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Let F be an infinite field with characteristic not equal to two. For a graph G=(V,E) with V={1,…,n}, let S(G;F) be the set of all symmetric n×n matrices A=[ai,j] over F with ai,j≠0, i≠j if and only if ij∈E. We show that if G is the complement of a partial k -tree and m?k+2, then for all nonsingular symmetric m×m matrices K over F, there exists an m×n matrix U such that UTKU∈S(G;F). As a corollary we obtain that, if k+2?m?n and G is the complement of a partial k-tree, then for any two nonnegative integers p and q with p+q=m, there exists a matrix in S(G;R) with p positive and q negative eigenvalues. 相似文献
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Let L be an n×n matrix with zero row and column sums, n?3. We obtain a formula for any minor of the (n−2)-th compound of L. An application to counting spanning trees extending a given forest is given. 相似文献
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This paper is devoted to a problem of finding the smallest positive integer s(m,n,k) such that (m+1) generic skew-symmetric (k+1)-forms in (n+1) variables as linear combinations of the same s(m,n,k) decomposable skew-symmetric (k+1)-forms. 相似文献
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For any n-by-n matrix A , we consider the maximum number k=k(A) for which there is a k-by-k compression of A with all its diagonal entries in the boundary ∂W(A) of the numerical range W(A) of A. If A is a normal or a quadratic matrix, then the exact value of k(A) can be computed. For a matrix A of the form B⊕C, we show that k(A)=2 if and only if the numerical range of one summand, say, B is contained in the interior of the numerical range of the other summand C and k(C)=2. For an irreducible matrix A , we can determine exactly when the value of k(A) equals the size of A . These are then applied to determine k(A) for a reducible matrix A of size 4 in terms of the shape of W(A). 相似文献
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In this paper, we give some necessary and sufficient conditions for the existence of Re-nnd and nonnegative definite {1,3}- and {1,4}-inverses of a matrix A∈Cn×n and completely described these sets. Moreover, we prove that the existence of nonnegative definite {1,3}-inverse of a matrix A is equivalent with the existence of its nonnegative definite {1,2,3}-inverse and present the necessary and sufficient conditions for the existence of Re-nnd {1,3,4}-inverse of A. 相似文献
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M. Gürdal 《Expositiones Mathematicae》2009,27(2):153-160
In the present paper we consider the Volterra integration operator V on the Wiener algebra W(D) of analytic functions on the unit disc D of the complex plane C. A complex number λ is called an extended eigenvalue of V if there exists a nonzero operator A satisfying the equation AV=λVA. We prove that the set of all extended eigenvalues of V is precisely the set C?{0}, and describe in terms of Duhamel operators and composition operators the set of corresponding extended eigenvectors of V. The similar result for some weighted shift operator on ?p spaces is also obtained. 相似文献
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An ACI-matrix over a field F is a matrix whose entries are polynomials with coefficients on F, the degree of these polynomials is at most one in a number of indeterminates, and where no indeterminate appears in two different columns. In 2011 Huang and Zhan characterized the m×n ACI-matrices such that all its completions have rank equal to min{m,n} whenever |F|?max{m,n+1}. We will give a characterization for arbitrary fields by introducing two classes of ACI-matrices: the maximal and the minimal full rank ACI-matrices. 相似文献