International Journal of Trends and Technology- Volume3 Issue1- 2012

On Strongly Multiplicative Graphs *M. Muthusamy 1, K.C. Raajasekar 2, J. Baskar Babujee 3 1SCSVMV University, Enathur, Kanchipuram – 631 561, India. 2Department of Mathematics, Presidency College, Triplicane, Chennai - 600 005, India. 3Department of Mathematics, Anna University, Chennai-600 025, India.

Abstract — A graph G with p vertices and q edges is said to be an induced subgraph of a strongly multiplicative graph. In this strongly multiplicative if the vertices are assigned distinct paper we study strongly multiplicative labeling for some numbers 1, 2, 3, …, p such that the labels induced on the edges special classes of graphs. by the product of the end vertices are distinct. We prove some of the special graphs obtained through graph operations such as + Definition 1: Cn (a graph obtained by adding pendent edge for each of d A graph G = (V(G), E(G)) with p vertices is said to the cycle Cn), (Pn  mK1) +N2, Pn + mK1 and Cn (cycle Cn with non-intersecting chords) are strongly multiplicative. be multiplicative if the vertices of G can be labeled with distinct positive such that label induced on the edges AMS Subject Classification: 05C78 by the product of labels of end vertices are all distinct. Keywords— Graph, labeling, path, cycle, relation, function. Definition 2: A graph G = (V(G), E(G)) with p vertices is said to I. INTRODUCTION be strongly multiplicative if the vertices of G can be labeled with p distinct integers 1, 2 ,..., p such that label induced on In this paper we deal with only finite, simple, connected the edges by the product of labels of the end vertices are all and undirected graphs obtained through graph operations. A distinct. labeling of a graph G is an assignment of labels to vertices or edges or both following certain rules. Labeling of graphs plays II. MAIN RESULTS an important role in application of in Neural Theorem 1: Networks, Coding theory, Circuit Analysis etc. A useful  survey on graph labeling by J.A. Gallian (2010) can be found The graph Cn has strongly multiplicative labeling. in [5]. All graphs considered here are finite, simple and Proof : Let V  v , v , v ,...... v , v ,.....v be the vertex undirected. In most applications labels are positive (or  1 2 3 n n1 2n nonnegative) integers, though in general real numbers could and E  E1  E2  E3 be the edge set where be used. Beineke and Hegde [4] call a graph with p vertices strongly E1  vivi1, 1  i  n, E2  vivni , 1  i  n and multiplicative if the vertices of G can be labeled with distinct E  v v  of the graph C  . Define a bijection integers 1, 2, . . . , p such that the labels induced on the edges 3 n 1 n by the product of the end vertices are distinct. They prove the f :V  1,2,3,.....2n such that following graphs are strongly multiplicative: trees; cycles; Case (i): When n is even wheels; Kn if and only if n ≤ 5; Kr,r if and only if r ≤ 4; and  n Pm× Pn. Beineke and Hegde [4] obtain an upper bound for the  4i  3 1  i  maximum number of edges λ(n) for a given strongly f v   2 i  n multiplicative graph of order n. It was further improved by 4n  i 3 1  i  n C. Adiga, H. N. Ramaswamy, and D. D. Somashekara [2] for  2 greater values of n. It remains an open problem to find a and nontrivial lower bound for λ(n). Seoud and Zid [7] prove the following graphs are strongly multiplicative: wheels; rK for  n n  4i  2 1  i  all r and n at most 5; rKn for r ≥ 2 and n = 6 or 7; rKn for r ≥ 3 2 f vni    and n = 8 or 9; K for all r; and the corona of P and K for n 4,r n m 4n  i 4 1  i  n all n and 2 ≤ m ≤ 8.Germina and Ajitha [6] prove that K2 + Kt,  2 quadrilateral snakes, Petersen graphs, ladders, and unicyclic Define an induced function g : E  N, such that graphs are strongly multiplicative. Acharya, Germina, and Ajitha [1] have shown that every graph can be embedded as gviv j   f vi  f v j   viv j  E and vi , v j V We show that the labeling of edges within the edge sets and * Department of Mathematics, among the edge sets are distinct. If it is assumed in each case Rajalakshmi Engineering College, Thandalam–602105, India.

ISSN: 2231-5373 http://www.internationaljournalssrg.org Page 13

International Journal of Mathematics Trends and Technology- Volume3 Issue1- 2012 that the induced label of the edges are same then we arrive at 8n  7 contradiction. gives i = j a contradiction as i  j  is not possible. 4 n n For the edges in E1 : Sub Case (c) For 1  i  and  1  j  n also i  j n 2 2 Sub Case (a) For i  j and 1  i, j  in E : 2 2 2 If we assume that We see that gv v   16i  20i  6 gvivi1   gv jv j1  i ni g v v  16 j 2  20 j  6  16n2  32nj  28n  8 j  12  f vi f vi1   f v j f v j1   j n j   

4i  3 4i 1 3  4 j  3 4 j  1 3  gvivni   gv jvn j 

1 Clearly the labeling of edges of E and that of E are all gives i = j a contradiction as i  j  is not possible. 1 2 2 distinct as the labeling of edges of E1 are all odd and those of n are even. Also edges of & and & are also Sub Case (b) For i  j and 1  i, j  n E2 E1 E3 E2 E3 2 distinct as edge in E3 is with the minimum odd label 3. If we assume that gvivi1   gv jv j1   Cn has strongly multiplicative labeling for n even.  f vi f vi1   f v j f v j1  4n  i 34n  i 1 3  4n  j 34n  j  1  3

4n 1 gives i = j a contradiction as i  j  is not possible. 2 n n Sub Case (c) For 1  i  and  1  i  n also i  j 2 2 in E1 2 We have g v i v i  1   16 i  8 i and g v v   16 j 2  8 j   16 n 2  8n  32 nj  j j1 Clearly 16i2  8i  16 j 2  8 j 16n2  8n  32nj

 gv v   gv v  i i1 j j1

For the edges in E2 : n Fig. 1. Strongly multiplicative labeling for C  Sub Case (a) For i  j and 1  i, j  8 2 Case (ii): When n is odd If we assume that gvivni   gv jvn j  The bijection f : V  1,2,3,.....2nis defined as the  f v f v  f v f v  i   ni   j   ni  following 4i  34i  2  4 j  34 j  2  n 1 4i  3 1  i  5  2 gives i = j a contradiction as is not possible. f v   i  j  i  n 1 4 4n  i 3 1  i  n n  2 Sub Case (b) For i  j and  1  i, j  n and 2  n 1 If we assume that gvivni   gv jvn j  4i  2 1  i   2 f vni     f vi  f vni   f v j f vn j  n 1 4n  i 4 1  i  n 4n  i 34n  i  4  4n  j 34n  j  4  2

ISSN: 2231-5373 http://www.internationaljournalssrg.org Page 14

International Journal of Mathematics Trends and Technology- Volume3 Issue1- 2012

n  f v f v   f v f v  The proof follows as above, replacing 1  i  by i n2 j n2 2  in  2  jn  2 n  1 n n  1  i  j a contradiction. 1  i  and 1  i  n by 1  i  n . 2 2 2 For the edges in E : Thus C  has strongly multiplicative labeling for n odd. 2 n For i  j, 1  i, j  n  3  Hence Cn has strongly multiplicative labeling for all n. If we assume gvivn1   gv jvn1 

 f vi f vn1   f v j f vn1   in  1  jn  1

 i  j a contradiction

For the edges in E3 : For i  j, 1  i, j  3

If we assume gvnvni   gvnvn j 

 f vn f vni   f vn  f vn j   nn  i  nn  i  i  j a contradiction

Now we have to show that the edges between different edge sets are distinct.

For the edges in E1 and E2 :  Fig.2. strongly multiplicative labeling for C9 For i  j, 1  i, j  n  3 Theorem 2: The graph P  mk  N has strongly  2 1  2 If we assume gvivn2   gv jvn1  multiplicative labeling.  f vi f vn2   f v j f vn1  Proof: Let V  v1, v2 , v3,...... vm ,vm1,.....vm4, where  in  2  jn 1 n  m  4 be the vertex set and E  E1  E2  E3 be the 2i  j edge set where E  v v , 1  i  n  3,  i  j  a contradiction 1 i n2 n E2  vi vn1 , 1  i  n  3 and For the edges in E and E : E3  vnvn1,vnvn2 ,vnvn3 of the graph P2  mk1  N2 . 1 3 For Define a bijection f :V  1,2,3,.....n such that i  j, 1  i  n  3, j  1,2,3

If we assume gvivn2   gvnvn j  f vi   i, 1  i  n

Define an induced function g : E  N, such that  f vi f vn2   f vn f vn j  gv v   f v  f v   v v  E and v , v  V n  2 n  j i j i j i j i j   a contradiction We show that the labeling of edges within the edge sets and n i among the edge sets are distinct. If it is assumed in each case that the induced label of the edges are same then we arrive at For the edges in E2 and E3 : contradiction. For i  j, 1  i  n  3, j  1,2,3 For the edges in E : 1 If we assume gv v   gv v  For i  j, 1  i, j  n  3 i n1 n n j

 f vi f vn1   f vn  f vn j  If we assume gvivn2   gv jvn2 

ISSN: 2231-5373 http://www.internationaljournalssrg.org Page 15

International Journal of Mathematics Trends and Technology- Volume3 Issue1- 2012

n  1 n  j  in  jn   a contradiction. n i  i  j a contradiction This implies all the edge labeling are distinct. Hence the graph P  mk  N has strongly multiplicative labeling. Now we have to show that the edges between different edge 2 1 2 sets are distinct. For the edges in E and E : 1 2 For i  j, 1  i, j  n  2

If we assume gvivn1   gv jvn 

 f vi f vn1   f v j f vn   in 1  jn i  n  a contradiction i  j

For the edges in E1 and E3 : For 1  i  n  2

If we assume gvivn1   gvnvn1 

 f vi f vn1   f vn  f vn1  Fig.3. Strongly multiplicative labeling for P  5k  N 2 1 2  in 1  nn 1 Theorem 3: The graph P  mk has strongly multiplicative 2 1  i  n a contradiction labeling. For the edges in E and E : Proof : Let V  v ,v , v ,...... v , be the vertex set and 2 3 1 2 3 n For 1  i  n  2 E  E1  E2  E3 be the edge set where If we assume gvivn   gvnvn1  E1  vivn1 , 1  i  n  2, E2  vi vn , 1  i  n  2 and  f vi f vn   f vn  f vn1  E  v v of the graph P  mk . 3  n n1 2 1  in  nn  1 Define a bijection f : V  1,2,3,.....n such that  i  n 1 a contradiction

f vi   i, 1  i  n This implies all the edge labeling are distinct. Hence the graph Define an induced function g : E  N, such that P2  mk1 has strongly multiplicative labeling. gviv j   f vi f v j  forall viv j  E and vi , v j  V We show that the labeling of edges within the edge sets and among the edge sets are distinct. If it is assumed in each case that the induced label of the edges are same then we arrive at contradiction.

For the edges in E1 : For i  j, 1  i, j  n  2

If we assume gvivn1   gv jvn1 

 f vi f vn1   f v j f vn1   in  1  jn  1  i  j a contradiction

For the edges in E : Fig.4. Strongly multiplicative labeling for P  6k 2 2 1 For i  j, 1  i, j  n  2

If we assume gvivn   gv jvn 

 f vi f vn   f v j f vn 

ISSN: 2231-5373 http://www.internationaljournalssrg.org Page 16

International Journal of Mathematics Trends and Technology- Volume3 Issue1- 2012

Definition 4[3]: Let G  V , E : R1, R2 . The vertex set n Sub Case (c) Edges of E1 for 1  i  are with odd labels V  1,2,.....,n and the edge set is defined by the relations 2 n R1 : b  a  1  a, b  V and those of 1  j  n are with even labels. Thus the R1 and R2 such that . 2 R2 : a  b  n  1 edge labels of these categories are distinct. If n  0(mod 2) , we get cycle C with d  n  2 2 n non-intersecting chords and when n  1 (mod 2) we get cycle C with d  n  3 2 non-intersecting chords. For the edges in E : n 2 d n The graph obtained by this relation is Cn , n  5. For i  j and 1  i, j  1 d 2 Theorem 5: The graph Cn , n  5 with non If we assume that gvi1vni1   gv j1vn j1  intersecting chords has strongly multiplicative labeling.  f vi1 f vni1   f v j1 f vn j1  Proof: Let V  v1,v2 , v3,...... vn, be the vertex set and d 2i 112n  n  i 1 2  2 j 112n  n  j 1 2 E  E1  E2  E3 be the edge set of the graph Cn with non 1 intersecting chords, where E  v v , 1  i  n, gives i = j a contradiction as i  j  is not possible 1 i i1 2  n  Now we have to show that the edges between different edge E2  vi1vni1, 1  i  1 and E3  vn v1. Define a  2  sets are distinct. bijection f :V  1,2,3,.....n such that For the edges in E and E : Case (i):When n is even 1 2 n  n The labels of edges of E and those of E for 1  i  are 2i 1 1  i  2 1 2  2 f v   distinct as the labels of are with even numbers and those i  n E2 2n  i 2 1  i  n n  2 of E1 for 1  i  are with odd numbers. Define an induced function g : E  N, such that 2 n n gv v   f v f v   v v  E and v , v  V. We show Also i  j and for 1  i  n and 1  j  1 i j i j i j i j 2 2 that the labeling of edges within the edge sets and among the gv v   f v f v   2i2  i  2n2  2n  4ni  3i edge sets are distinct. i i1 i i1 If it is assumed in each case that the induced label of the edges gv v   f v f v   2 j 2  j are same then we arrive to a contradiction. j1 n j1 j1 n j1 gvivi1   gv j1vn j1 

For the edges in E1 : Moreover gvnv1   f vn f v1  n Sub Case (a) For i  j and 1  i, j  1 = 2 2 d Thus all the edges of Cn , n  5 with non intersecting If we assume that g v v  g v v  i i1   j j1  d chords are distinct. Hence C , n  5 with non intersecting  f v f v   f v f v  n i i1 j j1 chords has strongly multiplicative labeling for n even. 2i 12i  11  2 j 12 j 1 1 gives i = j a contradiction. n Sub Case (b) For i  j and 1  i, j  n 2

If we assume that gvivi1   gv jv j1 

 f vi f vi1   f v j f v j1  2n  i 22n  i 1 2  2n  j 22n  j 1 2 gives i = j a contradiction as i  j  2n 1 is not possible.

ISSN: 2231-5373 http://www.internationaljournalssrg.org Page 17

International Journal of Mathematics Trends and Technology- Volume3 Issue1- 2012

3 Fig. 6. C9 with non intersecting chords

Theorem 6: The Union of two strongly multiplicative graphs is also a strongly multiplicative graph. Proof: Let G and G be two strongly multiplicative graphs 1 2 with number of vertices n and n respectively. The graph 1 2 G  G will have n  n vertices. Since G is strongly 1 2 1 2 1 multiplicative the induced labeling of the edges are distinct.

Relabel the vertices of G2 as n1  1 for the vertex with label 1, n  2 for the vertex of label 2 and so on. As vertices are 1 3 relabeled, the induced edge labeling will be added by the Fig. 5. C with non intersecting chords 8 2 quantity n1  i  j n1 for all 1  i, j  n2 . The addition of 2 the quantity n  i  j n with the labels of the edges of 1   1 G2 will still result in the label of edges distinct. As G2 is also

strongly multiplicative graph, G  G is strongly 1 2 multiplicative graph. Case (ii):When n is odd III. REFERENCES: We define the bijection f :V  1,2,3,.....n such that [1] B. D. Acharya, K. A. Germina, and V. Ajitha,  n  1 Multiplicatively indexable graphs, in Labeling of Discrete  2i  1 1  i  Structures and Applications, Narosa Publishing House, f v   2 New Delhi, 2008, 29-40. i  n  1 2n  i  2  1  i  n [2] C. Adiga, H. N. Ramaswamy, and D. D. Somashekara,  2 A note on strongly multiplicative graphs, Discuss. Math., With the edge set being E  E  E  E where 24 (2004) 81-83. 1 2 3 [3] J. Baskar Babujee, L. Shobana, “Graph from Relations  n  1  and its Labeling”, European Journal of Scientific E1  vivi1, 1  i  n, E2  vi1vni1, 1  i   1  2  Research ,Vol.64 No.3 (2011), pp. 463-471. [4] L. W. Beineke and S. M. Hegde, Strongly multiplicative and E1  vnv1. graphs, Discuss. Math. Graph Theory, 21 (2001) 63-75. The proof follows as above. [5] Gallian, J.A., “A Dynamic Survey of Graph Labeling”, d Electronic Journal of Combinatorics [2010]. Thus C with non intersecting chords has strongly n [6] K. A. Germina and V. Ajitha, Strongly multiplicative multiplicative labeling for n odd. graphs, unpublished. d Hence C with non intersecting chords has strongly [7] M. A. Seoud and A. Zid, Strong multiplicativity of n unions and corona of paths and complete graphs, Proc. multiplicative labeling for all n. Math. Phys. Soc. Egypt, 74 (1999) 59-71.

ISSN: 2231-5373 http://www.internationaljournalssrg.org Page 18