# Sign pattern matrices that admit M, N, P or inverse M matrices

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1 Sign pattern matrices that admit M, N, P or inverse M matrices C Mendes Araújo, Juan R Torregrosa CMAT - Centro de Matemática / Dpto de Matemática Aplicada Universidade do Minho / Universidad Politécnica de Valencia Portugal / Spain / Keywords: Sign pattern matrix, N matrix, P matrix, M matrix, inverse M matrix AMS classification: 15A15 (primary), 15A48 (secondary) Abstract In this paper we identify the sign pattern matrices that occur among the N matrices, the P matrices and the M matrices We also address to the class of inverse M matrices and the related admissibility of sign pattern matrices problem 1 Introduction In qualitative and combinatorial matrix theory, a methodology based on the use of combinatorial information such as the signs of the elements of a matrix is very often useful in the study of some properties of matrices A matrix whose entries are chosen from the set {+,, 0} is called a sign pattern matrix A zero pattern is a sign pattern matrix whose entries are all Research supported by Spanish DGI grant number MTM Research supported by the Portuguese Foundation for Science and Technology (FCT) through the research program POCTI Corresponding author 1

2 equal to 0 Given an n m real matrix A = (a ij ), we denote by sign(a) the sign pattern matrix obtained from A by replacing each one of its positive entries by + and each one of its negative entries by For an n m sign pattern matrix P, we define the sign pattern class C(P ) by C(P ) = { A R n n : sign(a) = P } A permutation pattern is simply a sign pattern matrix with exactly one entry in each row and column equal to +, and the remaining entries equal to 0 A product of the form S T P S, where S is a square permutation pattern and P is a sign pattern matrix of the same order as S, is called a permutation similarity A square sign pattern matrix whose entries lying outside the main diagonal are equal to zero is called a diagonal pattern, and a product of the form DP D, where D is a diagonal pattern with no zero entries in the main diagonal and P is a sign pattern matrix of the same order as D, is called a diagonal similarity Note that S T P S and DP D are again sign pattern matrices A sign pattern matrix P is said to require a certain property P referring to real matrices if all real matrices in C(P ) have the property P, and is said to allow that property P if some real matrix in C(P ) has the property P In the literature, one can find, in the last few years, an increasing interest in problems that arise from the basic question of whether a certain sign pattern matrix requires (or allows) a certain property (see, for instance, [2], [4], [5]) In this paper, we shall consider certain classes of real matrices, namely the class of N matrices, the class of P matrices, the class of M matrices and, finally, the class of inverse M matrices Our aim is to determine which sign pattern matrices are admissible for each one of these classes of real matrices In other words, we shall focus on the question which sign pattern matrices allow the property of belonging to the class of N matrices (respectively, P matrices, M matrices, inverse M matrices)? An n n real matrix A is called an N matrix if all its principal minors are negative while A is said to be a P matrix if all its principal minors are positive The class of P matrices generalizes many important classes of matrices, such as the M matrices and inverse M matrices Denote by Z n the set of all square real matrices of order n whose off-diagonal entries are non-positive A matrix A is called an M matrix if A Z n and A is 2

3 positive stable Throughout this paper, M matrices are not allowed to be singular A nonsingular matrix A is said to be an inverse M-matrix if A 1 is an M matrix As in the case for P matrices, there are many different equivalent conditions for a matrix to be an M matrix We shall use the following: a matrix A Z n is an M matrix if and only if A is a P matrix or, equivalently, A is nonsingular and A 1 0 ([1], Chapter 2) We say that P is a Z n sign pattern matrix if P = sign(a) for some A Z n We write A > 0 if A is entrywise positive S, Recall that an n n matrix A is reducible if for some permutation matrix [ ] SAS T B C =, 0 D where B and D are square matrices, or if n = 1 and A = 0 Otherwise, A is irreducible We define reducible and irreducible sign pattern matrices in a natural way: an n n sign pattern matrix P is reducible if all matrices A C(P ) are reducible, and irreducible otherwise An n n sign pattern matrix P = (p ij ) is said to be transitive if for any set of distinct vertices {i 1, i 2,, i k } {1, 2,, n}, the following condition holds: p i1 i 2, p i2 i 3,, p ik 2 i k 1, p ik 1 i k 0 p i1 i k 0 We denote by J n m the n m matrix whose components are all equal to 1 Observe that J k m J m n = mj k n for all positive integers k, m, n For an n n matrix A, the submatrix of A lying in rows α and columns β, α, β {1,, n}, is denoted by A[α β], and the principal submatrix A[α α] is abbreviated to A[α] Then, a real n n matrix A is an N matrix if det A [α] < 0, for all α {1,, n}, and a P matrix if det A [α] > 0, for all α {1,, n} In [7], the authors show that any N matrix is diagonally similar to an N matrix in S n, where S n = { A = (a ij ) sign(a ij ) = ( 1) i+j+1, for all i, j {1,, n} } 3

4 This class has as its canonical representative A n = (a ij ) with a ij = ( 1) i+j+1 for all i, j {1,, n} We say that P is an S n sign pattern matrix if P = sign(a n ) 2 Sign pattern matrices that admit N matrices In the next result, which, as we will see, can be derived from the fact mentioned above, we characterize all of the admissible sign pattern matrices for the class of N matrices Theorem 21 Let P be a sign pattern matrix There exists a matrix A in the class C(P ) such that A is an N matrix if and only if P is diagonally similar to an S n sign pattern matrix Proof Suppose that P is an n n sign pattern matrix such that there exists an N matrix A in C(P ) We know that sign(a) = P and that A is diagonally similar to an N matrix B S n Therefore, we can assert that there exists a diagonal matrix D such that B = DAD 1 Let E = sign(d) It is obvious that sign(b) = EP E Observe that Q = EP E is the S n sign pattern matrix and that it is diagonally similar to P Conversely, assume that P is diagonally similar to the S n sign pattern matrix Q and consider the following matrix B B = 1 a a 2 ( 1) n a n 1 a 1 a ( 1) n 1 a n 2 a 2 a 1 ( 1) n 2 a n 3 ( 1) n a n 1 ( 1) n 1 a n 2 ( 1) n 2 a n 3 1 in S n, where a > 1 It is not difficult to prove, by applying Proposition 32 of [7], that B is in fact an N matrix Note that sign(b) = Q Let E be a diagonal pattern such that P = EQE and let D be the diagonal matrix whose diagonal entries are equal to 1 or 1 and sign(d) = E Since sign(dbd 1 ) = P and since the class of N matrices is invariant under diagonal similarity, we can conclude that A = DBD 1 is an N matrix that belongs to the class C(P ) 4

5 Taking into account this result and the consequent characterization of the admissible sign pattern matrices for the class of N matrices, the natural question that arises now is whether we are able to give a conclusive answer to similar problems referring to classes of matrices defined by means of the signs of principal minors and of the signs of the entries of the matrices 3 Sign pattern matrices that admit P and M matrices In this section we derive a characterization of the admissible sign pattern matrices for the class of P matrices and also for its subclass of M matrices Theorem 31 Let P = (p ij ) be an n n sign pattern matrix There exists a P matrix A in C(P ) if and only if p ii = + for all i {1,, n} Proof Let P = (p ij ) be an n n sign pattern matrix such that there exists a P matrix A in C(P ) We know that sign(a) = P and that all the principal minors of A are positive In particular, det A[{i}] > 0 for all i {1,, n}, which allows us to conclude that p ii = + Conversely, assume that P = (p ij ) is a square sign pattern matrix of order n with p ii = +, i = 1,, n For each ε > 0 consider the n n matrix A ε = (a ij ) given by a ij = 1 if i = j 0 if i j and p ij = 0 ε if i j and p ij = + ε if i j and p ij = It is obvious that A ε belongs to the sign pattern class C(P ) {1,, n} Note that det A ε [α] = 1 + p α (ε), Let α where p α (ε) is the null polynomial or a polynomial on ε with null constant term Since p α (ε) tends to zero as ε tends to zero, for sufficiently small values of ε, A ε is a P matrix belonging to the class C(P ) From the preceding result and the previously mentioned characterization of M matrices, one can easily describe the admissible sign pattern matrices for the class of M matrices: 5

6 Theorem 32 Let P = (p ij ) be an n n sign pattern matrix Then, there is an M matrix A in C(P ) if and only if P is a Z n sign pattern matrix and p ii = + for all i {1,, n} 4 Sign pattern matrices that occur among inverse M matrices In this section, necessary conditions for the admissibility for inverse M matrices are presented and some partial results related to the characterization of the admissible sign pattern matrices are achieved for this particular class of matrices We shall now focus on the class of inverse M matrices Note that if a nonsingular matrix A has nonnegative entries, A 1 Z n and A 1 has positive main diagonal entries, then A is an inverse M matrix Obviously, if a sign pattern matrix allows the property of belonging to the class of inverse M matrices, it must be nonnegative The following lemma will be useful for the study of the admissible sign pattern matrices for this class of matrices Lemma 41 Given 0 < a < 1, the following n n matrix 1 a a a a a 1 a a a a a 1 a a A = a a a 1 a a a a a 1 is an inverse M matrix Proof Observe that A = (1 a)i n + aj n n Setting B = ci n + dj n n, it is a simple computation to show that AB = I n, and hence that B = A 1, if and only if c = (1 a) 1 ac and d = (1 a + an) Given a real number a, denote by C n,a the n n matrix whose main diagonal entries are equal to 1 and the remaining entries are all equal to a 6

7 Simple calculations yield that C n,a J n m = (1 + (n 1)a)J n m, J n m C m,a = (1 + (m 1)a)J n m, and hence, Cn,aJ 1 n m = (1 + (n 1)a) 1 J n m and J n m Cm,a 1 = (1 + (m 1)a) 1 J n m for all a R and n, m N It is well known that an irreducible matrix A Z n is an M matrix if and only if A 1 > 0 (see [1]) The next result follows from this fact Theorem 42 Let P = (p ij ) be an n n nonnegative irreducible sign pattern matrix Then, there is an inverse M matrix A in C(P ) if and only if P is positive Proof Let us assume that there exists an inverse M matrix A in C(P ) Since P is irreducible, A is an irreducible matrix and, consequently, so is A 1 By the result mentioned above, A > 0 and, therefore, P is positive Conversely, assume P is positive Consider 0 < a < 1 and the inverse M matrix A = C n,a This matrix belongs to C(P ) Using the definition of a transitive sign pattern matrix, we can restate a result due to Lewin and Neumann Theorem 43 ([6]) If A is an inverse M matrix, sign(a) is transitive This particular theorem will be very useful in the upcoming results addressing the characterization of the admissible reducible sign-pattern matrices for the class of inverse M matrices It is well known that any matrix is permutation similar to a block triangular matrix with irreducible diagonal blocks, the Frobenius normal form Therefore, we can reduce the general case to the case of block upper triangular sign pattern matrices P = P 11 P 12 P 1k 0 P 22 P 2k 0 0 P kk whose diagonal blocks P 11, P 22,, P kk are irreducible sign pattern matrices These diagonal blocks are the so called irreducible components of P 7

8 Corollary 44 Let P = (p ij ) be an n n sign pattern matrix of the form P 11 P 12 P 1k 0 P P = 22 P 2k 0 0 P kk where P 11, P 22,, P kk are irreducible sign pattern matrices of sizes q 1 q 1, q 2 q 2,, q k q k, respectively If there exists an inverse M matrix A in C(P ), then P 11, P 22,, P kk are positive, and each one of the blocks P ij, i j, is either positive or the zero pattern Proof Let A = A 11 A 12 A 1k 0 A 22 A 2k 0 0 A kk be an inverse M matrix in C(P ) Since A 11, A 22,, A kk are also inverse M matrices, it follows from Theorem 42 that P 11, P 22,, P kk are positive By applying Lewin and Neumann s result, we know that P is transitive Suppose P ij (i j) is not the zero pattern It is clear, than, that it has at least one positive component p rs Note that the first row of P ij corresponds to the (q q i 1 + 1) th row of P (take q 0 = 0), while the last row of P ij corresponds to the (q q i 1 + q i ) th row of P Moreover, the first column of P ij corresponds to the (q q j 1 + 1) th column of P and the last column of P ij corresponds to the (q q j 1 + q j ) th row of P Hence, r {q q i 1 + 1,, q q i 1 + q i } and s {q 1 + +q j 1 +1,, q 1 + +q j 1 +q j } Let p tu be any component of P ij different from p rs Clearly, t {q 1 + +q i 1 +1,, q 1 + +q i 1 +q i }, u {q q j 1 + 1,, q q j 1 + q j } and t r or u s Observe that p tr is an element of P ii and p su is a component of P jj We have p tr = p su = + since P ii and P jj are positive Therefore, p tr, p rs, p su 0 and, by the transitivity of P, p tu 0 Then, P ij is positive The converse of the previous result is also true for k = 2 8

9 Lemma 45 Any n n sign pattern matrix P of the form [ ] P 11 P 12 P =, 0 P 22 where P 11 and P 22 are positive sign pattern matrices of size q q and (n q) (n q), respectively, and P 12 is either positive or the zero pattern, is admissible for the class of inverse M matrices Proof If P 12 = 0, it is easy to prove, using Lemma 41 that [ ] C q,a 0 A =, 0 C n q,a with 0 < a < 1, is an inverse M matrix Note that A C(P ) If P 12 is positive, let A be the following matrix [ ] Given that A 1 = A = [ C 1 q,a C q,a aj q (n q) 0 C n q,a ac 1 0 Cn q,a 1 q,aj q (n q) Cn q,a 1 we only have to show that acq,aj 1 q (n q) Cn q,a 1 0 Recall that and Hence C 1 q,aj q (n q) = (1 + (q 1)a) 1 J q (n q) J q (n q) C 1 n q,a = (1 + (n q 1)a) 1 J q (n q) acq,aj 1 q (n q) Cn q,a 1 = a (1 + (q 1)a)(1 + (n q 1)a) J q (n q) and, therefore, A is an inverse M matrix in C(P ) Next we show that when P has more than two irreducible components and there are no zero patterns except for those below the principal block diagonal, P is also admissible for the class of inverse M matrices ], 9

10 Proposition 46 Let P be an n n sign pattern matrix of the form P 11 P 12 P 1k 0 P P = 22 P 2k 0 0 P kk where P 11, P 22,, P kk are sign patterns matrices of sizes q 1 q 1, q 2 q 2,, q k q k, respectively If P ij is positive for all j i, then there exists an inverse M matrix A in C(P ) Proof For 0 < a < 1, let A = A 11 A 12 A 1k 0 A 22 A 2k 0 0 A kk be the matrix defined by A ii = C qi,a (i = 1,, k) and A ij = aj qi q j (j > i) We claim that A 1 = (B ij ) with B ii = A 1 ii and, for j > i, B ij = a(1 a) j i 1 (q i a + (1 a)) (q i+1 a + (1 a)) (q j a + (1 a)) J q i q j From Lemma 41 it follows that each B ii has negative off-diagonal entries and positive diagonal entries If each B ij, j > i, is as described above, it is obvious that A is an inverse M matrix The proof follows by induction on the number k of diagonal blocks The case k = 2 is studied in the proof of the previous lemma Suppose, now, that, given k > 2 the result is valid for k 1 Apply this to submatrices A[{1,, n q k }] and A[{q 1 + 1,, n}] We only have to show, then, that B 1k is of the referred form Note that B 1k = B 11 A 1k A 1 kk B 12A 2k A 1 kk B 13A 3k A 1 kk B 1k 1A k 1k A 1 kk Recall B 11 = A 1 11 and B 1j = a(1 a) j 2 (q 1 a + (1 a)) (q 2 a + (1 a)) (q j a + (1 a)) J q 1 q j, 10

11 for all j {2,, k 1} Hence, B 1k is given by the expression A 1 11 A 1kA 1 kk a (q 1 a + (1 a)) (q 2 a + (1 a)) J a q 1 q 2 (q k a + (1 a)) J q 2 q k a(1 a) (q 1 a + (1 a)) (q 2 a + (1 a)) (q 3 a + (1 a)) J a q 1 q 3 (q k a + (1 a)) J q 3 q k a(1 a) k 3 (q 1 a + (1 a)) (q 2 a + (1 a)) (q k 1 a + (1 a)) J q 1 q k 1 a (q k a + (1 a)) J q k 1 q k By taking M = (q 1 a + (1 a)) (q 2 a + (1 a)) (q k 1 a + (1 a)) (q k a + (1 a)), we get B 1k = ( a (q 2 a + (1 a)) (q k 1 a + (1 a)) + +a 2 q 2 (q 3 a + (1 a)) (q k 1 a + (1 a)) + +a 2 (1 a)q 3 (q 4 a + (1 a)) (q k 1 a + (1 a)) + + +a 2 (1 a) k 3 q k 1 )M 1 J q1 q k Let N be the real scalar which in the equality above is right hand side multiplied by M 1 J q1 q k Observe that N = a 2 q 2 (q 3 a + (1 a)) (q k 1 a + (1 a)) a(1 a) (q 3 a + (1 a)) (q k 1 a + (1 a)) + +a 2 q 2 (q 3 a + (1 a)) (q k 1 a + (1 a)) + +a 2 (1 a)q 3 (q 4 a + (1 a)) (q k 1 a + (1 a)) + + +a 2 (1 a) k 3 q k 1 = a(1 a) (q 3 a + (1 a)) (q k 1 a + (1 a)) + +a 2 (1 a)q 3 (q 4 a + (1 a)) (q k 1 a + (1 a)) + + +a 2 (1 a) k 3 q k 1 = a 2 (1 a)q 3 (q 4 a + (1 a)) (q k 1 a + (1 a)) a(1 a) 2 (q 4 a + (1 a)) (q k 1 a + (1 a)) + +a 2 (1 a)q 3 (q 4 a + (1 a)) (q k 1 a + (1 a)) + + +a 2 (1 a) k 3 q k 1 11

12 = a(1 a) 2 (q 4 a + (1 a)) (q k 1 a + (1 a)) + = +a 2 (1 a) 2 q 4 (q 5 a + (1 a)) (q k 1 a + (1 a)) + + +a 2 (1 a) k 3 q k 1 = a(1 a) k 2, which concludes the proof We end this section with the following remark Remark: The irreducible sign pattern matrices that occur among the inverse M matrices are described in Theorem 42 Since every matrix is permutation similar to a block triangular matrix with irreducible diagonal blocks, the general question is whether sign pattern matrices of the form P 11 P 12 P 1k 0 P P = 22 P 2k, 0 0 P kk whose diagonal blocks P 11, P 22,, P kk are the irreducible components of P, are admissible for the class of inverse M matrices Corollary 44 states that P ii > 0, for all choices of i, and P ij = 0 or P ij > 0, for all j > i, is a necessary condition for the admissibility of such sign pattern matrices P We strongly believe that this is also a sufficient condition When k = 2 or when all P ij > 0, the result follows from Lemma 45 and from Proposition 46 The question that remains with no answer is whether there exists an inverse M matrix in the class C(P ) where P is a sign pattern matrix as described above, with P ii > 0, for all choices of i, P ij = 0 or P ij > 0, for all j > i, and at least one of these latter blocks is a zero pattern Observe that if P ij is a zero pattern, the transitivity of P may imply that some other blocks, above the principal block diagonal, are also zero patterns It is, however, apparently hard to describe all the possibilities and, consequently, to achieve a conclusive answer Nevertheless, it is clear that those patterns that can be expressed as direct sums of patterns where all of the blocks above the diagonal are positive work as well 12

13 Acknowledgments The authors would like to thank the referee for the valuable comments, for the simpler proof of Lemma 41 and for the suggestions to improve the readability of the paper References [1] A Berman and RJ Plemmons, Nonnegative matrices in the Mathematical Sciences, SIAM (1994) [2] M Fiedler and R Grone, Characterizations of sign patterns of inversepositive matrices, Linear Algebra and Appl, 40: (1981) [3] RA Horn and CR Johnson, Matrix Analysis, Cambridge University Press, New York (1955) [4] CR Johnson, Sign patterns of inverse nonnegative matrices, Linear Algebra and Appl, 55:69 80 (1983) [5] CR Johnson, FT Leighton and HA Robinson, Sign patterns of inverse-positive matrices, Linear Algebra and Appl, 24:75 83 (1979) [6] M Lewin and M Neumann, On the inverse M matrix problem for (0, 1) matrices, Linear Algebra and Appl, 30:41 50 (1980) [7] C Mendes Araújo, JR Torregrosa and AM Urbano, N-matrix completion problem, Linear Algebra and Appl, 372: (2003) 13

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