, then the graph is called 1-factorizable and any such partition is called a 1-factorization. In this paper, 1-factorizability of the replacement product RH of graphs and H is studied. As an application we show that fullerene C 60 and C 4 C 8 nanotorus are 1-factorizable. 1. Introduction raphs considered in this paper are finite, simple and undirected. Let be a simple graph. The vertex set, edge set, maximum degree and minimum degree of will be denoted by V(), E(), (), δ() respectively. The order of is V(), i.e. the number of vertices of. A graph is called regular if δ() = () and the latter integer is called the degree of and denoted by d(). A d-regular graph on n vertices is called an (n, d)-graph. A graph is called cubic if it is 3-regular. We denote by K n and C n the complete graph of order n and the cycle of order n, respectively. We denote by N (x) the neighborhoods of x V(), that is, the set of vertices adjacent to x. A spanning subgraph of a graph is a subgraph H of such that V(H) = V(). An r-regular spanning subgraph of is called an r-factor. A 1-factorization of is a set of edge-disjoint 1-factors of whose union is E(). The graph is said to be 1-factorizable if it has a 1-factorization. A necessary condition for a graph to be 1-factorizable is that is a regular graph of even order. The concept of 1-factorization can be expressed by the concept of edge coloring. An edge coloring of a graph is a map θ : E() C, where C is a set, called the color set, and θ(e) θ( f ) for any pair e and f of adjacent edges of. If α is a color, the set θ 1 ({α}), i.e. the set of edges of colored α, is called the α-color class. If C = m we say that θ is an m-edge coloring. The least integer m for which an m-edge coloring of exists is called the edge chromatic index of and denoted by χ (). It is easily seen that χ () () for any graph. Vizing [9] proved that 2010 Mathematics Subject Classification. Primary 05C15; Secondary 05C70 Keywords. Replacement product; One-factorization; Fullerene; Nanotorus. Received: 12 April 2011; Accepted: 23 August 2011 Communicated by Dragan Stevanović This research was in part supported by a grant from IPM (No ). This research was also in part supported by the Center of Excellence for Mathematics, University of Isfahan. addresses: (Alireza Abdollahi), (Amir Loghman)

2 A. Abdollahi, A. Loghman / Filomat 27:1 (2013), Theorem 1.1 (Vizing s Theorem). χ () () + 1 for any graph. If in a regular graph, we have χ () = () then edges of each color class consists a 1-factor and is 1-factorizable. We call a graph Hamiltonian if it contains a spanning cycle. Such a cycle is called a Hamiltonian cycle. Any Hamiltonian graph of even order has a 1-factor, for if s a Hamiltonian cycle of and we select alternate edges of C, we eventually end up with a 1-factor of. A 2-factor all of whose components are even is called an even 2-factor. We shall also need the following well known sufficient condition for the existence of a Hamilton cycle in a graph due to Dirac [4]. Theorem 1.2 (Dirac s Theorem). Let be a graph of order at least three such that δ() V() 2. Then is Hamiltonian. Let A and B be finite groups. Assume that B acts on A, namely we are fixing a homomorphism ϕ from B to the automorphism group of A and for elements a A, b B we denote by a b the element a bϕ the action of b on a. We also use a B to denote the orbit of a under this action. The Cayley graph C(H, S) of a group H and a generating set S = S 1 := {s 1 s S}, is an undirected graph whose vertices are the elements of H, and where {, h} is an edge if 1 h S. The Cayley graph C(H, S) is S -regular Factorizations of replacement products In this section we describe the replacement product and investigate 1-factorizability of replacement product. Let be any (n, k)-graph and let [k] = {1,..., k}. By a random numbering of we mean a random numbering of the edges around each vertex of by the numbers in {1,..., k}. More precisely, a random numbering of is a set φ consisting of bijection maps φ x : N (x) [k] for any x V(). Thus the graph has (k!) n random numberings. Example 2.1. Suppose = C(A, S) is a Cayley graph. Then the edges around each vertex of are naturally labeled by the elements of S: if {x, y} E() then φ x (y) = f (x 1 y), where f is a bijection map from S to [ S ]. Definition 2.2. Let be an (n, k)-graph and let H be a (k, k )-graph with V(H) = [k] = {1,..., k} and fix a random numbering φ of. The replacement product R φ H is the graph whose vertex set is V() V(H) and there is an edge between vertices (v, k) and (w, l) whenever v = w and kl E(H) or vw E(), φ v (w) = k and φw (v) = l. Note that the definition of R φ H clearly depends on φ. Thus for given any two regular graphs and H as above, there are ( V(H)!) V() replacement products which are not necessarily isomorphic. It follows from the definition that R φ H is a regular graph and in fact it is a (nk, k + 1)-graph (see [1, 7, 8]). Example 2.3. Let = K 5 and H = C 4 and let φ, φ be as shown in Figure 1. Then the replacement product R φ H and R φ H are not isomorphic, (see Figure 2) since the determinants of the adjacency matrices of R φ H and R φ H are 9 and 12, respectively. Lemma 2.4. Let be an (n, k)-graph. Then is 1-factorizable if and only if there exists a random numbering φ such that for any edge xx of we have φ x (x ) = φ x (x). Proof. : Let have a 1-factorization. Then, since is k-regular, there exists an edge coloring θ : E() [k] for. Now define the map φ x from N (x) to [k] for any x V() as φ x (y) = θ(xy) for all y N (x). Clearly φ x is a bijection and we have φ x (x ) = θ(xx ) = θ(x x) = φ x (x)

3 A. Abdollahi, A. Loghman / Filomat 27:1 (2013), Figure 1: = K 5 and H = C 4 Figure 2: (A) is R φ H and (B) is R φ H for any xx E(). Thus φ = {φ x x V()} is the desired random numbering of. : Suppose that has a random numbering φ with the mentioned property in the lemma. Let β be the map from E() to [k] defined by β(xy) = φ x (y) for any xy E(). Since φx (y) = φy (x) (β(xy) = β(yx)), then β is well-defined. Now we show that β is an edge coloring for. To do this, suppose e, f be two adjacent edges of E(). Then there exist x, y, y V() with e = xy, f = xy and y y. Hence φ x (y) φx (y ), as φ x is one-to-one. This implies that β(e) β( f ), as required. If is a 1-factorizable graph, a random numbering φ is called a 1-factorizable numbering whenever for any edge xx of we have φ x (x ) = φ x (x). Thus it follows from Lemma 2.4 that every 1-factorizable graph has a 1-factorizable numbering. We now consider the case when the two components of the product graph are Cayley graphs = C(A, S A ) and H = C(B, S B ). Furthermore, suppose that B acts on A in such a way that S A = α B for some α S A. So the edges around each vertex of are naturally labeled by the elements of B. This enables us to define the replacement product of and H. The following result is well-known and we give a proof by using Lemma 2.4.

4 A. Abdollahi, A. Loghman / Filomat 27:1 (2013), Lemma 2.5. Let be a cubic graph. Then the following properties are equivalent: (i) is 1-factorizable. (ii) has an even 2-factor. (iii) R φ C 3 is 1-factorizable for any 1-factorizable numbering φ. Proof. (i) (ii) Suppose that is 1-factorizable. Then, since is 3-regular, there exists an edge coloring θ : E() [3]. For any two colors α, β, the subgraph generated by the union of α- and β-color classes is a spanning subgraph of which is the union of some cycle subgraphs C 1,..., C s of with disjoint vertices and even length. If has an even 2-factor then contains a spanning subgraph s i=1 where are disjoint cycles of even length. Thus are 1-factorizable and by Lemma 2.4, there exists a random numbering φ Ci such that for any edge ab of we have φ a (b) = φ b (a). Now define φ x for each x V() as follows: φ x (y) = { φ x (y) xy E( ) 3 otherwise. It is easy to see that φ is a random numbering of satisfying the conditions of Lemma 2.4. (i) (iii) We know is 1-factorizable if only if there exists a 1-factorizable numbering φ such that for any xy E() we have φ x (y) = φy (x). Assume is 1-factorizable and φ is a 1-factorizable numbering. If V(C 3 ) = [3] = {1, 2, 3} then we define φ (x,i) R φ C 3 for (x, i) V(R φ C 3 ) as follows: { φ (x,i) [3]-{i,j} x = y, ij E(C3 ) R φ C 3 ((y, j)) = φ x (y) xy E(), where (y, j) N Rφ C 3 ((x, i)). It is obvious that φ (x,i) ((y, j)) = φ(y,j) ((x, i)). Now Lemma 2.4 completes the RH RH proof. Corollary 2.6. Let be a 1-factorizable graph and φ is 1-factorizable numbering of. Assume also that 1 denotes R φ C 3 and recursively let r = r 1 R φ r 1 C 3. Then { r } r 1 is an infinite family of 1-factorizable cubic graphs. Theorem 2.7. Let be an (n, k)-graph and let H be a (k, k )-graph with V(H) = [k]. If H has a 1-factorization, then the replacement product R φ H is 1-factorizable for any random numbering φ of. Proof. Let RH = R φ H. By Lemma 2.4, there exists a set φ H = {φ a H : N H(a) [k ] a V(H)} such that for any ab E(H) we have φ a H (b) = φb (a). Now define φ(x,a) H RH for any (x, a) V(RH) and (y, b) N RH(x, a) as follows: φ (x,a) { φ RH ((y, b)) = a H (b) k + 1 x = y otherwise. It is obvious that φ (x,a) ((y, b)) = φ(y,b) ((x, a)). Now Lemma 2.4 completes the proof. RH RH Example 2.8. Let C n be a cycle of length n. Then for any random numbering φ Cn, we have C n R φcn K 2 C 2n If n is odd then C n is not 1-factorizable but C n R φcn K 2 C 2n is 1-factorizable. If n is even then C n 1 is not 1- factorizable. Therefore for any random numbering φ Kn, the graph K n R φkn C n 1 is cubic having an even 2-factor. Then by Lemma 2.5, K n R φkn C n 1 is a 1-factorizable graph. Theorem 2.9. Let be an (n, m)-graph. Then the replacement product R φ K m is 1-factorizable whenever one of the following conditions holds: (a) m is even and φ is any random numbering of. (b) is 1-factorizable and φ is any 1-factorizable numbering of.

5 A. Abdollahi, A. Loghman / Filomat 27:1 (2013), Proof. The complete graph K m is 1-factorizable whenever m is even. Then by Theorem 2.7, R φ K m is 1-factorizable for any random numbering φ of. If m is odd and is 1-factorizable then by Lemma 2.4, there exists a random numbering φ such that for x V() and y N (x), we have φ x (y) = φy (x). For any (x, i) V(R φ K m ) and (y, j) N Rφ K m ((x, i)), where i, j V(K m ) = [m], we define φ (x,i) R φ K m as follows: φ x (y) xy E() φ (x,i) R φ K m ((y, j)) = i+j 2 x = y and i + j is non zero and even m x = y and i + j = 0 i+j+m 2 x = y and i + j is odd where i + j is the remainder when i + j is divided by m. It is easy to see that φ (x,i) RK m ((y, j)) = φ (y,j) RK m ((x, i)). This completes the proof. For the replacement product RC k, k 4, we derive the following result. Theorem Let be an (n, k)-graph and C k a cycle of length k 4. Then the replacement product R φ C k is 1-factorizable if one of the following conditions holds: (a) k is even and φ is any random numbering of (b) has an even 2-factor H = s i=1 and φ Ci = φ Ci, where φ Ci are 1-factorizable numbering of. Proof. If k is even then C k = ( k) has 1-factorization and by Theorem 2.7, R φ C k is 1-factorizable. Let have an even 2-factor. Thus there exist disjoint cycles (1 i s) with even lengths, whose s i=1 is a spanning subgraph of the graph. Cycles = (a i1 a i2... a ini ), 1 i s, are 1-factorizable and n = s i=1 n i. Because each n i is even then by Lemma 2.4, there are φ Ci where for any edge a il, a im of we have φ a il (a im ) = φ a im (a il ). Let φ a i1 (a i2 ) = φ a i2 (a i1 ) = 1 thus we define cycle T i as follows: T i = ((a i1, 1)(a i2, 1)(a i2, k)(a i2, k 1)... (a i2, 3)(a i2, 2)(a i3, 2)(a i3, 3)... (a i3, k 1)(a i3, k)(a i3, 1)... (a ini, 1)(a ini, k)(a ini, k 1)... (a ini, 3)(a ini, 2)(a i1, 2)(a i1, 3)... (a i1, k 1)(a i1, k)) T i are cycles of length kn i and s i=1 T i is a spanning subgraph of the replacement product R φ C k, where φ Ci = φ Ci. Then by Lemma 2.5, R φ C k has 1-factorization., Figure 3: (A) is Icosahedron and (B) is Fullerene C 60

6 A. Abdollahi, A. Loghman / Filomat 27:1 (2013), Corollary Let be an (n, k)-graph and 2k n. Then by Dirac s Theorem, the replacement product RC k is 1-factorizable. Let us end the paper with two applications of the above result. Figure 4: Coloring the edges of C 60. Figure 5: A TC 4 C 8 (R) tori (a) Top view (b) Side view Example A fullerene graph (in short a fullerene) is a 3-connected cubic planar graph, all of whose faces are pentagons and hexagons. By Euler formula the number of pentagons equals 12. The first fullerenes, C 60 and C 70, were isolated in The smaller version, C 60, is in the shape of a soccer ball. raph-theoretic observations on structural properties of fullerenes are important in this respect [3, 5, 6]. Suppose F is a fullerene C 60 and A is the icosahedron ((12, 5)-graph). By Figure 3(A), C = (a 1 a 3 a 2 a 6 a 5 a 10 a 11 a 7 a 12 a 8 a 9 a 4 ) is a Hamiltonian cycle of A and A is hamiltonian. If φ A for any edges a i a j are defined in Table I, then AR φa C 5 = F is Hamiltonian, see Figure 3(B). By Lemma 2.5, F is 1-factorizable. There are many different ways to color the edges of the graph C 60 (for example see Figure 4). Example Consider C 4 nanotorus S. Then S = C k C m, where C n is the cycle of order n and S be an (km, 4)- graph ( S is the Cartesian product of cycles). Suppose T is a T = TC 4 C 8 [k, m](r) nanotorus (which k is the number of squares in every row and m is the number of squares in every column, Figure 5), [2]. Then T is an (4km, 3)-graph. It is easy to see that T = SR φs C 4, for some of random numbering φ S of graph S. Because C 4 is 1-factorizable then by Theorem 2.10, nanotorus T is 1-factorizable.

7 A. Abdollahi, A. Loghman / Filomat 27:1 (2013), φ a 1 A (a 4) φ a 1 A (a 3) φ a 1 A (a 2) φ a 1 A (a 6) φ a 1 A (a 5) φ a 4 A (a 10) φ a 4 A (a 9) φ a 4 A (a 3) φ a 4 A (a 1) φ a 4 A (a 5) φ a 3 A (a 9) φ a 3 A (a 8) φ a 3 A (a 2) φ a 3 A (a 1) φ a 3 A (a 4) φ a 2 A (a 8) φ a 2 A (a 7) φ a 2 A (a 6) φ a 2 A (a 1) φ a 2 A (a 3) φ a 6 A (a 7) φ a 6 A (a 11) φ a 6 A (a 5) φ a 6 A (a 1) φ a 6 A (a 2) φ a 5 A (a 11) φ a 5 A (a 10) φ a 5 A (a 4) φ a 5 A (a 1) φ a 5 A (a 6) φ a 12 A (a 11) φ a 12 A (a 7) φ a 12 A (a 8) φ a 12 A (a 9) φ a 12 φ a 11 A (a 6) φ a 11 A (a 6) φ a 11 A (a 7) φ a 11 A (a 12) φ a 11 φ a 7 A (a 5) φ a 7 A (a 2) φ a 7 A (a 8) φ a 7 A (a 12) φ a 7 A (a 11) φ a 8 A (a 4) φ a 8 A (a 3) φ a 8 A (a 9) φ a 8 A (a 12) φ a 8 A (a 7) φ a 9 A (a 3) φ a 9 A (a 4) φ a 9 A (a 10) φ a 9 A (a 12) φ a 9 A (a 8) φ a 10 A (a 2) φ a 10 A (a 5) φ a 10 A (a 11) φ a 10 A (a 12) φ a 10 A (a 9) A (a 10) A (a 10) Table 1: Table I Acknowledgement. The authors are grateful to the referee for his/her invaluable comments improving the original version of the paper. References [1] N. Alon, A. Lubotzky, and A. Wigderson, Semi-direct product in groups and zig-zag product in graphs: connections and applications (extended abstract). In 42nd IEEE Symposium on Foundations of Computer Science (Las Vegas, NV, 2001), pages IEEE Computer Society, [2] A. R. Ashrafi and A. Loghman, Computing Padmakar-Ivan index of TUC 4 C 8 (S) Nanotorus, J. Comput. Theor. Nanosci. 3(3) (2006) [3]. Brinkmann, B.D. McKay, Construction of planar triangulations with minimum degree 5, Discrete Math. 301 (2005) [4].A. Dirac, Some theorems on abstract graphs, Proc. London Math. Soc., 2(1952), [5] J.E. raver, The independence numbers of fullerenes and benzenoids, European J. Combin. 27 (2006) [6] J.E. raver, Encoding fullerenes and geodesic domes, SIAM. J. Discrete Math. 17 (2004) [7] C. A. Kelley, D. Sridhara and J. Rosenthal, Zig-Zag and Replacement product graphs and LDPC codes, Advances in Mathematics of Communications, 2(4)(2008) [8] O. Reingold, S. Vadhan and A. Wigderson, Entropy waves, the zig-zag graph product, and new constant-degree expanders, Ann. of Math. 2,(155) (2002), [9] V.. Vizing, On an estimate of the chromatic index of a p-graph, Metody Diskret Analiz, 3(1964),

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