# 1 Introduction. A Partition Problem for Sets of Permutation Matrices

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1 A Partition Problem for Sets of Permutation Matrices Richard A. Brualdi Department of Mathematics, University of Wisconsin, Madison, WI Hanley Chiang and Chi-Kwong Li Department of Mathematics, College of William and Mary, Williamsburg, VA Introduction Consider a set P of permutation matrices of order n. What is the smallest integer m such that P can be partitioned into subsets P 1, P 2,..., P m such that {P : P Pi }, (i = 1, 2,..., m) are (0,1)-matrices? Let G(P) be a graph with vertex set P with an edge joining two permutation matrices P, Q P provided P and Q have a 1 in common (that is, a 1 in the same position). The integer m equals the chromatic number χ(g(p)). Natural sets P of permutation matrices arise by choosing A = [a ij ] to be a (0, 1)-matrix and P = P A = {P : P A, P is a permutation matrix}. (1) (Here the inequality P A is interpreted entrywise.) In this case the sets P i in the partition must satisfy {P : P Pi } A. A more restrictive problem requires that {P : P Pi } = A (i = 1, 2,..., m). (2) If (2) holds, then {P : P PA } = ma, and we say that P A has a perfect partition. The cardinality of the set P A equals the permanent of A defined, as usual, by: per(a) = (i 1,i 2,...,i n) S n a 1i1 a 2i2 a nin, where the summation is over the symmetric group S n of all permutations of {1, 2,..., n}. Suppose that P A has a perfect partition. Then there are two consequences for the structure of A. First, there is an integer k such that all row and column sums of A equal k, and this integer k satisfies the equation per(a) = mk. Second, a perfect partition implies that 1

2 each 1 of A belongs to m permutation matrices P A, and hence, where A(i, j) denotes the submatrix of A obtained by deleting row i and column j, that pera(i, j) = m if a ij = 1, that is, the permanental minors of the 1 s of A all equal the same constant m. Let G A = G(P A ). Since the chromatic number of G A equals the minimal number of independent sets into which P A can be partitioned, we have χ(g A ) per(a) α(g A ), (3) where α(g A ) is the maximal size of an independent set of G A. We can have equality in (3) only if α(g A ) per(a). If P A has a perfect partition, then the integer m in (2) equals χ(g A ). Since χ(g A ) is an integer, (3) implies that χ(g A ) per(a). (4) α(g A ) By a theorem of Folkman and Fulkerson [2] (see also Theorem in [1]), the independence number α(g A ) equals min { } sum(akl ) k + l n : k + l > n where the minimum is taken over all pairs of integers k and l with n < k + l 2n and k l submatrices A kl of A, and sum(a kl ) is the sum of the entries of A kl. There is a geometrical interpretation of the perfect partition problem. Recall that a necessary condition for the existence of a perfect partition for P A is that the sum of matrices in P A is a multiple of A. Thus, the average of P A, which can also be viewed as the centroid of the convex hull of P A, has the form γa. Clearly, every element in P A is an extreme point of the convex hull of P A. (To see this, note that every element X in P A has the same Frobenius norm (trace XX t ) 1/2 and therefore cannot be written as a convex combination of the others.) If A has row sums and column sums all equal to k, then one needs at least k elements in P A whose average (regarded as the centroid of the convex hull of the k elements) is equal to γa; if the desired partition is a partition of P A in k-element sets, then each of them has the same average as that of P A. In the subsequent discussion, let J n be the n n matrix of all 1 s. In the next section we consider perfect partitions of S n = P Jn (where we now regard S n as the set of n n permutation matrices) and the alternating group A n of all n n even permutation matrices (permutation matrices with determinant equal to 1). In Section 3, we consider the set D n = P Jn In of n n derangement permutation matrices; we present some partial results and open problems. Additional open questions are discussed in the final section. 2

3 2 Partitioning S n and A n We have α(j n ) = n and per(j n ) = n!, and it is easy to show that X S n X = (n 1)!J n. Can we partition S n into (n 1)! subsets so that the sum of the matrices in each subset is J n? The answer is affirmative. Proposition 2.1 The set S n = P Jn is a disjoint union of (n 1)! subsets such that the sum of the matrices in each subset is J n. Hence S n has a perfect partition. Proof. Let H = {I n, P,..., P n 1 } where P is the basic n n circulant matrix (5) Then H is a cyclic group with n elements whose sum is the matrix J n. There are (n 1)! cosets of H in S n. Each coset has the form QH = {QP j : j = 0,..., n 1} for some Q S n. Clearly, the sum of the matrices in each coset is also the matrix J n. Now we consider the group A n of even permutation matrices. We have A n = n!/2, and it is not hard to show that X A n X = [(n 1)!/2]J n if n 3. Can we partition A n into (n 1)!/2 subsets so that the sum of the matrices in each subset is J n? We have the following result. Proposition 2.2 Suppose n 3. The set A n can be partitioned into (n 1)!/2 subsets so that the sum of the matrices in each subset is J n. Proof. We consider three cases according to n. Case 1. If n 3 is odd, then the basic circulant matrix P is in A n. Thus H = P is a subgroup of A n with (n 1)!/2 cosets, and the sum of the matrices in each coset is J n. Case 2. If n = 4k for some positive integer k, we can prove by induction that: There is a subgroup H in A n with n elements whose sum equals J n, and hence the cosets of the group H will be a desired partition. When k = 1, let H 4 be the subgroup of A 4 containing all the elements of order 2 or 0 (H 4 is the 2-Sylow subgroup of A 4 ). One can readily check that the the sum of the matrices in H 4 sum up to J 4. Now, suppose the result is true for n = 4k for some k 1. Consider the case when n = 4(k + 1). By the induction assumption, there is a group H 4k of A 4k such that the sum of the matrices in H 4k is J 4k. Let H = {A B : A H 4, B H 4k }, where X Y = (x ij Y ) denotes the usual tensor product of two matrices. Then H is a subgroup of A n with n = 4(k + 1) elements whose sum is the matrix J n. By induction, our claim is proved. 3

4 Case 3. Let n = 2m for some odd integer m. We consider the subgroup K of A n consisting of matrices of the form A B, where A and B are m m permutation matrices. There are (m!) 2 /2 such matrices. To see this, if we allow A and B to be arbitrary matrices in S m, there will be (m!) 2 such matrices in S n. Since half of them are odd permutations, we see that K has (m!) 2 /2 elements as asserted. We claim that K can be partitioned into m((m 1)!) 2 /2 subsets such that each subset has m elements summing up to J m J m. To this end, let P S m be the basic circulant. Let G = P, and let Q 1 G,..., Q r G be the cosets of G in S m, where r = (m 1)!, Q 1,..., Q r/2 A m and Q j / A m for j > r/2. For each i, j = 1,..., r/2, consider the following m-element subsets of A n : S ij1 = {(Q i Q j )(P P ) k : k = 0,..., m 1}, S ij2 = {X(I m P ) : X S ij1 }, S ij3 = {X(I m P 2 ) : X S ij1 },...,..., S ijm = {X(I m P m 1 ) : X S ij1 }. We get m(r/2) 2 disjoint m-element subsets of K. Next, for each i, j = r/2 + 1,..., r, consider S ij1 = {(Q i Q j )(P P ) k : k = 0,..., m 1}, S ij2 = {X(I m P ) : X S ij1 }, S ij3 = {X(I m P 2 ) : X S ij1 },...,..., S ijm = {X(I m P m 1 ) : X S ij1 }. We get another m(r/2) 2 disjoint m-element subsets of K. Consequently, we get mr 2 /2 = m((m 1)!) 2 /2 disjoint m-element subsets of K. Moreover, the matrices in each subset sum up to J m J m as desired. ( ) 0m I Now, consider the matrix R obtained by switching the first two rows of m. Then R A m. Let H = K {RX : X K}. One easily checks that H is the subgroup of A n consisting of matrices of the form ( ) A 0 0 B or ( ) 0 C. D 0 Moreover, for each set S ijk defined above, we may construct T ijk = S ijk {RX : X S ijk }. Then each T ijk will have n = 2m elements summing up to J n, and these T ijk form a partition of the subgroup H. Now, let H, W 1 H, W 2 H,..., W t H be the cosets of H in A n, where t+1 = A n / H. Each coset W s H is a disjoint union of W s T ijk s, and each W s T ijk has n elements summing up to J n. I m 0 m 4

5 Corollary 2.3 The set S n has a perfect partition in which each part of the partition consists of all even permutation matrices or all odd permutation matrices. Proof. As in the proof of Proposition 2.1, the coset of odd permutations also can be partitioned into sets summing to J n. 3 Partitioning D n = P Jn I n : Partial Result Let L n = J n I n. For n = 2,..., 5 we show that D n = P Ln can be partitioned into subsets each with n 1 matrices that sum to L n. In the following discussion, we identify a permutation σ in disjoint cycle representation with the corresponding permutation matrix in S n. For example, (1, 2)(3, 4) represents the permutation obtained from the identity matrix by interchanging the first and second rows, and also the third and fourth rows. Then σ S n is a derangement if and only if σ(i) i for i = 1,..., n. Moreover, the elements in a set of derangements {σ 1,..., σ n 1 } D n sum to L n if and only if σ r (i) σ s (i) for r s and for all i = 1,..., n. We have the following partial result for the partition problem of P Ln. Proposition 3.1 The set D n has a perfect partition if n 5. Proof. If n = 2, then D n = {L n } is a singleton. If n = 3, then the members of D n = {(1, 2, 3), (1, 3, 2)} sum to L n. For n = 4, a permutation belongs to D n if and only if it is a 4-cycle or a product of two disjoint transpositions. If F 1 = {(1, 2)(3, 4), (1, 3, 2, 4), (1, 4, 2, 3)}, F 2 = {(1, 3)(2, 4), (1, 2, 3, 4), (1, 4, 3, 2)}, F 3 = {(1, 4)(2, 3), (1, 2, 4, 3), (1, 3, 4, 2)}, then D 4 = 3 k=1f k and the members of each F k sum to L 4. For n = 5, a permutation belongs to D n if and only if it is of the form (i 1, i 2, i 3, i 4, i 5 ) or (i 1, i 2 )(i 3, i 4, i 5 ). Let D 5 D 5 be the set of derangements of the form (i 1, i 2, i 3, i 4, i 5 ) and let D 5 D 5 be the set of derangements of the form (i 1, i 2 )(i 3, i 4, i 5 ). Observe that D 5 = 24 and D 5 = 20. We show that D 5 and D 5 can be partitioned into 6 and 5 subsets, respectively, such that the members of each subset sum to L 5. Let τ 1 = (1, 2, 3, 4, 5), τ 2 = (1, 2, 3, 5, 4), τ 3 = (1, 2, 4, 3, 5), τ 4 = (1, 2, 4, 5, 3), τ 5 = (1, 2, 5, 3, 4), and τ 6 = (1, 2, 5, 4, 3). If T k = {τ k, τk 2, τk 3, τk 4 }, then the collection of subsets T 1,..., T 6 forms a partition of D 5 such that the members of each T k sum to L 5. Now, consider the following subsets of D 5: R 1 = {(1, 2)(3, 4, 5), (1, 3)(2, 5, 4), (1, 4)(2, 3, 5), (1, 5)(2, 4, 3)} 5

6 R 2 = {(2, 1)(3, 5, 4), (2, 3)(1, 4, 5), (2, 4)(1, 5, 3), (2, 5)(1, 3, 4)} R 3 = {(3, 1)(2, 4, 5), (3, 2)(1, 5, 4), (3, 4)(1, 2, 5), (3, 5)(1, 4, 2)} R 4 = {(4, 1)(2, 5, 3), (4, 2)(1, 3, 5), (4, 3)(1, 5, 2), (4, 5)(1, 2, 3)} R 5 = {(5, 1)(2, 3, 4), (5, 2)(1, 4, 3), (5, 3)(1, 2, 4), (5, 4)(1, 3, 2)}. Then the collection of subsets R 1,..., R 5 forms a partition of D 5 such that the members of each R k sum to L 5. This completes the partition of D 5. The problem of partitioning D n with n 6 is more difficult. In the following, we describe several different approaches we considered. First, we divide the set D n into subsets according to different cycle decompositions, and we attempt to show that each of these subsets admits a partition into (n 1)-element subsets with elements summing to L n. In particular, when n = 5, the partition was done in this way. When we apply this idea to D 6, we get the following subsets: T 1 : the set of length-6 cycles 120 elements; T 2 : the set of permutations obtained by the product of a 2-cycle and a 4-cycle 90 elements; T 3 : the set of permutations obtained by the product of two 3-cycles 40 elements; T 4 : the set of permutations obtained by the product of three 2-cycles 15 elements. For each subset, the sum of its elements (say, denoted by X) will be a multiple of L n because all of the diagonal entries of X are zeroes and P XP t = X for every permutation matrix P. However, this approach to partitioning D n fails when n = 6. One can check that the set T 4 cannot be partitioned into three 5-element subsets such that the elements in each subset sum up to L 6. An alternative idea is to select 15 elements τ 1,..., τ 15 from T 1 and construct disjoint subsets U i = {τ j i : j = 1,..., 5} so that each of them has elements summing up to L 6. Note that each U i will have two elements in T 1, two elements in T 3, and one element in T 4. If this is done, then we are left with 90 elements in T 1, the entire set T 2, and 10 elements in T 3. Another scheme is to select one element in T 4 and four elements in T 1 of the form τ 1, τ1 1, τ 2, τ2 1 to form a set whose elements sum up to L 6. Here is an example: (1, 2)(3, 4)(5, 6), (1, 3, 5, 2, 6, 4), (1, 4, 6, 2, 5, 3), (1, 6, 3, 2, 4, 5), (1, 5, 4, 2, 3, 6). In fact, one can construct 15 sets of such form and use up the 15 elements in T 4 together with 60 elements in T 1. It is also possible to use two elements in T 3 and three elements in T 1 to form a set whose elements sum up to L 6. Here is an example: (1, 2, 3)(4, 5, 6), (1, 3, 2)(4, 6, 5), (1, 4, 2, 5, 3, 6), (1, 5, 2, 6, 3, 4), (1, 6, 2, 4, 3, 5). One can actually construct 20 subsets of this form and use up the 40 elements in T 3 together with 60 elements in T 1. 6

7 One may want to use the two schemes in the last two paragraphs to exhaust the elements in T 1, T 3, and T 4, but this strategy seems to be impossible. Of course, even if it can be done, one must still partition the elements in T 2 into 18 sets, each of which has elements summing up to L 6. Here is an example of such a set: (1, 2)(3, 4, 5, 6), (1, 3)(2, 4, 6, 5), (1, 4)(2, 5, 3, 6), (1, 5)(2, 6, 4, 3), (1, 6)(2, 3, 5, 4). It is unclear whether one can construct 18 disjoint subsets of T 2 with the desired property. Thus, the problem of finding a perfect partition for P Ln seems difficult. We close this section with a statement of the problem and some related questions: Problem 3.2 For n 6, is there a perfect partition for P Ln or P Ln A n? Problem 3.3 For n 6, is there a perfect partition for the collection of permutations in P Ln with some specific cycle decomposition? For example, can the set of permutations obtained by the product of a 2-cycle and an (n 2)-cycle be partitioned into subsets such that the elements of each subset sum up to L n? The answer is no for n = 4, yes for n = 5, and unknown for n 6. 4 Additional Problems We continue to use P to denote the basic circulant as defined in (5). Note that J n = n 1 k=0 P k and L n = n 1 k=1 P k. For any subsets K {0, 1,..., n 1}, let P K = k K P k. A general question is: Problem 4.1 Determine K {0, 1,..., n 1} so that P PK perfect partition. (respectively, P PK A n ) has a By the results in the previous sections, we see that both problems in Problem 4.1 have affirmative answers if K = n. If K = 1, then both problems also have affirmative answers trivially. If K = 2, then we have the following proposition. Proposition 4.2 Let A = I n + P k with 0 < k < n. Then P A admits a perfect partition. Proof. Write P k in disjoint cycle notation. There are two cases. Case 1. If (n, k) is relatively prime, then P k is just one long cycle, and I n and P k are the only two elements in P A, which admits a trivial perfect partition. Case 2. If d > 1 is the greatest common divisor of n and k, and m = n/d, then P k is the product of d cycles of length m. Now, we can rewrite A = I n + P k as the direct sum of d 7

8 m m matrices, each of which is I m + Q, where Q is the m m basic circulant. In this form, one readily checks that X P A if and only if X = X 1 X d such that X j {I m, Q}. Thus, there are 2 d matrices in P A. Moreover, P A has a perfect partition consisting of sets of the form {X, A X} with X P A. If K = n 1, we basically have the P Ln problem, and we only have partial results. If K = 3, even the necessary condition for a perfect partition may not hold. Here is an example which can be verified readily. Example 4.3 For n = 5 there are 13 matrices in P A for A = I n +P +P 2 or A = I n +P 2 +P 3. In either case, a perfect partition is impossible. Note that in general, if K = n 2, then P K = J n P r P s. Replacing P K by P j P K for a suitable j {0,..., n 1}, we may assume that (r, s) = ( l, l) with 1 l n/2, or (r, s) = (0, 1). For example, for n = 5, we only need to consider the cases in Example 4.3. If n is even and (r, s) = ( l, l) with 1 l n/2, then up to a permutation equivalence, i.e., replace A by RAS for some suitable R, S S n, we can assume that P K = J n (I n/2 J 2 ), which can be viewed as a generalization of L n. In general, if n = km, we consider L n,k = J n (I m J k ). We have the following. Proposition 4.4 Suppose n 4 and n = km. Then per (L n,k ) is a multiple of (n k). Moreover, if n > k 2, then PLn,k A n = per (Ln,k )/2 is also a multiple of (n k). Proof. Use Laplace expansion about the first row of L n,k. Note that all of the submatrices of L n,k obtained by deleting the first row and jth column with k < j n are permutationally equivalent and have the same permanent, say, r. Thus, per (L n,k ) = (n k)r. Next, suppose n > k 2. Then for each σ P Ln,k, we have (1, 2)σ P Ln,k, and either σ or (1, 2)σ is an even permutation. Thus, half of the elements in P Ln,k belong to A n. Next, consider the Laplace expansion of per (L n,k ) as in the first paragraph of the proof. We claim that r is even. To this end, suppose A is obtained from L n,k by deleting its first row and (k + 1)st column. Note that for any permutation σ P A, we have σ(1, 2) P A, and either σ or σ(1, 2) is an even permutation (in S n 1 ). Thus, P A = r is even. Consequently, P Ln,k A n = per (Ln,k )/2 = (n k)(r/2) is also a multiple of (n k). Note that the second assertion of the above proposition is not valid for P Ln. As shown in Section 3, the number of even and odd permutations in P Ln may be different: n : P Ln : P Ln A n :

9 Nevertheless, for (n, k) = (3, 1), (4, 1), (5, 1) it is not hard to find a perfect partition for P Ln A n ; see the results in the last section. In general, we have the following. Problem 4.5 Determine whether there is a perfect partition for P Ln,k A n ). (respectively, P Ln,k Notice that finding a perfect partition for P Ln,n/2 is the same as finding a perfect partition for P A with A = J n/2 J n/2. Examining Case 3 in the proof of Proposition 2.2, we have the following. Proposition 4.6 Suppose n is even. There is always a perfect partition for P Ln,n/2 (respectively, P Ln,n/2 A n ). Answering Problem 4.5 for other values of (n, k) is not so easy. For (n, k) = (6, 2), we have an affirmative answer. Proposition 4.7 There is a perfect partition for P L6,2. Proof. Let L 6,2 = (A ij ) 1 i,j 3, where A ii = 0 2 for i = 1, 2, 3, and A ij = J 2 for i j. We first show that P L6,2 = 80. Every permutation matrix in P L6,2 is determined by selecting exactly one nonzero entry from each row and column of L 6,2. Consider the number of ways to construct a matrix X P L6,2 if the 1 in the (1, 3) position of L 6,2 is selected to be in X. In the following discussion, a nonzero entry of L 6,2 is said to be available if no other nonzero entry in its row or column has been selected to be in X. We consider two cases, depending on the nonzero entry selected from the second row of L 6,2. Case 1. If the (2, 4) entry of L 6,2 is selected to be in X, then the remaining four nonzero entries of X must be obtained by selecting two nonzero entries each from the A 23 and A 31 submatrices of L 6,2. The nonzero entries from each of these two submatrices can be selected in one of two ways: either entirely on the submatrix diagonal or entirely off of the submatrix diagonal. Thus, there are 2 2 = 4 possible ways to construct X in this case. Case 2. If the (2, 4) entry of L 6,2 is not selected, then there are two available nonzero entries in the second row of L 6,2 that can be selected; both choices lie in A 13. Each choice sequentially forces the selection of one of two available nonzero entries each from the A 23, A 21, and A 31 submatrices, thereby determining the final selection of the only available nonzero entry from the A 32 submatrix. Thus, there are 2 4 = 16 ways to construct X in this case. Combining the two preceding cases, there are = 20 matrices in P L6,2 with a 1 in the (1, 3) position. By analogous arguments, one can show that there are 20 matrices in P L6,2 with a 1 in the (1, k) position for k = 4, 5, 6. Thus, P L6,2 = 4 20 = 80. Next, we show that P L6,2 admits a perfect partition. Let T 2 S 2 correspond to the permutation (1, 2), and let R 3 S 3 correspond to the permutation (1, 3, 2). Let W = {R 3 I 2, R 3 T 2, R t 3 I 2, R t 3 T 2 }. 9

10 Then W, (3, 4)W, (5, 6)W, and (3, 4)(5, 6)W are disjoint subsets of P L6,2 such that the matrices in each subset sum up to L 6,2, and each of the four subsets contains exactly one matrix from Case 1 above. The remaining 64 matrices in P L6,2 can be partitioned as follows. Recall that each of the 16 matrices X 1,..., X 16 from Case 2 above has exactly one nonzero entry from every A ij in L 6,2 with i j. Now, we associate each matrix X r from Case 2 with three other matrices X r,2, X r,3, and X r,4 in P L6,2 as determined in the following manner: X r,2 : From each nonzero A ij, select the entry horizontally adjacent to the entry that was selected to be in X r. X r,3 : From each nonzero A ij, select the entry vertically adjacent to the entry that was selected to be in X r. X r,4 : From each nonzero A ij, select the entry diagonal to the entry that was selected to be in X r. Then we have 16 disjoint sets of the form {X r, X r,2, X r,3, X r,4 } such the matrices in each set sum up to L 6,2. For example, the following four matrices in P L6,2 constitute a set in the partition: X r = , X r,2 = , X r,3 = , X r,4 = None of the 64 matrices partitioned into sets of the form {X r, X r,2, X r,3, X r,4 } were previously used up in sets of the form σw, because all matrices belonging to sets of the form σw in the partition have either two or zero entries from each nonzero A ij in L 6,2. We thus have a perfect partition of P L6,2. We close the paper with some general questions. Problem 4.8 For which n n (0,1)-matrices A does P A have a perfect partition? Note that such matrices A must be regular, and if k is the constant row and column sum, k must be a factor of the permanent of A. In addition, the permanental minors of the 1 s of A are constant. 10

11 Problem 4.9 Determine a good upper bound on the chromatic number χ(g A ) of the permutation graph of a regular matrix A. More specifically, find a constant c n such that per(a) χ(g A ) c n. k An even more general problem is the following. Problem 4.10 Let P = {P i : i I} be a set of permutation matrices of order n. Let A be a multiset of (0, 1)-matrices of order n. When is there a partition of I into sets I 1, I 2,..., I m such that the matrices {P j : j I i }, (i = 1, 2,..., m), are the matrices in A, including multiplicities? The problem discussed in this paper concerns sets of permutation matrices P A where A is a (0, 1)-matrix and A is the multiset consisting of A with a certain multiplicity. References [1] R.A. Brualdi and H.J. Ryser, Combinatorial Matrix Theory, Cambridge, [2] J. Folkman and D.R. Fulkerson, Edge colorings in bipartite graphs, Combinatorial Mathematics and Their Applications (R.C. Bose and T. Dowling, eds.), University of North Carolina Press, Chapel Hill,

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