Damir Vuki~evi} a, * and Ante Graovac b,c INTRODUCTION

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1 CROATICA CHEMICA ACTA CCACAA 77 (3) (2004) ISSN CCA-2951 Origina Scientific Paper Which Vaence Connectivities Reaize Monocycic Moecues: Generating Agorithm and Its Appication to Test Discriminative Properties of the Zagreb and Modified Zagreb Indices Damir Vuki~evi} a, * and Ante Graovac b,c a Department of Mathematics, Facuty of Science, University of Spit, Nikoe Tese 12, HR Spit, Croatia b Department of Chemistry, Facuty of Science, University of Spit, Nikoe Tese 12, HR Spit, Croatia c Ru er Bo{kovi} Institute, P.O. Box 180, HR Zagreb, Croatia RECEIVED JANUARY 20, 2004; REVISED APRIL 2, 2004; ACCEPTED APRIL 4, 2004 Key words vaence connectivities monocycic graphs Zagreb index modified Zagreb index Vaence connectivities in hydrogen suppressed graphs are characterized by 10-tupes of quantities m ij where m ij is the number of edges that connect vertices of vaences i and j. It is shown which 10-tupes are reaizabe by monocycic graphs and this finding is used to compare discriminative properties of the Zagreb and modified Zagreb indices. INTRODUCTION Connectivity of atoms in the moecue is convenienty described by the corresponding moecuar graph. The atoms are represented by vertices and bonds by edges of graphs. Such approach enabes one to reate properties of moecues to their connectivities and has been a subject of intensive research in the ast few decades. 1,2 Moecuar graphs can be numericay characterized in a variety of ways. The simpest numbers ascribed to a graph are the number of its vertices and its edges. Each vertex x in a given graph G can be characterized by the number of its neighboring vertices, d G (x), which is caed a vertex degree and which obviousy paraes the chemica notion of the vaence of atoms. Connectivity of vertices in a graph can be further characterized by specifying the numbers m ij of edges that connect (adjacent) vertices of degrees i and j. Obviousy, the unique sequence of m ij s is ascribed to any graph, but the opposite generay does not hod, e.g. there is no graph with m 12 = 1 and a other m ij being equa to zero. This nontrivia probem has been recenty attacked in a series of papers. Firsty, the probem of the existence of graphs with a given sequence of m ij s for i, j=1,2,4 was soved 3 and after that it was generaized for i, j=1,2,3,4. 4 The agorithms obtained are rather invoved, but for specia casses of graphs one shoud hope to deveop particuar and faster agorithms. Indeed, such an agorithm was deveoped for connected acycic graphs (trees) with i, j=1,2,3,4. 5 In this paper, monocycic graphs, i.e. connected graphs having ony one cyce and an arbitrary number of trees attached to it are studied. * Author to whom correspondence shoud be addressed. (E-mai: vukicevi@pmfst.hr)

2 482 D. VUKI^EVI] AND A. GRAOVAC In the second section basic definitions and notations are introduced and in the third section necessary and sufficient conditions of m ij s for the existence of monocycic graph(s) are determined. These conditions are summarized in Theorem 5. Based on this theorem and Theorems 6 8, an efficient generating agorithm is deveoped that gives a m ij sequences corresponding to monocycic graphs with a given number of vertices. Finay, in the fifth section, this agorithm is utiized to test discriminative properties of the Zagreb and modified Zagreb indices in monocycic graphs. PRELIMINARIES Here we consider connected graphs with the maxima degree at most 4 and with a finite number of vertices. Further, we restrict our attention to monocycic graphs, i.e. to the graphs having ony one cyce. The set of a monocycic graphs is denoted by T. To each graph GT a unique sequence (m 11, m 12, m 13, m 14, m 22, m 23, m 24, m 33, m 34, m 44 ) can be associated in such a way that in graph G there are exacty m ij edges that are adjacent with vertices of degrees i and j. In this way, a function :TN 10 0 is defined, where N 10 0 is the set of a 10-tupes of nonnegative integers (i.e. N 0 = N0, where N is the set of natura numbers). It can be easiy seen that this function is not a surjection (or mapping onto), e.g. there is no graph GT such that m 11 = 4 and m 12 = m 13 = m 14 = m 22 = m 23 = m 24 = m 33 = m 34 = m 44 =0. Therefore, it is of interest for chemistry to find a necessary and sufficient condition for an arbitrary sequence (m 11, m 12, m 13, m 14, m 22, m 23, m 24, m 33, m 34, m 44 ) to beong to the set (T). In the second section, we find these necessary and sufficient conditions. The foowing equation: m 11 + m 12 + m 13 + m 14 + m 22 + m 23 + m 24 + m 33 + m 34 + m 44 = n hods for monocycic graphs with n vertices. In the fourth section, we use the resut of the previous sections to find a fast agorithm that generates a the sequences (m 11, m 12, m 13, m 14, m 22, m 23, m 24, m 33, m 34, m 44 ) for prescribed n = m ij N (T). Note that if we try to derive these sequences (m 11, m 12, m 13, m 14, m 22, m 23, m 24, m 33, m 34, m 44 ) expicity, we woud need to construct a trees with maxima degrees smaer than or equa to 4. It can be easiy shown that this is a nonpoynomia probem. Using our theorems, we can generate these sequences in poynomia time. This is a key feature of our agorithm, because one of the most important goas of theoretica computer science is to repace as many nonpoynomia agorithms as possibe by poynomia ones. This agorithm promises important chemica appications, e.g. as expained in the fifth section, it enabes a comparison of discriminative properties of the Zagreb and modified Zagreb indices. 57 years ago, chemists 6 noticed that information given by a graph can be compressed in a singe number, the so caed moecuar descriptor. Of course, there is an infinite number of ways to do this and ony those descriptors which correate we with physica, chemica and bioogica properties of moecues are of interest. These indices, the so caed topoogica inidices, have found enormous appications in QSPR (Quantitive Structure Property Reationship) and QSAR (Quantitive Structure Activity Reationship) and other studies, and the reader is referred to recent monographs for an overview. 7,8 Many of these indices are uniquey defined by a sequence (m 11, m 12, m 13, m 14, m 22, m 23, m 24, m 33, m 34, m 44 ). A we known such index is the Zagreb index 9 defined by M 2 (G) = d G (x) d G (y) = i j ij (G), xy, E( G) 1 i j4 where ij (G) denotes the number of edges that connect vertices of degree i and degree j. Properties and appications of this index and its reationship with other topoogica indices have been recenty reviewed. 10 The foowing modification of the Zagreb index, the so caed modified Zagreb index, is proposed: 10,11 1 m ij ( G) *M 2 (G) = = xy, E( G) dg() x dg() y 1 i j4 ij Obviousy, if two graphs G and G' have equa sequences (G) and (G'), they cannot be discriminated by any of the two above indices. On the other hand, one or/and the other of these indices can be the same for nonequa sequences. Using the agorithm deveoped in the fourth section, in the fifth section we anayze how we these indices distinguish sequences m,m'(t). Herein, we use the standard graph theory terms and notations. Let G be a graph with vertex set V(G) and edge set E(G). Let A,B V(G) where A and B are disjoint sets. By GA we denote the subgraph of G induced by the vertex set A, bye G (A) we denote the set of edges of G with both adjacent vertices in A, bye G (A,B) we denote the set of edges of G with one adjacent vertex in A and the other in B. We aso put e G (A) =E G (A) and e G (A,B) =E G (A,B). Let G be a graph and S a set of edges in compement of G. By G + S we denote the graph obtained from G by adding the set S. We aso define functions ij :TN 0, for each 1 i j 4by ij (G) = m ij if and ony if exacty m ij edges connect vertices with degrees i and j in graph G. Aso by A n we denote the set of a monocycic graphs with n vertices having the maxima degree at most 4. The edge connecting vertex with itsef is caed a oop and the pair of edges having the same termina vertices are caed parae edges. At the end of this section, we give the foowing definition:

3 VALENCE CONNECTIVITIES REALIZABLE BY MONOCYCLIC GRAPHS 483 Definition 1. Let p,q,rn 0 and n 3,n 4 N such that p+q+r = n 3 +n 4.SetF(n 3, n 4, p, q, r) is the set of graphs G such that V(G) = N 3 N 4 ; N 3 = x 1,, x n3 ; N 4 = y 1, y 2, y n4 ; e G (N 3 )=p; e G (N 3, N 4 )=q; e G (N 4 )=r; d G (x) 3, for each x N 3 ; and d G (y) 4, for each y N 4. Further, G is a simpe graph except in the foowing cases in which it has one of the foowing: a singe oop or a singe pair of parae edges: 1) (n 3 =1)and(p = 1). There is a singe oop adjacent to x 1. 2) (n 4 = 1) and (r = 1). There is a singe oop adjacent to y 1. 3) (n 3 =2)and(p = 2). There is a pair of parae edges adjacent to x 1 and x 2. 4) (n 4 =2)and(r = 2). There is a pair of parae edges adjacent to y 1 and y 2. 5) (n 3 =1)and(p =0)and(r = 0). There is a pair of parae edges adjacent to x 1 and y 1. 6) (n 4 =1)and(p =0)and(r = 0). There is a pair of parae edges adjacent to x 1 and y 1. Set F (n 3, n 4, p, q, r) is the auxiiary famiy of graphs that wi be needed to obtain further resuts. NECESSARY AND SUFFICIENT CONDITIONS OF VALENCE CONNECTIVITIES FOR THE EXISTENCE OF A MONOCYCLIC MOLECULAR GRAPH In this section, we give a mathematica background of the agorithm that wi be deveoped in the next section, i.e. we give the necessary and sufficient conditions of m ij for the existence of a monocycic moecuar graph. We start with few emmas: Lemma 2. Let k, N and et a 1,a 2,,a k, b 1,,b 1 N 0 and et max b 1,,b 1 min b 1,,b 1 1 k a bi, i1 q min min i,, i1 then there is a simpe bipartite graph G with q edges and partition casses A = x 1,,x k and B = y 1,,y such that d G (x i ) a i for each i = 1,,k and d G (y i ) b i for each i = 1,,. Proof. We prove the caim by induction on k. If k =1, then the caim is trivia. Let us prove the inductive step. Distinguish three cases: 1) min d, G ( ak), bi =. i1 k1 In this case, we have q min min ai,, bi 1, i1 i1 therefore there is, by inductive hypothesis, a bipartite graph G' with partition casses a 1,,a k 1 and B such that d G (x i ) a i for each i = 1,,k 1 and d G (y i ) b i 1 for each i = 1,,. Graph G=G'+a k b 1,,a k b has the required properties. 2) min d, G ( ak), bi = d G (a k ). i1 Without oss of generaity, we may assume that b 1 b 2 b. In this case, we have maxb 1 1,, b dg (x k ) 1,b dg (x k )+1,,b minb 1 1,, b dg (x k ) 1,b dg (x k )+1,,b 1; q d G (a k ) min k1 dg ( xk ) min ai,, bi 1 bi i1 i1 idg ( xk ) 1, and therefore there exists, by inductive hypothesis, a bipartite graph G' with partition casses a 1,,a k 1 and B such that d G (x i ) a i for each i = 1,,k 1, and d G (y i ) b i 1 for each i = 1,, d G (x k ), and d G (y i ) b i for each i = d G (x k ) + 1,,. Graph G=G'+a k b 1,, a k b dg (x k ) has the required properties. 3) min d, G ( ak), bi = b i. i1 i1 This case is trivia. Lemma 3. Let p,q,r N 0 and n 3,n 4 N such that p+q+r = n 3 +n 4.If ( p n3) and ( rn4) and ( prn3n4 1) and ( q3n3 2p) and ( q4n4 2r, ) then F (n 3, n 4, p, q, r). Proof. Let us describe a graph G F(n 3, n 4, p, q, r). If n 3 = 1 and p = 0, then GN 3 is a graph with no edges; if n 3 = 1 and p = 1 then GN 3 is a graph with a singe vertex and a oop; if n 3 = p and n 3 = 2, then GN 3 is a graph with two vertices and two parae edges adjacent to them; if n 3 = p and n 3 2, then GN 3 is a cyce; if n 3 2 and p n 3 2, then GN 3 is a graph such that (G(N 3 )) 1; if n 3 2 and n 3 2 p n 3 1, then GN 3 is an acycic graph such that (G(N 3 )) 1 and that (G(N 3 )) 2. If n 4 = 1 and r = 0, then GN 4 is a graph with no edges; if n 4 = 1 and r = 1, then GN 4 is a graph with a singe vertex and a oop; if n 4 = r and n 4 = 2, then GN 4 is a graph with two vertices and two parae edges adjacent to them; if n 4 = r and n 4 2, then GN 4 is a cyce; if n 4 2 and r n 4 2, then GN 4 is a graph such that (G(N 4 )) 1;

4 484 D. VUKI^EVI] AND A. GRAOVAC if n 4 2 and n 4 2 r n 4 1, then GN 4 is an acycic graph such that (G(N 4 )) 1 and that (G(N 4 )) 2. We distinguish three cases: 1) (n 3 =1)and(p =0)and(r = 0). Note that q = n 3 + n 4 p r= n n 3 2p =3, hence 1 n 4 2. If n 4 = 1, then G is the graph with two vertices adjacent with two parae edges; and if n 4 =2, then G is the graph such that V(G) = x 1, y 1, y 2 and E(G) = x 1 y 1, x 1 y 1, x 1 y 2. 2) (n 4 =1)and(p =0)and(r = 0). This case can be soved anaogousy as the previous one. 3) (n 3 1) and (n 4 1) or (p 0) or (r 0). Note that max 4 d G (y 1 ),, 3 d G (y n4 ) min 4 d G (y 1 ),, 4 d G (y n4 ) 1. So, from the previous Lemma, it foows that it is sufficient to prove that n G G. i1 3 4 q min min 3d ( xi), n4, 4d ( yi) n i1 Note, that max 3 d G (x 1 ),, 3 d G (x n3 ) min 3 d G (x 1 ),, 3 d G (x n3 )1. Therefore, the above expression is equivaent to n n min 3 d ( x ), 4 d ( y ), n3n4 i1 i1 3 4 q min i i G G. Simpe computation shows that this is equivaent to q min 3n 3 2m 33,4n 4 2m 44, n 3 n 4. It remains to prove that q n 3 n 4. Distinguish three subcases: 3.1) (p 0) or (r 0). Note that (n 3 1)(n 4 1) 0, hence q=n 3 + n 4 p q r n 3 + n 4 1 n 3 n ) (n 3 1) and (n 4 1). Suppose to the contrary that q > n 3 n 4. Note that q= n 3 + n 4 p r n 3 + n 4. It foows that n 3 n 4 n 3 + n 4. Now, we have 2n 3 n 3 n 4 n 3 + n 4 and 2n 4 n 3 n 4 n 3 + n 4, which impies n 3 n 4 and n 4 n 3, which is a contradiction. Lemma 4. Let p,q,r N 0 and n 3,n 4 N such that p+q +r= n 3 + n 4.If ( p n3) and ( rn4) and ( prn3n4 1) and ( q3n3 2p) and ( q4n4 2r, ) then there is at east one connected graph in F (n 3, n 4, p, q, r). Proof. Let G' be a graph constructed in the previous Lemma. Let G be a set of graphs G'' F(n 3, n 4, p, q, r) and such that G''N 3 =G'N 3 and G''N 4 =G'N 4. Note that G is nonempty, since at east G' is in G.LetGbea graph in G with the smaest number of connected components. It remains to prove that G is connected. Suppose the contrary. Then, there is at east one component C (in G) and an edge e such that: a) e is not one of parae edges; b) e E G (N 3, N 4 ); c) e is contained in a cyce. Denote e = xy, such that x N 3 and y N 4. Let C' be any other component. Distinguish three cases: 1) V(C') N 3. Let c V(C') be an arbitrary vertex. Graph G xy + yc is in G and it has a smaer number of components than G, which is a contradiction. 2) V(C') N 4. Let c V(C') be an arbitrary vertex. Graph G xy + xc is in G and it has a smaer number of components than G, which is a contradiction. 3) V(C ') N 3 and V(C ') N 4. There is an edge x'y' such that x' N 3 and y' N 4 in C '. Graph G xy + x'y' +xy' +x'y is in G and it has a smaer number of components then G, what is a contradiction. We have obtained a contradiction in each case, so our caim is proved. Now, we sha prove four theorems that cover four different cases: 1) m 13 + m 23 + m 33 + m 34 0 and m 14 + m 24 + m 34 + m 44 0; 2) m 13 + m 23 + m 33 + m 34 = 0 and m 14 + m 24 + m 34 + m 44 0; 3) m 13 + m 23 + m 33 + m 34 0 and m 14 + m 24 + m 34 + m 44 =0; 4) m 13 + m 23 + m 33 + m 34 = 0 and m 14 + m 24 + m 34 + m 44 =0. Theorem 5. Let m =(m 11, m 12, m 13, m 14, m 22, m 23, m 24, m 33, m 34, m 44 ) N such that m 13 + m 23 + m 33 + m 34 0 and that m 14 + m 24 + m 34 + m There is a moecuar

5 VALENCE CONNECTIVITIES REALIZABLE BY MONOCYCLIC GRAPHS 485 graph G such that (G) = m if and ony if the foowing hods: ( m11 0) and [( m12 m23 m24 0) or ( m22 0) ] and ( n2, n3, n4 N0 ) and ( x 0) and ( m33 n3) and ( m33 xm24 n3) and ( m44 n4 ) and ( m44 xm23 n4 ) and ( m33 m34 m44 x n3 n4 ) and ( m 33+m44 n+n 3 4 1) and ( m33 m44 xm23 n3 n4 1) and ( m33 m44 xm24 n3 n4 1), and (( n3 1) or (( m34 n4) and ( m33 0))) and (( n4 1) or (( m34 n3) and ( m44 0))) and (( n3 2) or ( m33 n3)) and (( n4 2) or ( m44 n4 )) and (( n3 1) or ( xm24 0) or ( m22 1) ) and (( n4 1) or ( xm23 0) or ( m22 1)) and (( n3 1) or ( n4 1) or ( xm23 0) or ( xm24 0) or ( m22 2)) where n 2 =(m 12 +2m 22 + m 23 + m 24 )/2 n 3 =(m 13 + m 23 +2m 33 + m 34 )/3 n 4 =(m 14 + m 24 + m 34 +2m 44 )/4 x =(m 23 + m 24 m 12 )/2. Proof. Let us first prove the necessity. We sha define graph G with the required properties. Note that: m m 22 + m 23 + m 24 m 23 + m 24 m 12 (mod 2), therefore x N 0. By a tedious check of tweve inequaities, it can be proved that max x m 24,0 m23 x, n3 n4 m33 m44 1,, n4m44xm23, min 2. n3n4 1m33 m44xm23,n3m 33 2 Hence, by putting p' = max x m 24,0N 0,weget max x m 24,0p' m23 x, n3n4 m33m44 1,, n 4 m 44 x m 23, min. 2. n3 n4 1m33 m44xm23,n3m 33 2 Again, by a tedious check of eight inequaities, it can be proved that max 0, p' +x m 23 min x p', p' x+m 24, n 4 m 44, n 3 + n 4 1 m 33 m 44 p'. Therefore, by putting r' = max 0, p' +x m 23 N 0, we get max 0, p' +x m 23 r' min x p', p' x+m 24, n 4 m 44, n 3 + n 4 1 m 33 m 44 p'. Since x p'+r', it foows that q'=x p' r' N 0. Denote p=m 33 + p', q=m 34 + q', and r=m 44 + r'. Note that ( p n3) and ( rn4) and ( prn3n4 1) and ( q3n3 2p) and ( q4n4 2r, ) hence there is a connected graph G' F(n 3, n 4, p, q, r) constructed in the previous emma. Let us seect p' edges in E G' (N 3 ), q' edges in E G' (N 3, N 4 ) and r' edges in E G' (N 4 ) such that the remaining graph is simpe. Note that this is aways possibe, because ( m33 n3) and ( m44 n4 ) and ( m33 m34 m44 n3 n4 ) and ( m33 m44 n3 n4 1) and (( n3 1) or (( m34 n4 ) and ( m33 0))) and ( 2m33 m34 n3) and ( 2m34 m34 n4 ). and (( n3 1) or (( m34 n4) and ( m33 0))) and (( n4 1) or (( m34 n4) and ( m44 0))) and (( n3 2) or ( m33 n3)) and (( n4 2) or ( m44 n4 )) and ( 2m m n ) and ( m m n ) Let G'' be a graph obtained by repacing each of the seected edges by a path of ength 2, by adding to each vertex x in N 3 exacty 3 d G' (x) neighbors of degree 1, and by adding to each y vertex in N 4 exacty 4 d G' (y) neighbors of degree 1. Note that: 3n 3 2p q m 23 2p' q' 0 and that: 4n 4 2r q m 24 2r' q' 0, therefore it is possibe to seect m 23 2p' q' edges that connect vertices of degrees 1 and 3; and to seect m 24 2q' r' edges that connect vertices of degrees 1 and 3. Let G''' be a graph obtained by repacing each of these vertices by a path of ength 2. Note that: (G''') = (m 11, m 12, m 13, m 14,0,m 23, m 24, m 33, m 34, m 44 ) Distinguish three cases: 1) G''' is a simpe graph.

6 486 D. VUKI^EVI] AND A. GRAOVAC If m 22 = 0, it is sufficient to take G = G'''. Otherwise, denote by z an arbitrary vertex of degree 2 and denote neighbors of z by z' and z''. Graph G is defined by V(G) = V(G''') z 0, z 1,, z m22 \ z E(G) = E(G''') z'z 0, z 0 z 1, z 1 z 2,, z m22 1z m22,z m22 z''\ z'z, z''z 2) G''' contains no oops and a singe pair of parae edges. We have one of the foowing two subcases: 2.1) (n 3 =1)and(x m 24 0), and vertices adjacent to parae edges have degrees 2 and ) (n 4 =1)and(x m 23 0), and vertices adjacent to parae edges have degrees 2 and 4. In both of these subcases denote by z a vertex of degree 2 adjacent to these parae edges and by z' another vertex adjacent to these edges. Graph G is defined by V(G) = V(G''') z 0, z 1,, z m22 \z E(G) = E(G''') z'z 0, z 0 z 1, z 1 z 2,, z m22 1z m22, z m22 z'\ z'z, z'z. 3) G''' contains no oops and it has two pairs of parae edges. We have (n 3 =1)and(n 4 =1)and(x m 23 0) and (x m 24 0). One pair of parae edges is adjacent to vertices of degree 2 (denote it by z) and 3 (denote it by z') and the other is adjacent to vertices of degree 2 (denote it by w) and 4 (denote it by w'). Note that, in this case, m Graph G is defined by V(G) = V(G''') w 0, w 1 z 0, z 1,, z m22 1\z,w E(G) = E(G''') z'z 0, z 0 z 1, z 1 z 2,, z m22 2z m22 1z', w'w 0, w 0 w 1 w 1 w'\z'z, z'z, w'w, w'w A the cases are exhausted and the necessity is proved. Now, et us prove sufficiency. Let G be a graph with the required properties. Note that G has n j vertices of degree j, for each j = 2,3,4, so indeed n 2, n 3, n 4 N 0. Since G is connected and n 3, n 4 0, it foows that (m 12 + m 23 + m 24 0) or (m 22 =0) and that m 11 = 0. Since G is simpe, it foows that ((n 3 1) or ((m 34 n 3 ) and (m 33 = 0))); and that ((n 4 1) or ((m 34 n 4 ) and (m 44 = 0))); and that ((n 3 2) or (m 33 n 3 )) and ((n 4 2) or (m 44 n 4 )). Denote by N i the set of vertices of degree i (in G) for each i = 2,3,4. Note that GN 3 and GN 4 have at most one cyce and not both of them contain a cyce. Hence, m 33 n 3, m 44 n 4 and m 33 + m 44 n 3 + n 4 1. Denote by P the set of a maxima induced paths in G such that its a interior vertices have degree 2 (in G). Denote by P ij, i j the set of paths in P such that its termina vertices have degrees i and j. Note that P is partitioned in the sets P 13, P 14, P 33, P 34 and P 44. Denote P' =P 33 P 34 P 44. Note that there are exacty x paths in P', so indeed x 0. Let G' be a graph obtained by a contraction of a paths in P' to a singe edge. Graph G'N 3 N 4 is a monocycic graph, therefore m 33 + m 34 + m 44 + x = n 3 + n 4. Note that G'N 3 and G'N 4 have at most one cyce and not both of them contain a cyce, therefore n 3 + n 4 m 33 + P 33 + m 44 1 m 33 + m 44 + (x m 24 ) 1 n 3 + n 4 m 33 + P 44 + m 44 1 m 33 + m 44 + (x m 23 ) 1 Let G'' be a graph obtained by a contraction of a paths in P 33 to a singe edge. Since G''N 3 has at most one cyce, it foows that n 3 m 33 + P 33 m 33 +(x m 24 ). Aso if n 3 = 1 and m 22 =0,then0=P 33 x m 24, hence (n 3 1) or (x m 24 0) or (m 22 1). Anaogousy, et G''' be a graph obtained by a contraction of a paths in P 44 to a singe edge. Since G'''N 4 has at most one cyce, it foows that n 4 m 44 + P 44 m 44 +(x m 23 ). Aso, if n 4 = 1 and m 22 =0,then0=P 44 x m 23, hence (n 4 1) or (x m 23 0) or (m 22 1). Anaogousy, it can be proved that (n 3 1) or (n 4 1) (x m 23 0) or (x m 24 0) or (m 22 2). This proves the theorem. Let us prove: Theorem 6. Let m =(m 11, m 12,0,m 14, m 22,0,m 24,0,0, m 44 ) N 0 10 such that m 14 +m 24 + m There is a monocycic moecuar graph G such that (G) = m if and ony if ( m11 0) and (( m22 0) or ( m12 m24 0)) and ( n2, n4) N0) and ( x 0) and ( m44 x n4 ) and (( n4 2) or ( m44 2)) and (( n4 1) or (( m22 0) and ( m44 0))) where n 2 =(m 12 +2m 22 + m 24 )/2 n 4 =(m 14 + m 24 +2m 44 )/4 x =(m 24 m 12 )/2.

7 VALENCE CONNECTIVITIES REALIZABLE BY MONOCYCLIC GRAPHS 487 Proof. First et us prove the necessity. We sha define graph G with the required properties. Note that m m 22 + m 24 m 24 m 12 (mod 2), hence x N 0. Distinguish three cases: 1) n 4 =1. Let G' be the graph defined by V(G) =a, b 0, b 1,, b m22, c 1, c 2 E(G) =ab 0, b 0 b 1, b 1 b 2,, b m22 1b m22, bm 22 a, ac 1, ac 2 Graph G is obtained from G' by repacing arbitrary m 12 edges from the set ac 1, ac 2 by paths of ength 2. 2) n 4 =2. We start from graph G' defined by V(G') = { a1, a2, b1, c1, c2, c3, c4 }, m44 1 { a1, a2, b1, b2, c1, c2, c3, c }, m E(G') = { aa 1 2, ab 1 1, ab 2 1, ac 1 1, ac 1 2, ac 2 3, ac 2 4}, m441 { ab 1 1, a2b1, a1b2, a2b2, a1c1, a1c2, a2c3, a2c4}, m440. Let graph G'' be obtained from G' by repacing arbitrary m 12 edges from the set a 1 c 1, a 1 c 2, a 2 c 3, a 2 c 4 by paths of ength 2. If m 22 = 0, it is sufficient to take G = G''. Otherwise, define G by: V(G) = V(G'') d 0, d 1,, d m22 \ b 1 E(G) = E(G'') a 1 d 0, d 0 d 1, d 1 d 2,, d m22 1d m22, d m22 a 2 \ a 1 b, ba 2. 3) n 4 3. Let G' be a cyce with n 4 vertices. Let G'' be a graph obtained by repacing arbitrary x edges by paths of ength 2 and adding to each vertex in G' two neighbors of degree 1. Let G''' be a graph obtained from G'' by repacing arbitrary x edges adjacent to vertices of degree 1 by paths of ength 2. Note that (G''') = (m 11, m 12,0,m 14,0,0, m 24,0,0,m 44 ). If m 22 = 0, it is sufficient to take G = G'''. Otherwise, et z be a vertex of degree 2 in G''' and et z' and z'' be its neighbors. Graph G is defined by V(G) = V(G''') z 0, z 1,, z m22 \ z E(G) = E(G''') z'z 0, z 0 z 1, z 1 z 2,, z m22 1z m22, z m22 z' \ zz', zz''. A the cases are exhausted and the necessity is proved. Now, et us prove sufficiency. Let G be the graph with the required properties. Denote N 2, N 4, n 2, n 4, P, P 14 and P 44 as in the previous theorem. Obviousy, n 2, n 4 N 0 and since G is connected, it directy foows that (m 11 = 0) and ((m 22 =0)or(m 12 + m 24 0)). Note that x= P 44, hence (x 0) and (m 44 + x = n 4 ). If n 4 = 2, then m Therefore, we have (n 4 2) or (m 44 2). Aso, if n 4 = 1, then m 22 0 and m 44 = 0, hence indeed (n 4 1) or ((m 22 0) and (m 44 = 0)). This proves the theorem. By a compete anaogy, it can be proved that: Theorem 7. Let m =(m 11, m 12, m 13,0,m 22, m 23,0,m 33, 0, 0) N 0 10 such that m 13 +m 23 + m There is a monocycic moecuar graph G such that (G) = m if and ony if ( m11 0) and (( m22 0) or ( m12 m23 0)) and ( n2, n3 N0) and ( x 0) and ( m33 x n3) and (( n3 2) or ( m33 2)) and (( n3 1) or (( m22 0) and ( m33 0))) where n 2 =(m 12 +2m 22 + m 23 )/2 n 3 =(m 13 + m 23 +2m 33 )/3 x =(m 23 m 12 )/2. It can be easiy proved that: Theorem 8. Let m =(m 11, m 12,0,0,m 22,0,0,0,0,0) N There is a monocycic moecuar graph G such that (G) = m if and ony if (m 11 =0)and(m 12 =0)and(m ALGORITHM In this section, we present the main resut of our paper. We present an agorithm that generates the set (A n ). We use Theorem 5 to make function gen1; we use Theorem 6 to make function gen2; Theorem 7 is used to make function gen3; and Theorem 8 is incorporated within the code of the function gen. The code is presented in the programming anguage C++. The function xx is any user defined function that utiizes this agorithm.

8 488 D. VUKI^EVI] AND A. GRAOVAC

9 VALENCE CONNECTIVITIES REALIZABLE BY MONOCYCLIC GRAPHS 489 DISCRIMINATIVE PROPERTIES OF THE ZAGREB AND MODIFIED ZAGREB INDICES FOR MONOCYCLIC GRAPHS The aim of this section is to utiize the deveoped agorithm. We compare discriminative properties of the Zagreb, M 2, and modified Zagreb, *M 2, indices for monocycic graphs with the prescribed number of vertices, Let n be a natura number arger than 5. Define the foowing functions (M 2 ) n,(*m 2 ) n : (A n ) R, which map 10-tupes into rea numbers by We aso define (M 2 ) n ((G)) = M 2 (G) (*M 2 ) n ((G)) = *M 2 (G) P n = m 1, m 2 : m 1, m 2 (A n ), m 1 m 2 D n = m 1, m 2 P n : m 1, m 2 (A n ), (M 2 ) n (m 1 ) M 2 ) n (m 2 ) *D n = m 1, m 2 P n : m 1, m 2 (A n ), (*M 2 ) n (m 1 ) (*M 2 ) n (m 2 ) I n = m 1, m 2 P n : m 1, m 2 (A n ), (M 2 ) n (m 1 ) =(M 2 ) n (m 2 ) *I n = m 1, m 2 P n : m 1, m 2 (A n ), (*M 2 ) n (m 1 ) =(*M 2 ) n (m 2 ) The probabiity that the pair of eements of (A n ) wi be discriminated by the Zagreb indices is D n /P n and probabiity that they wi not be discriminated is I n /P n. Anaogousy, the probabiity that the pair of eements of (A n ) wi be discriminated by modified Zagreb indices is D* n /P n and probabiity that they wi not be discriminated is I* n /P n. Computations for graphs containing up to n = 60 vertices are given (right coumn). These resuts show that both the Zagreb and modified Zagreb indices have remarkabe discriminative properties with reative discrimination monotonousy increasing with n for both indices. (Note that here discrimination refers to the aowed 10-tupes rather than to the corresponding graph(s).) CONCLUSIONS The edges in the graph connect vertices of various degrees, which is characterized by quantities m ij. For chemicay interesting moecuar graphs (with maxima degrees at most 4), the m ij s can be represented by 10-tupes. In the present paper, theorems have been proved that show under which conditions monocycic graph(s) exist for a given arbitrary 10-tupe. These theorems have enabed n I n /P n D n /P n *I n /P n *D n /P n *I n /I n Not defined

10 490 D. VUKI^EVI] AND A. GRAOVAC deveopment of the agorithm that was appied to compare discriminative properties of the Zagreb and modified Zagreb indices. Acknowedement. The partia support of the Ministry of Science and Tehnoogy of the Repubic of Croatia (Grant No and Grant No ) is gratefuy acknowedged. REFERENCES 1. A. Graovac, I. Gutman, and N. Trinajsti}, Topoogica Approach to the Chemistry of Conjugated Moecues, Springer-Verag, Berin, N. Trinajsti}, Chemica Graph Theory, CRC Press, Boca Raton, 1983; 2nd revised ed D. Vuki~evi} and A. Graovac, Croat. Chem. Acta, in press. 4. D. Vuki~evi} and N. Trinajsti}, unpubished paper. 5. D. Vejan and D. Vuki~evi}, unpubished paper. 6. H. Wiener, J. Am. Chem. Soc. 69 (1947) R. Todeschini and V. Consonni, Handbook of Moecuar Descriptors, Wiey-VCH, Weinheim, M. V. Diudea, I. Gutman, and L. Jantschi, Moecuar Topoogy, Nova, Huntington, I. Gutman and N. Trinajsti}, Chem. Phys. Lett. 17 (1972) S. Nikoi}, G. Kova~evi}, A. Mii~evi}, and N. Trinajsti}, 10. Croat. Chem. Acta 76 (2003) B. Lu~i}, A. Mii~evi}, S. Nikoi} and N. Trinajsti}, On Modified Zagreb Indices, in: A. Graovac, B. Pokri}, and V. Smre~ki (Eds.), MATH/CHEM/COMP 2002 Book of Abstracts, ISBN , Zagreb, 2002, pp. 39. O egzistenciji monociki~kih moekua sa zadanim uvjetima na susjedstvo vaencija: generiraju}i agoritam i njegova primjena u testiranju diskriminativnih svojstava Zagreba~koga i modificiranoga Zagreba~koga indeksa Damir Vuki~evi} i Ante Graovac Susjedstva vaencija (stupnjeva) u moekuarnim grafovima (bez vodikovih atoma) su karakterizirana desetorkama brojeva m ij gdje m ij ozna~ava broj grana koje povezuju vrhove stupnjeva i i j. Pokazano je koje su desetorke kompatibine s monociki~kim grafovima i dokazani matemati~ki iskazi su rabjeni za usporedbu diskriminativnih svojstava Zagreba~koga i modificiranoga Zagreba~koga indeksa.