# Determinants in the Kronecker product of matrices: The incidence matrix of a complete graph

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1 FPSAC 2009 DMTCS proc (subm), by the authors, 1 10 Determinants in the Kronecker product of matrices: The incidence matrix of a complete graph Christopher R H Hanusa 1 and Thomas Zaslavsky 2 1 Department of Mathematics, Queens College (CUNY), Kissena Blvd, Flushing, NY 11367, USA Phone: Department of Mathematical Sciences, Binghamton University (SUNY), Binghamton, NY , USA, Abstract We investigate the least common multiple of all subdeterminants, lcmd(a B), of a Kronecker product of matrices, of which one is an integral matrix A with two columns and the other is the incidence matrix of a complete graph We prove that this quantity is the least common multiple of lcmd(a) to the power n 1 and certain binomials in the entries of A Résumé Nous examinons le plus petit commun multiple de tous les sous-déterminants, lcmd(a B), d un produit de Kronecker de matrices L une des matrices, A, est à entrées entières à deux colonnes et l autre est la matrice d incidence d un graphe complet Nous prouvons que le lcmd(a B) est égale au plus petit commun multiple du lcmd(a) à la puissance n 1 et de certains binômes des coefficients de A Keywords: Kronecker product, determinant, least common multiple, incidence matrix of complete graph, matrix minor In a study of non-attacking placements of chess pieces, Chaiken, Hanusa, and Zaslavsky [1] were led to a quasipolynomial formula that depends in part on the least common multiple of the determinants of all square submatrices of a certain Kronecker product matrix, namely, the Kronecker product of an integral m 2 matrix with the incidence matrix of a complete graph We give a concise expression for the least common multiple of the subdeterminants of this product matrix For matrices A = (a ij ) m k and B = (b ij ) n l, the Kronecker product A B is defined to be the mn kl block matrix a 11 B a 1k B a m1 B a mk B It is known (see [2], for example) that when A and B are square matrices of orders m and n, respectively, then det(a B) = det(a) n det(b) m The quantity we want to compute is lcmd(a B), where for an integer matrix M, the notation lcmd(m) denotes the least common multiple of the determinants of all subm to DMTCS c by the authors Discrete Mathematics and Theoretical Computer Science (DMTCS), Nancy, France

2 2 Christopher R H Hanusa and Thomas Zaslavsky square submatrices This is a much stronger question, as the matrices A and B are most likely not square and the result depends on all square submatrices of their Kronecker product We are interested in the case where A is an integral m 2 matrix and B is the incidence matrix of a complete graph on n vertices (Their Kronecker product has order mn 2 ( n 2) ) For a simple graph G = (V, E), the incidence matrix D(G) is a V E matrix with a row corresponding to each vertex in V and a column corresponding to each edge in E For a column that corresponds to an edge e = vw, there are exactly two non-zero entries: one +1 and one 1 in the rows corresponding to v and w The sign assignment is arbitrary We concern ourselves with the complete graph K n, the graph on n vertices v 1,,v n with an edge between every pair of vertices As an example, one of the many incidence matrices for K 4 is the 4 6 matrix ( a11 a the Kronecker product D(K 4 ) = ; ) D(K a 21 a 4 ) is 22 a 11 a 11 a a 12 a 12 a a a 11 a 11 0 a a 12 a a 11 0 a 11 0 a 11 0 a 12 0 a 12 0 a a 11 0 a 11 a a 12 0 a 12 a 12 a 21 a 21 a a 22 a 22 a a a 21 a 21 0 a a 22 a a 21 0 a 21 0 a 21 0 a 22 0 a 22 0 a a 21 0 a 21 a a 22 0 a 22 a 22 We now introduce our main result after some notation Let A = (a ij ) be an m 2 matrix; this makes A D(K n ) an mn n(n 1) matrix with non-zero entries ±a ij We introduce new notation for some matrices ( that will ) arise naturally in our theorem For i, j [m] := {1, 2,, m}, we write A i,j ai1 a to represent i2 If I is a multisubset of [m], we a j1 a j2 define a Ik to be the product ( ) i I a ik If I and J are multisubsets of [m], we define A I,J to be the matrix ai1 a I2 In this notation, a J1 a J2 lcmd A = lcm ( LCM a ik, LCM deta i,j), i,k i,j where LCM denotes the least common multiple of non-zero quantities taken over all indicated pairs of indices Theorem 1 Let A be an m 2 matrix, not identically zero, and n 1 The least common multiple of all square minor determinants of A D(K n ) is lcmd ( A D(K n ) ) ( ( = lcm (lcmd A) n 1, LCM det A )), Is,Js (1) K (I s,j s) K

3 Determinants in the Kronecker product of matrices 3 where LCM denotes the least common multiple of non-zero quantities taken over all collections K = {(I s, J s )}) of pairs of multisubsets I s and J s of [m] satisfying I s = J s and I s J s = for all s and 2 I s n It is only necessary to take the LCM component over all maximal collections K, that is, collections K satisfying I s = n/2 Proof: Consider an l l submatrix N of A D(K n ) We wish to determine the determinant of N and show that it divides the right-hand side of Equation (1) We need consider only matrices N whose determinant is not zero, since a matrix with detn = 0 has no effect on the least common multiple Since D(K n ) is constructed from a graph, we will analyze N from a graphic perspective The matrix N is a choice of l rows and l columns from A D(K n ) This corresponds to a choice of l vertices and l edges from K n where we are allowed to choose up to m copies of each vertex and up to two copies of an edge Another way to say this is that we are choosing m subsets of V (K n ), say V 1 through V m, and two subsets of E(K n ), say E 1 and E 2, with the property that m i=1 V i = 2 k=1 V k = l From this point of view, if a row in N is taken from the first n rows of A D(K n ), this is thought of as placing the corresponding vertex of V (K n ) in V 1, and so on, up through a row in N from the last n rows of A D(K n ) corresponding to a vertex in V m We will say that the copy of v in V i is the i th copy of v and the copy of e in E k is the k th copy of e Under this framework, we will now perform elementary matrix operations on N in order to make its determinant easier to calculate We call the resulting matrix the simplified matrix of N Each copy of a vertex v has a row in N associated with it; two rows corresponding to two copies of the same vertex contain the same entries except for the different multipliers a ik For example, if v is a vertex in both V 1 and V 2, then there is a row corresponding to the first copy with multipliers a 11 and a 12 and a row corresponding to the second copy with the same entries multiplied by a 21 and a 22 In the case when there is a vertex in exactly two vertex sets V i and V j corresponding to two rows R i and R j in N, we perform the following operations depending on the multipliers a i1, a i2, a j1, and a j2 We first notice that deta i,j = a i1 a j2 a i2 a j1 is non-zero; otherwise, the rows R i and R j would be linearly dependent in N and detn = 0 Therefore either both a i1 and a j2 or a i2 and a j1 are non-zero In the former case, let us add a j1 /a i1 times R i to R j in order to zero out the entries corresponding to edges in E 1 The multipliers of entries in R j corresponding to edges in E 2 are now all deta i,j /a i1 Similarly, we can zero out the entries in R i corresponding to edges in E 2 Lastly, factor out deta i,j /a j2 a i1 from R j If on the other hand, either multiplier a i1 or a j2 is zero, then reverse the roles or i and j in the preceding argument There cannot be a vertex in three or more vertex sets since then the corresponding rows of N would be linearly dependent and det N would be zero The above manipulations ensure that the multiplier of every non-zero entry in N that corresponds to an i th vertex and a k th edge is a ik We assert that the simplified matrix of N has no more that two non-zero entries in any column For a column e corresponding to an edge e = vw in K n, each of v and w is either in one vertex set V i or in two vertex sets V i and V j If the vertex corresponds to two rows in N, the above manipulations ensure that there is only one copy of the vertex that has a non-zero multiplier in the column Another important quality of this simplification is that if a vertex is in more than one vertex set, then every edge incident with once instance of this repeated vertex is now in the same edge set Since we are assuming detn 0, N has at least one non-zero entry in each column or row If a row (or column) has exactly one non-zero entry, we can reduce the determinant by expanding in that row (or column) This contributes that non-zero entry as a factor in the determinant After reducing repeatedly

4 4 Christopher R H Hanusa and Thomas Zaslavsky v q e q v q+1 C Fig 1: An edge e q = v qv q+1 in the cycle C generated by block B When v q V i, v q+1 V j, and e q E k, the contributions y q and z q to detb are a ik and a jk, respectively in this way, we arrive at a matrix where each column has exactly two non-zero entries, and each row has at least two non-zero entries This implies that every row has exactly two non-zero entries as well After interchanging the necessary columns and rows and possibly multiplying columns by 1, the structure of what we will call the reduced matrix of N is a block diagonal matrix where each block B is a weighted incidence matrix of a cycle, such as y z 6 z 1 y z 2 y z 3 y z 4 y z 5 y 6 The determinant of a p p matrix of this type is y 1 y p z 1 z p Therefore, we can write the determinant of N as the product of powers of entries of A, powers of deta i,j, and binomials of this form In our situation, the entries y q and z q are the variables a ik, depending on in which vertex sets the rows lie and in which edge sets the columns lie If the vertices of K n corresponding to the rows in B are labeled v 1 through v p, this block of the block matrix corresponds to traversing the closed walk C = v 1 v 2 v p v 1 in K n (in this direction) As a result of the form of the simplified matrix of N, for a column that corresponds to an edge e q = v q v q+1 in E k traversed from the vertex v q in vertex set V i to the vertex v q+1 in vertex set V j, the entry y q is a ik and the entry z q is a jk (See Figure 1) Therefore each block B in the block diagonal matrix contributes detb = a ik a jk (2) e=v qv q+1 C e=v qv q+1 C e E k,v q V i e E k,v q+1 V j for some closed walk C in G We can simplify this expression by analyzing what exactly the a ik and a jk are Suppose that two adjacent edges e q 1 and e q in C are in the same edge set E k, and suppose that the vertex v q that these edges share is in V i (See Figure 2) In this case, both entries z q 1 and y q are a ik, which can then be factored out of each product in Equation (2) A particular case to mention is when the cycle C contains a vertex that has multiple copies in N (not necessarily both in C) In this case, the edges of C incident with this repeated vertex are both from the same edge set, as mentioned earlier After factoring out a multiplier for each pair of adjacent edges in the same edge set, all that remains inside the products in Equation (2) are the contributions of multipliers from vertices where the incident edges are from different edge sets

5 Determinants in the Kronecker product of matrices 5 v q e q 1 e q C Fig 2: Two adjacent edges e q 1 and e q, both incident with vertex v q in the cycle C generated by block B When both edges are members of the same edge set E k and v q is a member of V i, the contributions z q 1 and y q are both a ik, allowing this multiplier to be factored out of Equation (2) More precisely, when following the closed walk, let I be the multiset of indices i such that the walk C passes from an edge in E 2 to an edge in E 1 at a vertex in V i Similarly, let J be the multiset of indices j such that C passes from an edge in E 1 to an edge in E 2 at a vertex in V j Then what remains inside the products in Equation (2) after factoring out common multipliers is exactly deta I,J = i I a i1 j J a j2 j J a j2 a i2 There is one final simplifying step Consider a value i occurring in both I and J In this case, we can factor a i1 a i2 out of both terms This implies that the determinant of each block B of the block diagonal matrix is of the form ( ) ± deta I,J, (3) i,k a s ik ik where the exponents s i,k are non-negative integers and I and J are disjoint subsets of [m] of the same cardinality and 2 I + i,k s ik = p because the degree of detb is the order of B Notice that when I = J = 1 (say I = {i} and J = {j}), the factor deta I,J equals deta i,j Combining contributions from the simplification process and all blocks, we have detn = ± i,j for some non-negative exponents S ik (deta i,j ) Vi Vj i,k a S ik ik i I deta IB,JB, We now verify that this product divides the right hand side of Equation (1) The exponents S ik can be no larger than n because there are only n rows with entries a ik in A D(K n ), so as a polynomial in the variable a ik, every term in the expansion of the determinant has degree at most n Furthermore, it is not possible for the exponent of any a ik to be n The only way this might occur is if N were to contain all n vertices of V i and at least n edges of E k incident with the vertices of V i The corresponding set of columns is a dependent set of columns in N (the rank of D(K n ) is n), which would make detn = 0 Therefore detn contributes no more than n 1 factors of any a ik to lcmd(a D(K n )) Now let us determine the number of factors of deta i,j that may divide detn Notice that factors of deta i,j do not solely arise upon the conversion of N to the simplified matrix of N they also arise when I = J = 1 in Equation (3) We need to take this into account to determine how many factors of deta i,j may divide detn It is not possible for n factors deta i,j to arise through the initial simplification of N B

6 6 Christopher R H Hanusa and Thomas Zaslavsky If N contained all rows corresponding to vertices of V i and V j along with 2n columns of A D(K n ), then at least one edge set (E 1 or E 2 ) would contain n edges from K n The columns corresponding to these edges would form a dependent set of columns in N, making detn = 0 Because every deta i,j contribution from blocks must include at least one vertex in V i with no copies and one vertex in V j with no copies, there can be no more than n 1 V i V j copies of deta i,j from blocks Therefore there cannot be more than n 1 factors deta i,j in the contribution of detn to lcmd(a D(K n )) The factors of detn from the blocks B of the reduced matrix of N can be considered independently from the contributions of a ik and deta i,j to Equation (1) There may be contributions of deta i,j in both terms of the right hand side of Equation (1); however, they will not occur twice after application of the outer lcm operation Regarding the contribution of blocks B, the number of possible det A I,J factors is limited as well For every factor deta I,J, each vertex leading to an index in I and every vertex leading to an index in J must have no additional copies in the graph Hence each vertex of K n occurs at most once in some closed walk C generated by a block B Depending on the vertex set into which this vertex is placed, the vertex either contributes an index to either I B or J B So we are in a situation where for all B, I B = J B, I B J B =, and B ( I B + J B ) n, the product of which is the exact description of one product in the LCM K contribution of Equation (1) We have shown that for every matrix N, detn divides the right-hand side of Equation (1) We now show that there exist graphs that attain the claimed powers of factors Consider the path of length n 1, P = v 1 v 2 v n, as a subgraph of K n Create the (2n 2) (2n 2) submatrix N of A D(K n ) with rows corresponding to both an i th copy and a j th copy of vertices v 1 through v n 1 and edges corresponding to two copies of every edge in P Then a i a i a i1 a i1 0 0 a i2 a i N = 0 0 a i1 a i1 0 0 a i2 a i2 a j a j , a j1 a j1 0 0 a j2 a j a j1 a j1 0 0 a j2 a j2 with determinant (det A i,j ) n 1 The four quadrants of N are (n 1) (n 1) submatrices of A D(K n ) with determinants a n 1 i1, a n 1 i2, a n 1 j1, and a n 1 j2, respectively For all collections K = {(I s, J s )}) of pairs of multisubsets I s and J s of [m] satisfying I s = J s and I s J s = for all s and 2 I s n, we show that there is a submatrix N of A D(K n ) with determinant (I s,j s) K detais,js For all s starting with s = 1, choose a subgraph of K n with 2 I s vertices as follows If I s > 1, create a cycle C s of length 2 I s using the next I s unused vertices of K n If I s = 1 and the first unused vertex is vertex v q+1, then just create the edge e s = v q+1 v q+2 Choose the rows and columns of A D(K n ) for N using the framework of placing the vertices and edges of K n into vertex sets V i and edge sets E k For the cycle C s with vertices v q+1, v q+2,, v q+2 Is, place the odd-indexed vertices into a vertex set V i for every i I s and the even-indexed vertices into a vertex set V j for j J s Place the edges from a lower-indexed odd vertex to a higher-indexed even vertex in E 1 and

7 Determinants in the Kronecker product of matrices 7 place all other edges in E 2 When I s = {i} and J s = {j}, place vertex v q+1 in V i and vertex v q+2 in V j Place the edge v q+1 v q+2 in both E 1 and E 2 The submatrix N of A D(K n ) that arises from placing the vertices in lexicographic order and the edges in order following the cycles C s or edge e s is the block-diagonal matrix with blocks N s where each matrix N s is a 2 I s 2 I s matrix of the form a i a i12 a j11 a j a i22 a i a j21 a j a jl 1 a jl 2 if C s is a cycle and ( ) ai1 a i2 a j1 a j2 if e s is an edge The determinant of N s is exactly deta Is,Js for all s, so the determinant of N is (I det s,j s) K AIs,Js, as desired When understanding the right hand side of Equation (1), it may be instructive to notice that the LCM factor on the right hand side divides disjoint I, J: I = J =p (det A I,J ) n/2p, as the largest number of individual deta I,J factors that may occur for disjoint p-member multisubsets I and J of [m] is n/2p When m = 2, the only pair of disjoint p-member multisubsets of [m] is {1 p } and {2 p } From this, we have the following corollary Corollary 2 Let A be a 2 2 matrix, not identically zero, and n 1 The least common multiple of all square minor determinants of A D(K n ) is lcmd ( A D(K n ) ) = lcm ( (lcmd A) n 1, LCM n/2 ( (a11 a 22 ) p (a 12 a 21 ) p) n/2p ), p=2 where LCM denotes the least common multiple over the range of p We calculate a few examples, with matrices A that are needed for the chess-piece problem of [1] Example 1 When the chess piece is the bishop, A is the 2 2 matrix ( ) 1 1 A = 1 1

8 8 Christopher R H Hanusa and Thomas Zaslavsky We apply Corollary 2, noting that lcmd(a) = 2 We get lcmd ( A D(K n ) ) = lcm ( 2 n 1, LCM n/2 ( ( 1) p 1 p) n/2p ) The LCM generates powers of 2 no larger than 2 n/2, hence lcmd ( A D(K n ) ) = 2 n 1 p=2 Example 2 When the chess piece is the queen, A is the 4 2 matrix 1 0 A = Again, lcmd(a) = 2 We apply Theorem 1 Every pair (I, J) of disjoint p-member multisubsets of [4] has one of the following seven forms, up to the order of I and J: ({1 q }, {2 r, 3 s, 4 t }), ({2 r }, {1 q, 3 s, 4 t }), ({3 s }, {1 q, 2 r, 4 t }), ({4 t }, {1 q, 2 r, 3 s }), ({1 q, 2 r }, {3 s, 4 t }), ({1 q, 3 s }, {2 r, 4 t }), ({1 q, 4 t }, {2 r, 3 s }), where the sum of the exponents in each multisubset is p, and where q, r, s, and t may be zero When we calculate deta I,J for each pair (I, J), the presence of zeroes and ones in A simplifies our calculations For example, in the third case, where I = {3 s } and J = {1 q, 2 r, 4 t }, the matrix A I,J is as follows: ( ) 1 A I,J s 1 = s 1 q 0 r 1 t 0 q 1 r ( 1) t, where s = q + r + t = p If any entry in this matrix is zero, then the determinant is a product of 0 s, 1 s, and 1 s, which will not contribute to the LCM Therefore the only non-trivial case is when q = r = 0, in which case s = t = p, and deta I,J = ( 1) p 1 p = 0 or 2 This implies that the LCM of Equation (1) divides 2 n 1 We conclude that lcmd ( A D(K n ) ) = 2 n 1 Example 3 A more difficult example is the fairy chess piece known as a nightrider, which moves an unlimited distance in the directions of a knight Here A is the 4 2 matrix 1 2 A = The submatrices ( ) ( ) ( ) ,, and, with determinants 3, 4, and 5, respectively, lead to the conclusion that lcmd(a) = 60 Since the dimensions of A are as in Example 2, we have the same possibilities for disjoint pairs of p-member multisubsets of [4] It turns out that deta I,J has the same form in all seven cases: precisely ±2 u (2 2p 2u ± 1), where u is a number between 0 and p Furthermore, every value of u from 0 to p appears and every

9 Determinants in the Kronecker product of matrices 9 choice of plus or minus sign appears (except when u = p) in deta I,J for some choice of (I, J) We present two representative examples that support this assertion The case of I = {1 q } and J = {2 r, 3 s, 4 t } Then ( ) 1 A I,J q 2 = q 2 r 1 s 2 t 1 r ( 2) s ( 1) t, with q = r +s+t = p We can rewrite deta I,J as ±2 s 2 2p s = 2 s (2 2p 2s ±1) The only instance in where there is no choice of sign is when s = p and r = t = 0, in which case deta I,J simplifies to either 0 or 2 p+1 The case of I = {1 q, 2 r } and J = {3 s, 4 t } Then ( ) A I,J 1 = q 2 r 2 q 1 r 1 s 2 t ( 2) s ( 1) t, where q + r = s + t = p For this choice of I and J, deta I,J = ( 1) p 2 r+s 2 2p r s Since every deta I,J has the same form, and at most p/2n factors of type (2 2p 2u ±1) may occur at the same time, the LCM in Equation (1) is exactly ( LCM K (I s,j s) K for some N n We conclude that deta Is,Js ) = 2 N LCM 1 p n/2 0 u p 1 lcmd ( A D(K n ) ) = lcm(60 n 1, LCM 1 p n/2 0 u p 1 As a sample of the type of answer we get, when n = 8 this expression is (2 2p 2u ± 1) n/2p, (2 2p 2u ± 1) n/2p ) lcmd ( A D(K 8 ) ) = lcm(60 7, (4 ± 1) 8/2, (16 ± 1) 8/4, (64 ± 1) 8/6, (256 ± 1) 8/8 ) = The first few values of n give the following numbers: n lcmd ( A D(K 8 ) ) (factored) We hope to determine in the future whether lcmd(a B) has a simple form for arbitrary matrices A and B Our limited experimental data suggests this may be difficult However, we think at least some generalization of Theorem 1 is possible

10 10 Christopher R H Hanusa and Thomas Zaslavsky Another direction worth investigating is the number theoretic aspects of Theorem 1 Our initial goal was a formula for lcmd(a D(K n )) Theorem 1 gives a compact expression for this quantity, but not as simple as it could be without the least common multiples Improving it would require an understanding of when two multivariate binomials have a common divisor References [1] Seth Chaiken, Christopher RH Hanusa, and Thomas Zaslavsky, A q-queens problem In preparation [2] Roger A Horn and Charles R Johnson Topics in Matrix Analysis Cambridge University Press, New York 1991 vii pp

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