# Split Nonthreshold Laplacian Integral Graphs

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1 Split Nonthreshold Laplacian Integral Graphs Stephen Kirkland University of Regina, Canada Maria Aguieiras Alvarez de Freitas Federal University of Rio de Janeiro, Brazil Renata Raposo Del Vecchio Fluminense Federal University, Brazil Nair Maria Maia de Abreu Federal University of Rio de Janeiro, Brazil Abstract The aim of this article is to answer a question posed by Merris in European Journal of Combinatorics, 24(2003) , about the possibility of finding split nonthreshold graphs that are Laplacian integral, i.e., graphs for which the eigenvalues of the corresponding Laplacian matrix are integers. Using Kronecker products, balanced incomplete block designs, and solutions to certain Diophantine equations, we show how to build infinite families of these graphs. Keywords: Split graph, threshold graph, semiregular graph, Laplacian integral graph, block design. 1 Basic notions Let G = (V, E) be a simple graph such that, for i, j = 1, 2,..., n, v i V is a vertex, {v i, v j } E is an edge and its degree sequence is π(g) = (d 1, d 2,, d n ), where d i is the degree of v i. If all the vertices of G have the same degree, the graph is regular, while G is biregular if its degree sequence is constituted by only two distinct values. A graph G is bipartite if its 1

2 vertices can be partitioned into two sets in such a way that no edge joins two vertices in the same set. The Laplacian matrix of G is L(G) = D(G)-A(G), where A(G) is the adjacency matrix of G and D(G) is the diagonal matrix of the vertex degrees. The spectrum of L(G), ζ(g) = (µ 1,..., µ n 1, µ n ), is the sequence of eigenvalues of L(G) displayed in non-increasing order: µ 1... µ n 1 µ n. It is well-known that L(G) is a positive semidefinite and singular matrix, so µ n = 0. When each element of ζ(g) is an integer, G is called a Laplacian integral graph. A graph G is a cograph, also known as a decomposable graph if and only if no induced subgraph of G is isomorphic to P 4, [4]. These graphs can be constructed from isolated vertices by a sequence of operations of unions and complements. When a graph is not a cograph, it is called indecomposable. In [8], it is proved that any cograph is Laplacian integral. A graph G is a threshold graph if and only if it does not have an induced subgraph isomorphic to one of the forbidden graphs P 4, C 4 or 2K 2. Among the many interesting properties of threshold graphs is the fact that they are uniquely determined by their Laplacian spectra (see Theorem 6.1 of [6]). Because threshold graphs are cographs, their Laplacian eigenvalues are all integers. A split graph is one whose vertex set can be partitioned as the disjoint union of an independent set and a clique (either of which may be empty). It is shown in [2] that a graph is split if and only if it does not have an induced subgraph isomorphic to one of the following three forbidden graphs: C 4, C 5, and 2K 2. It follows from the definition that the complement of a split graph, as well as every induced subgraph of a split graph, is split, [9]. Based on the characterizations above, no nonthreshold split graph is a cograph. In a recent paper, Grone and Merris [7], demonstrate the existence of an infinite number of indecomposable Laplacian integral graphs. This result strengthens the interest of the following open problem posed by Merris in his paper [9]: There appear, on the other hand, to be many nondecomposable Laplacian integral graphs, and one might think that the natural place to look for them would be among those graphs closest to the thresholds. Yet, preliminary explorations have not yielded a single nonthreshold, Laplacian integral, split graph. Why are they so difficult to find? The question above motivates our investigation of graphs with those characteristics. Specifically, in this paper we look for nonthreshold split graphs that are Laplacian integral. We construct infinite families of such graphs, thus partially answering Merris s question. 2

3 In the next section, we build biregular split graphs from regular or biregular bipartite graphs. This allows us to obtain a characterization of Laplacian integral biregular split graphs. In Section 3, we use a generalization of the balanced incomplete block design in order to get Theorem 3.1, the main result of this paper. It shows how to obtain an infinite family of biregular split nonthreshold Laplacian integral graphs, for each generalized block design. This section ends with two example of these graphs, one of them with 52 vertices, 714 edges and the maximal clique with the size 28. A note on the complements of biregular split nonthreshold Laplacian integral graphs is presented in the Section 4. In the last section, we show how to obtain biregular split nonthreshold Laplacian integral graphs that are cospectral and nonisomorphic. 2 Characterization of Laplacian integral biregular split graphs This section is devoted to proving Theorem 2.2, which gives necessary and sufficient conditions for a biregular split graph to be Laplacian integral. First, we review some concepts and introduce a new one, the splitness of a split graph, which is useful in the characterization of bipartite split graphs. Definition 2.1. Let H = (V, E) be a bipartite graph with a vertex partition V = V 1 V 2 such that each vertex in V 1 has degree x and each vertex in V 2 has degree y. If x y, we say that H is an (x, y)-semiregular graph, or simply semiregular and, if x = y, we say that H is a y-regular bipartite graph, or simply regular bipartite. In both cases, this partition is called a degree partition of H. Although in the second case we can have more than one partition, in the first one, one degree partition is unique. It is interesting to note that every semiregular graph is bipartite biregular, but the converse is not necessarily true. For example, P 4 is bipartite biregular, but it is not semiregular. Definition 2.2. A graph G = (V, E) is a split graph if there is a partition of V = V 1 V 2 such that the induced subgraph < V 1 > is a complete graph and V 2 is an independent set. Such a partition, which may not be unique, is referred to as a split partition set of V. Example 2.1. The split graph G showed in Figure 1 has split partitions V = {1, 2, 3} {4, 5}, V = {1, 2, 3, 4} {5} and V = {1, 2, 3, 5} {4}. 3

4 Figure 1: G is a 4,3)-biregular split graph Theorem 2.1. Let G be a (t, y)-biregular connected split graph, where t > y > 0. Then V = V t V y, where V t = {v V d(v) = t} and V y = {v V d(v) = y}, is a split partition set of V. Proof: Let G be a (t, y)-biregular connected split graph where t > y > 0. Consider a split partition set of V = U 1 U 2 such that the induced subgraph < U 1 > is a maximal clique and U 2 is an independent set. Note that, since G is connected, there is a vertex v U 1 such that d(v) > U 1 1. So, t > U 1 1. Since < U 1 > is a maximal clique and t > y, it follows that y U 1 1. Moreover, each vertex in U 2 has degree y, for U 2 is an independent set. So U 2 V y. If all vertices in U 1 have degree t then U 1 = V t, U 2 = V y and the result follows. In the case that not every vertex of U 1 has degree t, then there is some vertex in U 1 with degree y. So, we can take U = {u U 1 d(u) = y}, a non-empty subset of U 1 and also V 1 = U 1 U and V 2 = U 2 U. Since U 1 induces a clique, then y U 1 1 and, as y U 1 1, we have y = U 1 1. So, if u U then u is not adjacent to any vertex of U 2. Since all vertices of U 2 have degree y, we have that y U 1 U. So, U = 1 and V 2 is an independent set. It follows that V = V 1 V 2 is a split partition set of V such that V 1 = V t and V 2 = V y, where y = U 1 1. Definition 2.3. Let G be a (t, y)-biregular connected split graph, where t > y > 0. The split partition set V = V t V y is called the split degree partition (or sdp) of the graph G. Example 2.2. The biregular split graph G in Figure 1 has split degree partition V = {1, 2, 3} {4, 5}. 4

5 Corollary 2.1. Let G be a (t, y)-biregular connected split graph, where t > y > 0. There is a unique bipartite graph H which is either the maximal semiregular spanning subgraph of G, or the maximal regular spanning subgraph of G such that there is a degree partition that is the split degree partition of G. Proof: Let G be a (t, y)-biregular connected split graph where t > y > 0 and consider its split degree partition V = V t V y. Let H be the bipartite graph obtained from G taking off all edges between the vertices in V t. If t > y + V t 1, H is the maximal semiregular spanning subgraph of G. Otherwise, H is the maximal regular bipartite spanning subgraph of G. In both cases, the degree partition of H is the sdp of G. Clearly, H is the unique bipartite spanning subgraph of G satisfying these properties. Definition 2.4. Let G be a (t, y)-biregular connected split graph, where t > y > 0. We define the splitness of G to be the bipartite graph H which is either the maximal semiregular spanning subgraph of G, or the maximal regular spanning subgraph of G such that its degree partition is the split degree partition of G. Example 2.3. Figure 2 shows the splitness of the (4,3)-biregular split graph G shown in Figure 1. Figure 2: The splitness of G in Figure 1 From Corollary 2.1, we note that each biregular split graph is obtained from a semiregular or a regular bipartite graph H, its splitness, by choosing one of the sets, either V 1 or V 2, and adding all the possible edges between their elements. Conversely, a semiregular bipartite or a regular bipartite 5

6 graph H can be obtained from a biregular split graph taking off all edges in the clique determined by its sdp. The following example illustrates the fact that every (x, y)-semiregular graph generates two non-isomorphic biregular split graphs. Example 2.4. The semiregular graph K 2,3 generates two biregular split graphs which are shown in Figure 3. Its adjacency matrix can be written as A = Figure 3: Biregular split graphs generated by K 2,3 Remark 2.1. The adjacency matrix of the splitness H of a split biregular graph G = (V, E), such that its sdp is V = V 1 V 2 with V 1 = p and V 2 = q, has the form ( ) 0 X A(H) = X T, 0 where X is a (0, 1)-submatrix of order p q. Moreover, there are parameters x, y such that Xsatisfiesthefollowingequalities :X1 q = x1 p ; X T 1 p = y1 q and 1 T p X1 q = px = qy, where I p is the identity matrix of order p and 1 q is the vector of order q with all elements equal to 1. Henceforth, we denote the p p all ones matrix by J p, and the p q all ones matrix by J p,q. 6

7 Corollary 2.2. A graph G is split biregular if, and only if, its Laplacian matrix has the form ( ) ( ) (p + x)ip J L(G) = p 0 (p + x)ip J A = p X 0 yi q X T, (2.1) yi q where A is the adjacency matrix of the splitness of G, and where the sdp V = V 1 V 2 is such that p = V 1 and q = V 2. Proof: The statement follows directly from Theorem 2.1. Finally, we present the main result of this section, a characterization theorem of biregular split Laplacian integral graphs. Theorem 2.2. Let G be a connected biregular split graph with Laplacian matrix given by (2.1) in Corollary 2.2. If X = J p,q, then the Laplacian spectrum of G is 0 with multiplicity 1, p with multiplicity q 1, and p + q with multiplicity p. If X J p,q, let r denote the rank of X, and for each nonzero eigenvalue τ of XX T, let m τ be equal to the maximum number of linearly independent τ-eigenvectors of XX T that are orthogonal to 1 p. Then the Laplacian spectrum of G is as follows: 0 with multiplicity 1; y with multiplicity q r; p + x with multiplicity p r; x + y with multiplicity 1; and for each nonzero eigenvalue τ of XX T, the roots of the equation µ 2 (p + x + y)µ + (p + x)y τ, each with multiplicity m τ. Proof: If X = J p,q then G = (K p K q ) and the result follows. Now suppose that X J p,q. Note that x + y is an eigenvalue of L(G) with ( ) x1p y1 q as its eigenvector. Further, if Ker(X) {0} and w is a nonzero vector in Ker(X) then ( ) 0 u = w satisfies L(G)u = ( ) Xw = y yi q w ( ) 0 = yu. w In this case, y is an eigenvalue of L(G) with multiplicity equal to q r. Similarly, if z is a non-zero vector in Ker(X T ) then z is orthogonal to 1 p, since G is connected and X1 q = x1 p. Then, for ( ) z v =, 0 7

8 L(G)v = (p + x)v. So, p + x is an eigenvalue of L(G) with multiplicity equal to p r. Finally, we suppose that τ is a non-zero eigenvalue of XX T with a corresponding eigenvector u that is orthogonal to 1 p. Then, for a, b R, ( ) a w = b is an eigenvector of ( p + x ) τ 1 y associated with eigenvalue µ if and only if, ( ) au v = bx T u is also an eigenvector of L(G) associated with eigenvalue µ. So, for each τ as above, the roots of µ 2 (p + x + y)µ + (p + x)y τ = 0 are eigenvalues of L(G). Further, the multiplicity of each root µ is equal to m τ. The following is immediate. Corollary 2.3. Let G be as in Theorem 2.2. Then G is Laplacian integral if and only if, for each non-zero eigenvalue τ of XX T with an associated eigenvector orthogonal to 1 p, (p + x y) 2 + 4τ is a perfect square. 3 Construction of Laplacian integral biregular split graphs The balanced incompleted block design, BIBD, is an important combinatorial concept, useful in several areas, especially Applied Statistics, see [1], [5] and [11]. We begin this section by giving a generalization of this concept that allows us to build an infinite family of Laplacian integral biregular split graphs. Definition 3.1. A generalized balanced incomplete block design, GBIBD, is a structure consisting of a set Y = {y 1,, y v }, v 2, and b distinct subsets of Y, {B 1,, B b }, called blocks, such that there are parameters r 1, λ 0, and v with 1 k v 1 for which the following hold: 1. each element of Y belongs to r blocks; 8

9 2. each block contains k elements; and 3. each pair of elements is simultaneously in λ blocks. The non-negative integers (v, b, r, k, λ) are called the design parameters and they satisfy: vr = bk and λ(v 1) = r(k 1). Since these equations hold, the (v, b, r, k, λ)-generalized block design can be denoted more simply as a (v, k, λ)- generalized block design. If v = b, we say that a corresponding GBIBD is a symmetric generalized incomplete block design. Observe that our generalization coincides with the usual definition, except that we allow the cases λ = 0 and k = v 1. and The v b incidence matrix M of a GBIBD is given by { 1, if y i B j ; m ij = 0, if y i / B j. For this matrix M, the following equations hold: MM T = (r λ)i v + λj v, M1 b = r1 v M T 1 v = k1 b. We note that each (v, b, r, k, λ)-generalized block design corresponds to another block design called the complement generalized block design. In order to obtain this complement, it is enough to take the blocks B i = Y B i, i = 1,, b. It is easy to see that if M is its incidence matrix then M = J v,b M. When v 2, the complement of a (v, b, r, k, λ)-gbbid has parameters (v, b, b r, v k, b 2r + k). If v = 2, the GBBID is selfcomplementary. Since every BIBD is a GBIBD, several examples of these structures can be found in [1], [5] and [11]. In Example 3.1, we present an instance of a GBIBD that is not a BIBD. Example 3.1. Let v = b = 5, r = k = 1 and λ = 0. We have the following (5, 1, 0) GBIBD with incidence matrix M =

10 Its complement is the (5, 4, 4) GBIBD with incidence matrix M = Let M be the incidence matrix of a (v, k, λ) GBIBD. For each α, β N, let X = M J α,β, where is the Kronecker product of matrices. Set p = αv and q = βb. We define the graph G(M, α, β) to be the graph on p + q vertices whose Laplacian matrix is given by ( ) (p + x)ip J L(G(M, α, β)) = p X X T. yi q The next lemma shows that each (v, k, λ) GBIBD yields an infinite family of biregular split nonthreshold graphs. Lemma 3.1. Let M be the incidence matrix of a (v, k, λ) GBIBD. For each α, β N, the graph G(M, α, β) is a biregular split nonthreshold graph. Proof: Let X = M J α,β, and note that we have X1 q = βr1 p and X T 1 p = αk1 q, where p = αv and q = βb. Since G(M, α, β) has Laplacian matrix ( ) (p + x)ip J p X X T, yj q it is clear that G(M, α, β) is a biregular split graph. It remains to prove that G(M, α, β) is a nonthreshold graph. Since the parameters of a (v, k, λ) GBIBD satisfy v 2 and 0 λ < r, for 1 i, i v there are 1 j, j b such that m ij = m i j = 1 and m ij = m i j = 0 in the incidence matrix M. Hence, the subgraph induced by the vertices corresponding to indices αi, αi, βj and βj is isomorphic to P 4. The next theorem is the main result of this paper. It shows how we can obtain an infinite family of biregular split nonthreshold Laplacian integral graphs, for each generalized block design. Theorem 3.1. Let M be the incidence matrix of a (v, k, λ) GBIBD. There are infinitely many Laplacian integral biregular split nonthreshold graphs of the form G(M, α, β). 10

11 Proof: From Lemma 3.1, for α, β N, G(M, α, β) is a biregular split nonthreshold graph. Thus, we need only show how to obtain appropriate parameters α and β in order to construct an infinite family of Laplacian integral graphs. To do this, we set X = M J α,β, and consider two cases: λ = 0 and λ > Case λ = 0 : For λ = 0, we have k = 1 = r, v = b. For each α, β N the Laplacian matrix of G(M, α, β) can be written as ( ) (αv + β)iαv J βv I v J α,β (I v J α,β ) T. αi βv It is easy to see that XX T = I v J α is a non-negative and reducible matrix with only one non-zero eigenvalue, namely τ = αβ, with multiplicity equal to v. It follows from Corollary 2.3 that G(M, α, β) is Laplacian integral if and only if, (p+x y) 2 +4xy = (x+(v 1)y) 2 +4xy is a perfect square, ie, if and only if the Diophantine equation (αv + β α) 2 + 4αβ = (β + (v 1)α) 2 + 4αβ γ 2 = 0, (3.1) is satisfied for some γ Z. Note that (α 0, β 0, γ 0 ) = (v+1, v, v 2 +v+1) is a particular solution of equation (3.1); thus, the split nonthreshold graph G(M, v + 1, v) is Laplacian integral. According to [3], the general solution of the equation (3.1) on Q is given by α = d[ s + (v + 1)t][(v + 1)s + (v 2 + 1)t], and β = d(2v + 1)(v + 2)[s v 1 v(v 1) t][s + 2v + 1 v + 2 t] where s, t Z and d Q. In particular, for each d N, positive integer solutions of the equation (3.1) can be obtained as follows: (a) for s and t of the same sign, for example, s > 0 and t > 0, choose s N such that v 1 2v+1t < s < (v + 1)t; 11

12 (b) otherwise, choose t < 0 < s such that v(v 1) v+2 t < s < v2 +1 v+1 t. 2. Case λ > 0 : In this case, XX T = β((r λ)i v + λj v ) J α is non-negative and irreducible. This matrix has three distinct eigenvalues, τ 1 = αβ kλ(v 1) k 1, τ 2 = αβλ(v k) k 1 and τ 3 = 0, provided that α 2, otherwise it has only two distinct eigenvalues, namely τ 1 and τ 2. From Theorem 2.2, G(M, α, β) is Laplacian integral if and only if, (p + x y) 2 + 4τ 2 is a perfect square. This is equivalent to having α 2 (v k) 2 αβλ(v k)(v + 1) βλ(v 1) ( ) 2 γ 2 = 0, (3.2) k 1 k 1 for some γ Z. Note that (α 0, β 0, γ 0 ) = (λ(2v 1), 2(k 1)(v k), λ(v k)(4v 1)) is a particular solution of equation (3.1) and hence the split nonthreshold graph G(M, λ(2v 1), 2(k 1)(v k)) is Laplacian integral. According to [3], the general solution of the equation (3.2) on Q is given by α = 3dλ(2v+1)(v k) 2 λ(v 1) λ(v 1)(2v 1) [s t][s+ 3(v k)(k 1) (2v + 1)(v k)(k 1) t], and β = 2d(v k) 3 (k 1)[s where s, t Z and d Q. λ(3v 1) (v k)(k 1) t][s + λv (v k)(k 1) t] Thus, for d N, positive integer solutions of the equation (3.1) can be obtained as follows: (a) for s and t having the same sign, for example, s > 0 and t > 0, λ(v 1) choose s N such that 3(v k)(k 1) t < s < λ(3v 1) (v k)(k 1) t; (b) otherwise, choose t < 0 < s such that λv v k t. λ(v 1)(2v 1) (2v+1)(v k)(k 1) t < s < The following examples present two cases of split nonthreshold Laplacian integral graphs. Figure 4 illustrates the case λ = 0, while Figure 5 gives an example for which λ > 0. 12

13 Example 3.2. Case λ = 0: Let v = 2, M = I 2, α = 2 and β = 3. As (β +(v 1)α) 2 +4αβ = 7 2, then G(M, 2, 3) is a split nonthreshold Laplacian integral graph. It is shown in Figure 4. Figure 4: G(M, 3, 2) is a split nonthreshold Laplacian integral graph Example 3.3. Case λ > 0 : Consider the (4, 2, 1) GBIBD with incidence matrix M = The graph G(M, 7, 4), displayed in Figure 5, is a split nonthreshold Laplacian integral graph on 52 vertices with 714 edges and the maximal clique of size A note on complements If G is a biregular split Laplacian integral graph, so is its complement, G. This happens since the spectrum of L(G) is ζ(g) = (n µ n 1,..., n µ 1, 0), where µ 1... µ n 1 µ n = 0 are the eigenvalues of L(G). In the case λ = 0, it is easy to see that G(I v, α, β) is isomorphic to G(M, β, α), where M = J v I v is the incidence matrix of the complement block design. In the case that we have an incidence matrix M for a GBIBD with λ > 0, there may not exist another incidence matrix M for a (v, k, λ ) GBIBD such that the complement of G(M, α, β) is G( M, α, β ), as we can see in 13

14 Figure 5: G(M, 7, 4) is a split nonthreshold Laplacian integral graph Example 3.3. The Laplacian matrix of the complement of the graph in Example 3.3 can be written as ( ) 38I24 J 24 X L(G) = X T, 12J 28 where X = M J 4,7 and M = Since M is not the incidence matrix of any GBIBD, we see that G can not be written as G( M, α, β ) for any GBIBD incidence matrix M. 5 Biregular split graphs with a common Laplacian spectrum Suppose that M is the incidence matrix of a (v, k, λ)-gbibd, where λ > 0. Select parameters α, β N, and form X = M J α,β. It is straightforward to determine that the rank of X coincides with the rank of M; since rank(m) = 14

15 rank(m T ) = rank(mm T ) and MM T = (r λ)i v + λj v, we find that the rank of X is v. Observe that the row sums of X are equal to βλ(v 1) k 1, while the column sums of X are equal to αk. As noted in the proof of Theorem 3.1, XX T = β((r λ)i v + λj v ) J α, so that XX T has just two nonzero αβ kλ(v 1) eigenvalues: τ 1 = k 1 and τ 2 = αβλ(v k) k 1. Observe that the quantity m τ1 of Theorem 2.2 is 0, since τ 1 is the (simple) Perron value for XX T, with corresponding eigenvector 1 αv. Similarly, we have m τ2 = v 1. Appealing to Theorem 2.2, we find that the spectrum (including multiplicities) of the Laplacian matrix of the graph G(M, α, β) is completely determined by the parameters v, k, λ, α, and β. In particular, suppose that D 1 and D 2 are two GBIBDs with the same parameters (v, k, λ), with incidence matrices M 1 and M 2, respectively. We can extend the definition of isomorphism of block designs in [11] to GBIBDs as follows: D 1 and D 2 are isomorphic if there are permutation matrices R and S, of orders v and b, respectively, such that M 1 = RM 2 S. The graphs G(M 1, α, β) and G(M 2, α, β) will share the same Laplacian spectrum. From Corollary 2.2, it follows that G(M 1, α, β) and G(M 2, α, β) are isomorphic if and only if there are permutation matrices P and Q of orders αv and βv, respectively, such that P (M 2 J α,β )Q = M 1 J α,β. We claim that this last condition holds if, and only if, D 1 and D 2 are isomorphic. Certainly if there exist permutation matrices R, S such that M 1 = RM 2 S, then we can find permutation matrices P, Q so that P (M 2 J α,β )Q = M 1 J α,β. To see the other direction of the claim, suppose that P (M 2 J α,β )Q = M 1 J α,β for some permutation matrices P and Q. Observe that both A 1 M 1 J α,β and A 2 M 2 J α,β can be partitioned as block matrices, with each block equal to either J α,β or the α β zero matrix. Further, the partitioning of the rows of A 1, A 2 corresponds to the sets of indices U i = {(i 1)α + 1, (i 1)α + 2,..., iα}, i = 1,..., v, while the partitioning of the columns of A 1, A 2 corresponds to the sets of indices W j = {(j 1)β + 1, (j 1)β + 2,..., jβ}, j = 1,..., b. From the fact that both A 1 and A 2 are block matrices, it follows that P can be taken to be a permutation that maps all indices in a single U i to a common U ki, for each i = 1,..., v, and Q can be take to be a permutation that maps all indices in a single W j to a common W lj, for each j = 1,..., b. It now follows readily that there are permuations matrices R, S such that RM 2 S = M 1. Now, suppose that we have λ > 0, and that D 1 and D 2 are nonisomorphic. Then for each α, β N, we find from the considerations above that G(M 1, α, β) and G(M 2, α, β) are nonisomorphic graphs with the same Lapla- 15

16 cian spectrum; moreover, if α, β are selected as in Theorem 3.1, then the nonisomorphic graphs G(M 1, α, β) and G(M 2, α, β) have the same integral Laplacian spectrum. As an example of this kind of construction, consider the following scenario. Recall that a Hadamard matrix of order 4n + 4 is a (1, 1) matrix H such that HH T = (4n + 4)I. By an appropriate sequence of row permutions, column permutations, signing of rows, and signing of columns, any Hadamard matrix H can be put into the following form: ( ) 1 1 T. 1 H It now follows readily that the matrix M = 1 2 (J + H) is a (0, 1) matrix of order 4n + 3 satisfying MM T = (n + 1)I + nj. Thus M is the matrix of a (4n + 3, 2n + 1, n) BIBD. Two Hadamard matrices H 1, H 2 are said to be equivalent if H 1 can be produced from H 2 by an appropriate sequence of row permutations, column permutations, row signings, and columns signing. It follows then that for nonequivalent Hadamard matrices H 1, H 2 of order 4n+4, the incidence matrices M 1, M 2 of the corresponding (4n+3, 2n+1, n) BIBD s will have the property that P M 2 Q M 1 for all 4n + 3n + 3 permutation matrices P and Q. Consequently, for α, β selected as in Theorem 3.1, G(M 1, α, β) and G(M 2, α, β) are nonisomorphic split nonthreshold graphs having the same integral Laplacian spectrum. As a particular instance, it is known (see [10]) that there are exactly five nonequivalent Hadamard matrices of order 16. These give rise to five incidence matrices M 1,..., M 5 for (15, 7, 3) BIBD s with the properties that none can be generated from another by row and/or column permutations. For each pair α, β chosen as in Theorem 3.1, this in turn gives rise to five nonisomorphic split nonthreshold graphs, G(M 1, α, β),..., G(M 5, α, β), all sharing a common integral Laplacian spectrum. Evidently taking different appropriate choices of α, β will generate an infinite family of collections of such graphs. Acknowledgement: The research of first author is supported in part by NSERC under grant number OGP The last author is indebted to CNPq(the Brazilian Council for Scientific and Technological Development) for all the support received for their research. 16

17 References [1] Assmus Jr, E. F. and Key, J. D.: Designs and their codes. Cambridge Univ. Press, [2] Brandstädt, A., Bang Le, V. and Spinrad, J.P.: Graph Classes: A Survey. SIAM Monographs on Discrete Mathematics and Applications, [3] Cohen, H.: Number Theory, Volume I: Tools and Diophantine Equations. Graduate Texts in Math. 239, Springer, [4] Corneil, D., Lerchs, H. and Burlingham, L., Complement reducible graphs, Discrete Appl. Math., 3 (1981) [5] Hall, M., Jr.,: Combinatorial Theory. John Wiley and Sons, Inc., New York, [6] Hammer, P.L. and Kelmans, A.K., Laplacian spectra and spanning trees of threshold graphs, Discrete Appl. Math., 65 (1996) [7] Grone, R. and Merris, R., Indecomposable Laplacian integral graphs, Linear Algebra and its Applications (2007), doi: /j.laa [8] Merris, R., Laplacian graph eigenvectors, Linear Algebra and its Applications, 278 (1998) [9] Merris, R., Split graphs, European Journal of Combinatorics, 24 (2003) [10] Sloan, N.J.A., A library of Hadamard matrices, njas/hadamard/. [11] van Lint, J. H. and Wilson, R. M.: A Course in Combinatorics. Cambridge Univ. Press,

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