# DISTANCE MAGIC CARTESIAN PRODUCTS OF GRAPHS

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1 1 2 Discussiones Mathematicae Graph Theory xx (xxxx) DISTANCE MAGIC CARTESIAN PRODUCTS OF GRAPHS Christian Barrientos Department of Mathematics, Clayton State University Sylwia Cichacz 1 AGH University of Science and Technology Dalibor Froncek Department of Mathematics and Statistics, University of Minnesota-Duluth Elliot Krop Department of Mathematics, Clayton State University and Christopher Raridan Department of Mathematics, Clayton State University Abstract A distance magic labeling of a graph G = (V, E) with V = n is a bijection l : V {1,..., n} such that the weight of every vertex v, computed as the sum of the labels on the vertices in the open neighborhood of v, is a constant. In this paper, we show that hypercubes with dimension divisible by four are not distance magic. We also provide some positive results by providing necessary and sufficient conditions for the Cartesian product of certain complete multipartite graphs and the cycle on four vertices to be distance magic. Keywords: Distance magic labeling, magic constant, sigma labeling, Cartesian product, hypercube, complete multipartite graph, cycle Mathematics Subject Classification: 05C76, 05C78.

2 2 C. Barrientos, S. Cichacz, D. Froncek, E. Krop and C. Raridan Introduction Definitions 37 For standard graph theoretic definitions and notation, we refer to Diestel [9]. 38 All graphs G = (V, E) are finite undirected simple graphs with vertex set V (G) 39 and edge set E(G). Given any vertex v, the set of all vertices adjacent to v is the 40 open neighborhood of v, denoted N(v) (or N G (v), if necessary), and the degree of v 41 is N(v). If every vertex in a graph G has the same degree r, the graph is called 42 r-regular. The closed neighborhood of v is N(v) {v}, denoted N[v] (or, N G [v]). 43 A distance magic labeling of a graph G of order n is a bijection l : V (G) {1,..., n} such that the weight of every vertex v, defined as w(v) = u N(v) l(v), 45 is a constant, which we call the magic constant, denoted simply as µ. Any graph 46 which admits a distance magic labeling is called a distance magic graph. Distance 47 magic graphs are analogue to closed distance magic graphs; see [3, 6]. 48 We use the definition of Cartesian product given in [12]. Given two graphs G 49 and H, the Cartesian product of G and H, denoted G H, is the graph with vertex 50 set V (G) V (H), where two vertices (g, h) and (g, h ) are adjacent if and only if 51 g = g and h is adjacent to h in H, or h = h and g is adjacent to g in G. 52 The cycle on n vertices is denoted C n. The complete graph on n vertices is 53 denoted K n. The complete bipartite graph with parts of cardinality m and n, 54 respectively, is denoted K m,n. The complete r-partite graph with n vertices in each 55 part is denoted K(n; r). The n-dimensional hypercube is denoted Q n. The vertices 56 of Q n are binary n-tuples and two vertices are adjacent if their corresponding tuples 57 differ in exactly one position. For integers 0 k n, we say that a vertex of Q n 58 belongs to row k, denoted r k, if the corresponding n-tuple contains exactly k entries 59 that are 1 s. For a vertex v r k, if 0 k n 1, we say the upper neighbors of v, 60 denoted N u (v), are those vertices in r k+1 that are adjacent to v, and if 1 k n, we 61 say the lower neighbors, denoted N l (v), are those in r k 1 that are adjacent to v. For a vertex v V (Q n ), let {v} denote the label on v and let N u {v} = x N u (v) {x} and N l {v} = x Nl(v) {x} denote the sum of the labels on the upper and lower 64 neighbors of v, respectively. Note that Q n also may be defined recursively in terms 65 of the Cartesian product: Q 1 = K 2 and Q n = Q n 1 K 2 for integers n History and Motivation Graph labelings have served as the focal point of considerable study for over forty years; see Gallian s survey [11] for a review of results in the field. For a detailed survey of previous work and open problems concerning distance magic labelings, see Arumugam, Froncek, and Kamatchi [5]. Some graph which are distance magic among (some) products can be seen in [2, 4, 6, 7, 8, 16, 18, 19]. The general question about characterizing graphs G and H such that G H is distance magic was posed in [5]. Some results along that line follow: Theorem 1 [18]. The Cartesian product C n C m is distance magic if and only if n = m 2 (mod 4). Theorem 2 [19]. (1) The Cartesian product P n C m, where n is an odd integer greater than 1 or n 2 (mod 4), has no distance magic labeling. (2) The Cartesian product K 1,n C m has no distance magic labeling. (3) The Cartesian product

3 Distance Magic Cartesian Products of Graphs K n,n C m, where n 2 and m is odd, has no distance magic labeling. (4) The Cartesian product K n,n+1 C m, where n is even and m 1 (mod 4), has no distance magic labeling. It was shown in [15, 16, 17, 20] that if G is an r-regular distance magic graph with n vertices, then the magic constant must be µ = r(n + 1)/2, implying that no graph with odd regularity can be a distance magic. That is, Q n for odd n is not distance magic. The concept of distance magic labelings has been motivated by the construction of magic rectangles (see [10, 13, 14]) since we can construct a distance magic labeling of K(n; r) by labeling the vertices in each part by the columns of the magic rectangle. Note, however, that lack of an n r magic rectangle does not imply that K(n; r) is not distance magic; for example, there is no 2 2 magic rectangle but Q 2 = K(2; 2) = K 2,2 is distance magic. In 2004, Acharya, Rao, Singh, and Parameswaran stated the following conjecture: Conjecture 3 [1]. For any even integer n 4, the n-dimensional hypercube Q n is not a distance magic graph. The following problem was given in [7]: Problem 4. If G is a regular graph, determine if G C 4 is distance magic. Notice that if G is an r-regular graph, then the necessary condition for H = G C 4 to be distance magic is that r is even (since H is (r + 2)-regular). In Section 2, we show that Q n, where n 0 (mod 4), is not distance magic. In Section 3, we provide some positive results by giving necessary and sufficient conditions for which K(n; r) C 4, where n 2, is distance magic Non-distance magic hypercubes Theorem 5. The hypercube Q n, where n 0 (mod 4), is not distance magic. Proof. Assume that Q n, where n 0 (mod 4), is distance magic with magic constant µ. Let k = n/2. By symmetry of the hypercube, we have that ( ) n µ = (N l {v} + N u {v}) = (N l {v} + N u {v}). (2.1) k 1 v r k 1 v r k+1 By considering the binary representation of the vertices of the hypercube, for 1 j k 1 and every vertex v r j, we have N u (v) = n j and N l (v) = j. Thus, ( ) n µ = (k + 1) {v} + (k 1) {v} k 1 v r k v r k 2 = (k + 1) {v} + (k 1) {v}, v r k v r k+2 which implies that {v} = {v}. (2.2) v r k 2 v r k+2

4 4 C. Barrientos, S. Cichacz, D. Froncek, E. Krop and C. Raridan Using (2.1) and (2.2) as the basis step, we perform induction on the hypercube rows. Assume that for some j, where 1 < j k/2, and all i j, {v} = {v}. (2.3) v r k 2(i 1) v r k+2(i 1) Now, by symmetry of the hypercube, ( ) n µ = (N l {v} + N u {v}) = (N l {v} + N u {v}), k 2i + 1 v r k 2i+1 v r k+2i+1 which implies (k + 2i 1) {v} + (k 2i + 1) {v} v r k 2i v r k 2(i 1) = (k + 2i 1) {v} + (k 2i + 1) {v}. v r k+2i v r k+2(i 1) Using (2.3) gives in particular, {v} = {v}; v r k 2i v r k+2i {v} = {v}. (2.4) v r 0 v r 2k Since both r 0 and r 2k contain only one vertex, (2.4) implies that the labels on these vertices are the same, which contradicts that Q n has a distance magic labeling Distance magic K(n; r) C 4 In this section the proof is based on an application of magic rectangles, which are a natural generalization of magic squares. A magic rectangle MR(a, b) is an a b array with entries from the set {1, 2,..., ab}, each appearing once, with all its row sums equal to a constant δ and with all its column sums equal to a constant η. Harmuth proved the following: Theorem 6 [13, 14]. A magic rectangle MR(a, b) exists if and only if a, b > 1, ab > 4, and a b (mod 2). To prove our main result in this section, we will need the following generalization of magic rectangles that was introduced in [10]. Definition 3.1. A magic rectangle set MRS(a, b; c) is a collection of c arrays (a b) whose entries are elements of {1, 2,..., abc}, each appearing once, with all row sums in every rectangle equal to a constant δ and all column sums in every rectangle equal to a constant η. Moreover, Froncek proved:

5 Distance Magic Cartesian Products of Graphs Theorem 7 [10]. If a b 0 (mod 2), a 2 and b 4, then a magic rectangle set MRS(a, b; c) exists for every c. Observation 8 [10]. If a magic rectangle set MRS(a, b; c) exists, then both MR(a, bc) and MR(ac, b) exist. In the following lemmas, we use C 4 = xuywx and we denote the vertices of K(n; r), the complete r-partite graph with n vertices in each part, by {v j i : i = 1,..., n and j = 1,..., r}, where we drop the subscript i if n = 1. Lemma 3.2. The Cartesian product K n C 4 is not distance magic. Proof. Notice that K n = K(1; n). Let H = K(1; n) C 4. Suppose H is distance magic and l is a distance magic labeling of H with magic constant µ. Let l(v j, u) + l(v j, w) = a j u,w and l(v j, x) + l(v j, y) = a j x,y for any j = 1,..., n. Since 0 = w(v j, x) w(v h, x) = l(v h, x) l(v j, x) + a h u,w a j u,w = w(v j, y) w(v h, y) = l(v h, y) l(v j, y) + a h u,w a j u,w, we obtain l(v h, x) l(v h, y) = l(v j, x) l(v j, y) for any j, h = 1,..., n. Therefore, l(v j, x) = k + l(v j, y) for some constant k and for any j = 1,..., n. On the other hand, µ = w(v j, y) = = w(v j, x) = = l(v p, y) + a j u,w l(v p, x) + a j u,w (k + l(v p, y)) + a j u,w, which implies k = 0 and l(v j, x) = l(v j, y), a contradiction. Lemma 3.3. The Cartesian product K(2; r) C 4 is not distance magic. Proof. Notice that K 2 = K(2; 1) is not distance magic by Lemma 3.2. Moreover, K 2,2 = C4 and C 4 C 4 is not distance magic by Theorem 1, so we assume that r > 2. Let H = K(2; r) C 4. Suppose that H is a distance magic graph with distance magic labeling l and magic constant µ. We have µ = w(v j 1, x) = = w(v j 2, x) = (l(v p 1, y) + l(vp 2, y)) + l(vj 1, u) + l(vj 1, w) (l(v p 1, y) + l(vp 2, y)) + l(vj 2, u) + l(vj 2, w), which implies that l(v j 133 1, u) + l(vj 1, w) = l(vj 2, u) + l(vj 2, w) for any j = 1,..., r. Analogously, we obtain that l(v j 134 1, x)+l(vj 1, y) = l(vj 2, x)+l(vj 2, y) for any j = 1,..., r.

6 6 C. Barrientos, S. Cichacz, D. Froncek, E. Krop and C. Raridan Since w(v j 1, x) = w(vj 1, y), we obtain that for any j = 1, 2,..., r. Hence (l(v p 1, x) + l(vp 2, x)) = (l(v p 1, y) + l(vp 2, y)) (r 1) (l(v j 1, x) + l(vj 2, x)) = (r 1) (l(v j 1, y) + l(vj 2, y)), j=1 j= implying that l(v j 1, x) + l(vj 2, x) = l(vj 1, y) + l(vj 2, y), a contradiction. Lemma 3.4. Let r > 1, n > 2. The Cartesian product K(n; r) C 4 is distance magic if and only if n is even. Proof. Let H = K(n; r) C 4. Notice that V (H) = 4nr and H is [n(r 1) + 2]- regular. Suppose that H is distance magic and l is a distance magic labeling of H with magic constant µ. Let l(v j i, u) + l(vj i, w) = aj u,w for any i = 1,..., n, j = 1,..., r. Then n µ = w(v j i, x) = l(v p h, x) + aj u,w, h= for any i = 1,..., n, j = 1,..., r. Analogously let l(v j i, x) + l(vj i, y) = aj x,y for any i = 1,..., n, j = 1,..., r. Observe that 2µ = w(vi h, x) + w(vi h, y) = n a p x,y + 2a h u,w (3.1) p=1,p h and 2µ = w(v j i, x) + w(vj i, y) = n a p x,y + 2a j u,w (3.2) for j = 1,..., r,i = 1,..., n. Thus subtracting equation 3.1 from 3.2 we obtain: r 154 n(a j x,y a h x,y) = 2(a j u,w a h u,w), for any j, h = 1,..., r. Analogously 2(a j x,y a h x,y) = n(a j u,w a h u,w) for any j, h = 1,..., r. Obviously for any j, h = 1,..., r we have (n 2)(a j x,y a h x,y) = (n 2)(a j u,w a h u,w). Since n 2 thus for any j = 1,..., r we have a j x,y + a j u,w = a for some constant a. If a j x,y = a j u,w = a/2 for any j = 1, 2,..., r, then since µ = w(v j i, z) = n h=1 l(vp h, z) + a/2 for any z {x, y, u, w} and i = 1,..., n, j = 1,..., r,

7 Distance Magic Cartesian Products of Graphs 7 it is easy to check that n 155 i=1 l(vj i, z) = na/4 for any z {x, y, u, w} and j = 1,..., r. 156 In this situation there exists distance magic labeling for the graph G is and only if 157 there exists a magic rectangle set MRS(2, n; 2r) with all its row sums equal to the 158 constant a/2 and with all its column sums equal to the constant na/4. If n is even, then a magic rectangle set MRS(2, n; 2r) exists by Theorem 7. Denote by z j i,h the entry in the i-th row and h-th column of the j-th rectangle from the set MRS(2, n; 2r), let: l(v j i, x) = zj i,1, l(vj i, y) = zj i,2, 159 l(v j i, u) = zj+r i,1, l(vj i, v) = zj+r i,2 for i = 1,..., n and j = 1,..., r. Obviously the labeling l is distance magic. Therefore we can assume now that n is odd. Suppose first that a j x,y = a/2 c for any j = 1, 2,..., r and some constant c. Thus a j x,y = a/2 + c for any j = 1, 2,..., r and moreover n i=1 (l(vj i, x) + l(vj i, y)) = n(a/2 + c), n i=1 (l(vj i, u) + l(vj i, v)) = n(a/2 c) for any j = 1, 2,..., r. Observe that: 2µ = w(v j i, x) + w(vj i, y) = n(r 1)(a/2 + c) + 2(a/2 c), 2µ = w(v j i, u) + w(vj i, v) = n(r 1)(a/2 c) + 2(a/2 + c). 160 Subtracting the above equation we obtain that c = 0, hence a j x,y = a j u,v = a/2 and 161 a distance magic labeling is impossible since there does not exist a magic rectangle 162 set MRS(2, n; 2r) for n being odd n must be even by Theorem 6 and Observation 8. Let now a j x,y = a/2 c j and a j 163 u,v = a/2 + c j for any j = 1, 2,..., r and some constants c j. Therefore n i=1 (l(vj i, x) + l(vj i, y)) = n(a/2 + cj ), n i=1 (l(vj i, u) l(v j i, v)) = n(a/2 cj ) for any j = 1, 2,..., r. Notice that 2µ = w(v j i, x) + w(vj i, y) = n (a/2 + c p ) + 2(a/2 c j ) (3.3) and 2µ = w(vi h, x) + w(vi h, y) = n (a/2 + c p ) + 2(a/2 c h ) (3.4) p=1,p h for j = 1,..., r,i = 1,..., n. Thus subtracting equation 3.3 from 3.4 we obtain: (n + 2)c h = (n + 2)c j for any j, h = 1,..., r. Hence c j = c for any j = 1, 2,..., r and a distance magic labeling does not exist. As a result of Lemmas 3.2, 3.3 and 3.4, we have the following theorems: Theorem 9. The Cartesian product K(n; r) C 4 is distance magic if and only if r > 1 and n > 2 is even. Theorem 10. The Cartesian product K(n; r) C 4 is distance magic if and only if there exists a magic rectangle set MRS(2, n; 2r).

8 8 C. Barrientos, S. Cichacz, D. Froncek, E. Krop and C. Raridan References [1] Acharya, B. D., Rao, S. B., Singh, T., and Parameswaran, V. Neighborhood magic graphs. National Conference on Graph Theory, Combinatorics and Algorithms, [2] Anholcer, M., and Cichacz, S. Note on distance magic graphs G C 4. Graphs and Combinatorics (2014), accepted, DOI: /s x. [3] Anholcer, M., Cichacz, S., and Peterin, I. Spectra of graphs and closed distance magic labelings, preprint (2014). [4] Anholcer, M., Cichacz, S., Peterin, I., and Tepeh, A. Distance magic labeling and two product graphs. Graphs and Combinatorics (2014), accepted, DOI: /s [5] Arumugam, S., Froncek, D., and Kamatchi, N. Distance magic graphs a survey. Journal of the Indonesian Mathematical Society JIMS Special Issue (2011), [6] Beena, S. On Σ and Σ labelled graphs. Discrete Math. 309, 6 (2009), [7] Cichacz, S. Group distance magic graphs G C 4. Discrete Appl. Math. (2014), accepted, DOI: /j.dam [8] Cichacz, S., and Görlich, A. Constant sum partitions of sets of integers and distance magic graphs, preprint (2013). [9] Diestel, R. Graph Theory, fourth ed., vol. 173 of Graduate Texts in Mathematics. Springer, Heidelberg, [10] Froncek, D. Handicap distance antimagic graphs and incomplete tournaments. AKCE Int. J. Graphs Comb. 10, 2 (2013), [11] Gallian, J. A. A dynamic survey of graph labeling. Electron. J. Combin. 5 (2013), DS 6, [12] Hammack, R., Imrich, W., and Klavžar, S. Handbook of product graphs, second ed. Discrete Mathematics and its Applications (Boca Raton). CRC Press, Boca Raton, FL, With a foreword by Peter Winkler. [13] Harmuth, T. Ueber magische Quadrate und ähnliche Zahlenfiguren. Arch. Math. Phys. 66 (1881), [14] Harmuth, T. Ueber magische Rechtecke mit ungeraden Seitenzahlen. Arch. Math. Phys. 66 (1881), [15] Jinnah, M. I. On Σ-labelled graphs. In Technical Proceedings of Group Discussion on Graph Labeling Problems, eds. B.D. Acharya and S.M. Hedge, (1999),

9 Distance Magic Cartesian Products of Graphs [16] Miller, M., Rodger, C., and Simanjuntak, R. Distance magic labelings of graphs. Australas. J. Combin. 28 (2003), [17] Rao, S. B. Sigma graphs a survey. In Labelings of Discrete Structures and Applications, eds. B.D. Acharya, S. Arumugam, and A. Rosa, Narosa Publishing House, New Delhi (2008), [18] Rao, S. B., Singh, T., and Parameswaran, V. Some sigma labelled graphs I. In Graphs, Combinatorics, Algorithms and Applications, eds. S. Arumugam, B.D. Acharya, and S.B. Rao, Narosa Publishing House, New Delhi (2014), [19] Seoud, M. A., Abdel Maqsoud, A. E. I., and Aldiban, Y. I. New classes of graphs with and without 1-vertex magic vertex labeling. Proc. Pakistan Acad. Sci. 46, 3 (2009), [20] Vilfred, V. Σ-labelled graphs and circulant graphs. PhD thesis, University of Kerala, Trivandrum, India, 1994.

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