# 22 Gutman and Cioslowski whereas for any bipartite graph X i = X n i+1, i =1; 2;...;n. Since hexagonal systems are bipartite graphs, we immediately de

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1 PUBLICATIONS DE L'INSTITUT MATHÉMATIQUE Nouvelle série tome 42 (56), 1987, pp BOUNDS FOR THE NUMBER OF PERFECT MATCHINGS IN HEXAGONAL SYSTEMS Ivan Gutman and Jerzy Cioslowski Abstract. Upper bounds for the number of perfect matchings in hexagonal systems are deduced, which depend on the number of vertices and edges. The results are obtained using graph spectral theory. A hexagonal unit cell is a plane region bounded by a regular hexagon. A hexagonal system is a finite connected plane graph with no cut-vertices in which every interior region is a hexagonal unit cell. A perfect matching of a hexagonal system H is a set of (disjoint edges of H which cover all vertices of H). Perfect matchings in hexagonal systems have been studied in a number of recent publications [6 11]. In addition to this, both perfect matchings and hexagonal systems play an important role in theoretical chemistry [5] and there are numerous chemical papers concerned with this matter (for review and further references see [4, 5]). The number of vertices edges and perfect matchings of a hexagonal system H will be denoted by n, m and k, respectively. An obvious necessary condition for the existence of perfect matchings in a graph is that the number of its vertices is even Therefore throughout this paper we shall assume that n is an even number. It is clear that in hexagonal systems n cannot be less than 6 and we shall often need the fact that n=2 2 > 0. The adjacency matrix A of H is a square matrix of order n whose rs entry is equal to 1 if the vertices r and s of H are adjacent and is otherwise equal to 0. The eigenvalues of A are called the eigenvalues of H. Let X 1 ;X 2 ;...;X n be the eigenvalue of H, labeled so that X i X j for i<j. For any graph with m edges [1], P n i=1 X2 i =2m AMS Subject Classification (1980): Primary 03C50.

2 22 Gutman and Cioslowski whereas for any bipartite graph X i = X n i+1, i =1; 2;...;n. Since hexagonal systems are bipartite graphs, we immediately deduce that P (1) n=2 i=1 X2 i = m viz.: (2) and (3) For hexagonal systems two further relations for the graph eigenvalues hold, P n=2 i=1 X4 i =9m 6n Q n=2 i=1 X i = k Formula (2) is obtained in [3] whereas (3) is first reported in [2]. For a complete proof of (3) see pp of [1]. Consider now non-negative quantities x 1 ;x 2 ;...;x n=2 which satisfy the condition P n=2 i=1 x2 = m: i Lemma 1. If (1 0 ) holds, then Q n=2 i=1 x i» (2m=n) n=4 : Furthermore, equality in (4) occurs if and only if x 1 = x 2 = = x n=2. Proof. If some of the x i 's are equal to zero, then the inequality (4) holds in a trivial manner. Suppose therefore that all x i 's are positive and find the extreme of the functional L, L = P n=2 i=1 ln x i: under the constraint (1 0 ). A straightforward use of standard variational calculus yields the condition x 1 i +2 x i =0 which must hold for all i =1; 2;...;n=2. From (6), x i =( 2 ) 1=2 and because of (1 0 ), x i =(2m=n) 1=2 for all i =1; 2;...;n=2. Lemma 1 follows now immediately. Λ Bearing in mind the analogy between eqs. (1) and (1 0 )aswell as between the l.h.s. of (3) and (4), we arrive at the following conclusion. matchings Theorem 1. For all hexagonal systems with n vertices, m edges and k perfect (7) k<(2m=n) n=4 : Let us consider now a somewhat more complicated problem and seek for the extremes of the functional Q n=2 i=1 x i under the constraint (1 0 )and P (2 0 ) n=2 i=1 x4 i =9m 6n:

3 Bounds for the number of perfect matchings in hexagonal systems 23 Here again we may focus our attention on the case when all x i 's are positive. A reasoning fully analogous to that used in the proof of Lemma 1 leads to the condition. (8) x 1 i +2 x i +4μx 3 i =0 which must be satisfied for all i =1; 2;...;n=2. Equation (8) is transformed into (9) 1+2 x 2 i +4μx 4 i =0: Whatever is the value of the Lagrange multipliers and μ, equation (9) has either two positive roots or no real root. From the analysis which follows it will be seen that (9) has real roots in some cases of interest for the present consideration. Suppose that A and B are positive real roots of (9), A>B>0. Then some of the x i 's will be equal to A and the rest of them will be equal to B. Because of Lemma 1 we must not choose all the x i 's to be mutually equal. Let therefore t among the x i 's be equal to A, 1» t» n=2 1. particular, in the case when t =1, (10a) x n=2 1 = A; x 1 = = x n=2 2 = B; x n=2 = B: in the case when t = n=2 1, (10b) x 1 = = x n=2 1 = A; x n=2 = B and in the case when 1 <t<n=2 1, Let, in (10c) x 1 = = x t 1 = A; x n=2 1 = A; x t = = x n=2 2 = B; x n=2 = B: The reason for such an unusual assignment will become clear later. Substituting any of the relations (10) back into (1 0 ) and (2 0 ), we obtain (11) ta 2 +(n=2 t)b 2 = m; ta 4 +(n=2 t)b 4 =9m 6n from which (12) A = A(t) =(2m=n + RS) 1=2 ; B = B(t) =(2m=n R=S) 1=2 where (13) R =(18mn 12n 2 4m 2 ) 1=2 n 1 ; S =[(n 2t)=(2t)] 1=2 : Lemma 9. For all values of the parameters n and m which can occur in hexagonal systems, 18mn 12n 2 4m 2 > 0. Proof. It is easily seen that 18mn 12n 2 4m 2 is positive if p p (14) (9 33)n=4 <m<(9 + 33)n=4; i.e. 1:6 < 2m=n < 7:4:

4 24 Gutman and Cioslowski Now 2m=n is the average vertex degree of the hexagonal system considered. Obviously, for all hexagonal systems 2m=n lies between 2 and 3 and therefore the inequalities (14) are certainly fulfilled. Λ As a consequence of Lemma 2, R, as defined in (13), is necessarily real. Therefore A(t) is real for all t, 0 < t < n=2 whereas B(t) is real for 0 < t» m 2 =(9m 6n). If the x i 's are chosen according to (10), then the functional L, in(5),becomes equal to L(t): (15) L(t) =t ln A +(n=2 t)lnb with A and B given by equations (12). Lemma. Provided A and B are real (i.e. for 0 <t» m 2 =(9m 6n)) L(t) is a monotonously decreasing function of the parameter t. Proof. Differentiate equations (11) with respect to t and calculate A 0 and B 0 as A 2 +2tAA 0 B 2 +2(n=2 t)bb 0 =0 A 4 +4tA 3 A 0 B 4 +4(n=2 t)b 3 B 0 =0 (16) A 0 = (A 2 B 2 )=(4tA); B 0 = (A 2 B 2 )=(2nB 4tB): From (15) we get which combined with (16) gives L 0 (t) =lna + ta 0 =A ln B +(n=2 t)b 0 =B L 0 (t) =ln(a=b) (A=B) 2 =4+(A=B) 2 =4: For A > B, L 0 (t) is negative. In order to see this, notice that if f(x) = ln x x 2 =4+x 2 =4, then f(1) = 0 and f 0 (x) = (x 2 1) 2 =(2x 3 ). Hence for x>1, f(x) < 0. Λ For t =1; 2;... ; [m 2 =(9m 6n)], L(t) is a stationary point if the functional L, equation (5). From Lemma 3 one may expect that for t =1,L(t) is a maximum. We nowshow that this is indeed the case. The quantities x 1 ;x 2 ;... ;x n=2 can be understood as variables which are to be determined by variational calculus. Because of (1 0 ) and (2 0 ), only n=2 2 of them are independent. Let these be x 1 ;x 2 ;... ;x n=2 2 and denote the dependent variables x n=2 1 and x n=2 by y and z, respectively. (Note that in all the relations (10) y = A and z = B.) Let f(x) be an arbitrary function of x, suchthatf(x) and its first and second derivatives exit for x = x i, i =1; 2;... ;n=2. Consider a functional F, F = F (x 1 ;x 2 ;... ;x n=2 2 )= P n=2 i=1 f(x i):

5 Bounds for the number of perfect matchings in hexagonal systems 25 Then for i; j =1; 2;... ;n=2 2, (17) = f 0 (x i f 0 f 0 2 F = ffi ij f 00 (x i y f 0 z f f 00 f 00 (y) where ffi ii =1andffi ij = 0 for i 6= j. As special case of (17) and (18) we get from (1 0 )and(2 0 ): (19) (20) (21) x 3 i + y + z =0 x i y ffi ij + @ 2 z + z 3ffi ij x 2 i + y 2 y + z 2 z =0: In what follows we have distinguish between three cases: (a) t = 1, (b) t = n=2 1 and (c) 1 <t<n=2 1. Since the examination is virtually the same in all the there cases, we shall consider here only case (c). Thus let 1 <t<n=2 1. Denote the sets f1;... ;t 1g and ft;... ;n=2 2g by I A and I B,respectively. In the case (c), equations (10c) hold and after substituting them into (19) we get for i 2 I A : from which A(1 =0; A 3 (1 )+B =0 = =0: For i 2 I B, + B(1 )=0; A + B 3 (1 )=0 and therefore = 1: Both (22) and (23), substituted back into (17) =0. Now, for i = j 2 I A we get from (20) and 2 2 i = 2B 6A2 A 3 AB i = 4A2 B 3 A 2 B

6 26 Gutman and Cioslowski which results 2 2 i =2u where (24) u = f 00 (A)+f 0 (A) B2 3A 2 A 3 AB 2 2A2 f 0 (B) B 3 A 2 B : Similarly, for i; j 2 I A, i 6= 2 F= = u, for i = j 2 I 2 2 i = 2w where (25) w = f 00 (B)+f 0 (B) A2 3B 2 B 3 A 2 B 2B2 f 0 (A) A 3 AB 2 for i; j 2 I B, i 6= 2 F= = w whereas for i 2 I A, j 2 I B or i 2 I B, j 2 I 2 F= =0. This means that the Hessian matrix of F has the block diagonal form» U 0 ; 0 W where 0 is a zero matrix and u u... u 2w w... w u 2u... u U = ; W = w 2w... w : u u... 2u w w... 2w Note that U and W are of order t 1 and n=2 t 1, respectively. Their eigenvalues are easily found. The eigenvalues of U are: u (t 2)-times and tu. The eigenvalues of W are: w (n=2 t 2)-times and (n=2 t)w. In the problem we are concerned with, f(x i )=lnx i, and then formulas (24) and (25) reduce to u = w =2(A 2 B 2 )(AB) 2. Consequently, u>0, w<0and we reach the following conclusion. Lemma 1. The Hessian matrix of L, equation (5), has t 1 positive and n=2 t 1 negative eigenvalues. We have demonstrated the validity of Lemma 4 assuming that 1 <t<n=2 1. The very same statement holds also in the case (a) t =1and(b)t = n=2 1. The proof of Lemma 4 in cases (a) and (b) is essentially the same as in the case (c) and will not be reproduced here. Λ Since Lemma 4 holds for all t, 1» t<n=2, we arrive at our main results. Lemma 5. L(1) equation (15), is a maximum if and only if t =1. Lemma 6. If (1 0 ) and (2 0 ) Q n=2 hold, then i=1» K, where (26) K =(2m=n + R p n=2 1) 1=2 (2m=n R= p n=2 1) (n 2)=4 and R is given by (13). matching, Theorem 2. For all hexagonal systems with n vertices, m edges and k perfect (27) k» K

7 Bounds for the number of perfect matchings in hexagonal systems 27 with equality if and only if n =6. Proof. The main statement of Theorem 2 is an immediate consequence of Lemma 6. Only one hexagonal system, namely that composed of a single hexagon has only two distinct positive eigenvalues (X 1 =2;X 2 = X 3 =1).Λ We demonstrate now that the upper bound given in Theorem 2 is better than the upper bound given in Theorem 1. Lemma 7. For all values of the parameters n and m which may occur in hexagonal systems K<(2m=n) n=4. Proof. Consider K, equation (26) as a function of the parameter R. Setting R =0into (26) one obtains just the upper bound from Theorem 1. ln =1=2 p n=2 1[(2m=n + R p n=2 1) 1 (2m=n R= p n=2 1) 1 ] which for R>0isobviously negative. Hence ln K and thus also K is a decreasing function of R. According to Lemma 2, R > 0 for all combinations of n and m which occur in hexagonal systems. Λ The upper bounds (7) and (27) both depend on n thenumberofvertices and m the number of edges. One may ask why our variational procedure did not result also in lower bounds for k. As a matter of fact, lower bounds of this type probably cannot be found at all. Namely, whereas the parameters n and m increase with the increasing size of a hexagonal system, the number of perfect matchings need not do so. Even if we disregard hexagonal systems for which k = 0, there still remains an arbitrarily large number of examples for which k<k 0, where k 0 is a constant greater than 9. For instance, the following hexagonal system has 9 perfect matchings for all h 3: REFERENCES [1] D. Cvetkovi»c, M. Doob and H. Sachs, Spectra of Graphs Theory and Application, Academic Press, New York, [2] M.J.S. Dewar and H.C. Longuet-Higgins, The correspondence between the resonance and molecular orbital theories, Proc. Roy. Soc. A214(1952), [3] I. Gutman, Some topological properties of benzenoid systems, Croat. Chem. Acta 46 (l974), [4] I. Gutman, Topological properties of benzenoid molecules, Bull. Soc. Chim. Beograd 47 (1982),

8 28 Gutman and Cioslowski [5] I. Gutman and O.E. Polansky, Mathematical Concepts in Organic Chemistry, Springer- Verlag, Berlin [6] P. John and J. Rempel, Counting perfect matchings in hexagonal systems, Proc. Internat. Conf. on Graph Theory, Eyba 1984, Teubner, Leipzig, 1985, pp [7] P. John and H. Sachs, Calculating the number of perfect matchings and Pauling's bond orders in hexagonal systems whose inner dual is a tree, Proc. Internat. Conf. on Graph Theory, Eyba, 1984, Teubner, Leipzig 1985, pp [8] P. John and H. Sachs, Wegesysteme und Linearfaktoren in hexagonalen und quadratischen Systemen, Graphen in Forschung und Unterricht, Franzbecker-Verlag, Bad Salzdetfurth, l985, pp [9] A.V. Kostochka, Kriterii sushchestvovaniya suvershennyh parosochetanii v shestiugol'nyh sistemah, Proc. 30. Int. Sci. Colloquium TH Ilmenau, Graphen und Netzwerke Theorie und Anwendungen, Ilmenau, 1985, pp [10] H. Sachs, Perfect matchings in hexagonal systems, Combinatorica 4 (1984), [11] H. Sachs and H. Zernitz, Ein O(n log n)-algorithmus zur Auffindung eines Linearfaktors in einem hexagonalen System, Zeszyty Naukowe Wyzsza Szkola Inz. Zielona Gora 75 (1984), Odsek za matematiku (Received ) Prirodno-matemati»cki fakultet Kragujevac Jugoslavija Georgetown University Washington, D.C USA

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