共查询到20条相似文献,搜索用时 15 毫秒
1.
《Linear and Multilinear Algebra》2007,55(6):521-533
For each k≥ 0, those nonsingular matrices that transform the set of totally nonzero vectors with k sign variations into (respectively, onto) itself are studied. Necessary and sufficient conditions are provided. The cases k=0,1,2,n-3,n-2,n-1 are completely characterized. 相似文献
2.
Raphael Loewy 《Linear and Multilinear Algebra》2001,48(4):355-382
Let k and n be positive integers such that k≤n. Let Sn(F) denote the space of all n×n symmetric matrices over the field F with char F≠2. A subspace L of Sn(F) is said to be a k-subspace if rank A≤k for every AεL.
Now suppose that k is even, and write k=2r. We say a k∥-subspace of Sn(F) is decomposable if there exists in Fn a subspace W of dimension n-r such that xtAx=0 for every xεWAεL.
We show here, under some mild assumptions on kn and F, that every k∥-subspace of Sn(F) of sufficiently large dimension must be decomposable. This is an analogue of a result obtained by Atkinson and Lloyd for corresponding subspaces of Fm,n. 相似文献
Now suppose that k is even, and write k=2r. We say a k∥-subspace of Sn(F) is decomposable if there exists in Fn a subspace W of dimension n-r such that xtAx=0 for every xεWAεL.
We show here, under some mild assumptions on kn and F, that every k∥-subspace of Sn(F) of sufficiently large dimension must be decomposable. This is an analogue of a result obtained by Atkinson and Lloyd for corresponding subspaces of Fm,n. 相似文献
3.
Juan M. Gracia Inmaculadade Hoyos Franscisco E. Velasco 《Linear and Multilinear Algebra》1999,46(1):25-49
We give safety neighbourhoods for the necessary conditions in the change of the Jordan canonical form of a matrix under small perturbations. We also obtain the minimum distance from an n × n complex matrix which has less than k nonconstant invariant factors (2≤ k≤ n) to the set of matrices which have more or equal to k. When k= 2, we get in particular the distance from a nonderogatory matrix to the set of derogatory matrices. 相似文献
4.
Chi-Kwong Li 《Linear and Multilinear Algebra》1987,21(1):63-73
A linear operator T on a matrix space is said to be unital if T(I) = I. In this note we characterize the unital lineart operators on matrix spaces that preserve the k-numerical radius. Using the results obtained we easily determine the structure of all linear operators on the space of n × n complex matrices that preserve the k-numerical range. This completes the work of Pierce and Watking, who obtained the results for the cases when n ≠ n2k. 相似文献
5.
R. Craigen 《Linear and Multilinear Algebra》1991,29(2):91-92
It is shown that any m×n±1 matrix may be embedded in a Hadamard matrix of order kl, where k and l are the least orders greater than or equal to m and nrespectively in which Hadamard matrices exist. 相似文献
6.
Let us denote ab=max(a,b) and ab=a+b for
and extend this pair of operations to matrices and vectors in the same way as in linear algebra. We present an O(n2(m+n log n)) algorithm for finding all essential terms of the max-algebraic characteristic polynomial of an n×n matrix over
with m finite elements. In the cases when all terms are essential, this algorithm also solves the following problem: Given an n×n matrix A and k{1,…,n}, find a k×k principal submatrix of A whose assignment problem value is maximum. 相似文献
7.
Steve Kirkland 《Linear and Multilinear Algebra》1993,34(3):343-351
Given a tournament matrix T, its reversal indexiR(T), is the minimum k such that the reversal of the orientation of k arcs in the directed graph associated with T results in a reducible matrix. We give a formula for iR(T) in terms of the score vector of T which generalizes a simple criterion for a tournament matrix to be irreducible. We show that iR(T)≤[(n-1)/2] for any tournament matrix T of order n, with equality holding if and only if T is regular or almost regular, according as n is odd or even. We construct, for each k between 1 and [(n-1)/2], a tournament matrix of order n whose reversal index is k. Finally, we suggest a few problems. 相似文献
8.
Some Ore-type Results for Matching and Perfect Matching in <Emphasis Type="Italic">k</Emphasis>-uniform Hypergraphs 下载免费PDF全文
Let S1 and S2 be two (k-1)-subsets in a k-uniform hypergraph H. We call S1 and S2 strongly or middle or weakly independent if H does not contain an edge e ∈ E(H) such that S1 ∩ e ≠∅ and S2 ∩ e ≠∅ or e ⊆ S1 ∪ S2 or e ⊇ S1 ∪ S2, respectively. In this paper, we obtain the following results concerning these three independence. (1) For any n ≥ 2k2-k and k ≥ 3, there exists an n-vertex k-uniform hypergraph, which has degree sum of any two strongly independent (k-1)-sets equal to 2n-4(k-1), contains no perfect matching; (2) Let d ≥ 1 be an integer and H be a k-uniform hypergraph of order n ≥ kd+(k-2)k. If the degree sum of any two middle independent (k-1)-subsets is larger than 2(d-1), then H contains a d-matching; (3) For all k ≥ 3 and sufficiently large n divisible by k, we completely determine the minimum degree sum of two weakly independent (k-1)-subsets that ensures a perfect matching in a k-uniform hypergraph H of order n. 相似文献
9.
EVA Achilles 《Linear and Multilinear Algebra》1977,5(1):63-70
The doubly stochastic matrices with a given zero pattern which are closest in Euclidean norm to Jnn, the matrix with each entry equal to 1/n, are identified. If the permanent is restricted to matrices having a given zero pattern confined to one row or to one column, the permanent achieves a local minimum at those matrices with that zero pattern which are closest to Jnn. This need no longer be true if the zeros lie in more than one row or column. 相似文献
10.
The least eigenvalue of a connected graph is the least eigenvalue of its adjacency matrix. We characterize the connected graphs of order n and size n + k (5≤k≤8 and n>k + 5) with the minimal least eigenvalue. 相似文献
11.
T. H. Pate 《Linear and Multilinear Algebra》1981,10(2):103-105
Let A be an nk × nk positive semi-definite symmetric matrix partitioned into blocks Aij each of which is an n × n matrix. In [2] Mine states a conjecture of Marcus that per(A) ≥ per(G) where G is the k × k matrix [per(Aij)]. In this paper we prove a weaker inequality namely that per(A) ≥ (k!)-1per(G). 相似文献
12.
L. N. Vaserstein 《Linear and Multilinear Algebra》1987,21(3):261-270
A theorem of Lagrange says that every natural number is the sum of 4 squares. M. Newman proved that every integral n by n matrix is the sum of 8 (-1)n squares when n is at least 2. He asked to generalize this to the rings of integers of algebraic number fields. We show that an n by n matrix over a a commutative R with 1 is the sum of squares if and only if its trace reduced modulo 2Ris a square in the ring R/2R. It this is the case (and n is at least 2), then the matrix is the sum of 6 squares (5 squares would do when n is even). Moreover, we obtain a similar result for an arbitrary ring R with 1. Answering another question of M. Newman, we show that every integral n by n matrix is the sum of ten k-th powers for all sufficiently large n. (depending on k). 相似文献
13.
Benjamin Doerr 《Discrete Mathematics》2002,250(1-3):63-70
In this article, we investigate the interrelation between the discrepancies of a given hypergraph in different numbers of colors. Being an extreme example we determine the multi-color discrepancies of the k-balanced hypergraph
on partition classes of (equal) size n. Let
. Set k0 k mod c and bnkc (n−n/c/k)k/c. For the discrepancy in c colors we showif k0≠0, and
, if c divides k. This shows that, in general, there is little correlation between the discrepancies of
in different numbers of colors. If c divides k though,
holds for any hypergraph
. 相似文献
14.
Byeong Moon Kim Byung Chul Song Woonjae Hwang 《Linear algebra and its applications》2007,420(2-3):648-662
A graph G = (V, E) on n vertices is primitive if there is a positive integer k such that for each pair of vertices u, v of G, there is a walk of length k from u to v. The minimum value of such an integer, k, is the exponent, exp(G), of G. In this paper, we find the minimum number, h(n, k), of edges of a simple graph G on n vertices with exponent k, and we characterize all graphs which have h(n, k) edges when k is 3 or even. 相似文献
15.
A k-connected graph G is said to be critically k-connected if G−v is not k-connected for any vV(G). We show that if n,k are integers with k4 and nk+2, and G is a critically k-connected graph of order n, then |E(G)|n(n−1)/2−p(n−k)+p2/2, where p=(n/k)+1 if n/k is an odd integer and p=n/k otherwise. We also characterize extremal graphs. 相似文献
16.
W. D. Wallis 《Linear and Multilinear Algebra》1986,19(4):387-388
It is shown that if A is any n×n matrix of zeros and ones, and if k is the smallest number not less than n which is the order of an Hadamard matrix, then A is a submatrix of an Hadamard matrix of order k2. 相似文献
17.
We show that a matrix is similar to a symmetric matrix over a field of characteristic 2 if and only if the minimum polynomial of the matrix is not the product of distinct irreducible polynomials whose splitting fields are inseparable extensions. When the field is not of characteristic 2, a known theorem is generalized by considering k, the number of elementary divisors of odd degree of the n × n A: If -1 is a sum of 2v squares and n differs from a multiple of 2v + 1 by at most ±k, then A is similar to a symmetric matrix. 相似文献
18.
Roy Meshulam 《Linear and Multilinear Algebra》1990,26(1):39-41
It is shown that if W is a linear subspace of real n × n matrices, such that rank (A) = k for all 0 ≠ A ∈ W, then dim W≤ n. If dim W = n.5≤ n is prime, and 2 is primitive modulo n then k =1. 相似文献
19.
Tams Lengyel 《Discrete Mathematics》1996,150(1-3):281-292
We partially characterize the rational numbers x and integers n 0 for which the sum ∑k=0∞ knxk assumes integers. We prove that if ∑k=0∞ knxk is an integer for x = 1 − a/b with a, b> 0 integers and gcd(a,b) = 1, then a = 1 or 2. Partial results and conjectures are given which indicate for which b and n it is an integer if a = 2. The proof is based on lower bounds on the multiplicities of factors of the Stirling number of the second kind, S(n,k). More specifically, we obtain
for all integers k, 2 k n, and a 3, provided a is odd or divisible by 4, where va(m) denotes the exponent of the highest power of a which divides m, for m and a> 1 integers.
New identities are also derived for the Stirling numbers, e.g., we show that ∑k=02nk! S(2n, k) , for all integers n 1. 相似文献
20.
Let Akbe the group of isometries of the space of n-by-n matrices over reals (resp. complexes, quaternions) with respect to the Ky Fan k-norm (see the Introduction for the definitions). Let Γ0 be the group of transformations of this space consisting of all products of left and right multiplications by the elements of SO(n)(resp. U(n), Sp(n)). It is shown that, except for three particular casesAk coincides with the normalizer of Γ in Δ group of isometries of the above matrix space with respect to the standard inner product. We also give an alternative treatment of the case D = Rn = 4k = 2 which was studied in detail by Johnson, Laffey, and Li [4]. 相似文献