共查询到20条相似文献,搜索用时 750 毫秒
1.
Kyriakos Keremedis 《Mathematical Logic Quarterly》1999,45(1):95-104
We find topological characterizations of the pseudointersection number ?? and the tower number t of the real line and we show that ?? < t iff there exists a compact separable T2 space X of π-weight < ?? that can be covered by < t nowhere dense sets iff there exists a weak Hausdorff gap of size K < t, i. e., a pair ({A : i ≠ k}, {BJ : j ε K}) C [W]W X [U]W such that A = {Ai : i ε K} is a decreasing tower, B = {Bj : j ε K) is a family of pseudointersections of A, and there is no pseudointersection S of A meeting each member of B in an infinite set. 相似文献
2.
Sang-Gu Lee 《Linear algebra and its applications》2011,435(9):2097-2109
We investigate simultaneous solutions of the matrix Sylvester equations AiX-XBi=Ci,i=1,2,…,k, where {A1,…,Ak} and {B1,…,Bk} are k-tuples of commuting matrices of order m×m and p×p, respectively. We show that the matrix Sylvester equations have a unique solution X for every compatible k-tuple of m×p matrices {C1,…,Ck} if and only if the joint spectra σ(A1,…,Ak) and σ(B1,…,Bk) are disjoint. We discuss the connection between the simultaneous solutions of Sylvester equations and related questions about idempotent matrices separating disjoint subsets of the joint spectrum, spectral mapping for the differences of commuting k-tuples, and a characterization of the joint spectrum via simultaneous solutions of systems of linear equations. 相似文献
3.
J H Pathak 《Proceedings Mathematical Sciences》1989,99(3):217-220
LetA andB be two reduced commutative rings with finitely many minimal prime ideals. If the polynomial algebrasA[X
1
…X
n
]=B[Y
1
…Y
n
] whereX
i
,Y
iF are variables overA andB respectively, then there exists an injective ring homomorphism ϕ:A→B such thatB is finitely generated over ϕ(A). 相似文献
4.
V. V. Makeev 《Journal of Mathematical Sciences》2011,175(5):572-573
Let X be an affine cross-polytope, i.e., the convex hull of n segments A
1
B
1,…, A
n
B
n
in
\mathbbRn {\mathbb{R}^n} that have a common midpoint O and do not lie in a hyperplane. The affine flag F(X) of X is the chain O ∈ L
1 ⊂⋯ ⊂ L
n
=
\mathbbRn {\mathbb{R}^n} , where L
k
is the k-dimensional affine hull of the segments A
1
B
1,…, A
k
B
k
, k ≤ n. It is proved that each convex body K ⊂
\mathbbRn {\mathbb{R}^n} is circumscribed about an affine cross-polytope X such that the flag F(X) satisfies the following condition for each k ∈{2,…, n}:the (k−1)-planes of support at A
k
and B
k
to the body L
k
∩ K in the k-plane L
k
are parallel to L
k
−1.Each such X has volume at least V(K)/2
n(n−1)/2. Bibliography: 5 titles. 相似文献
5.
For positive integers t?k?v and λ we define a t-design, denoted Bi[k,λ;v], to be a pair (X,B) where X is a set of points and B is a family, (Bi:i?I), of subsets of X, called blocks, which satisfy the following conditions: (i) |X|=v, the order of the design, (ii) |Bi|=k for each i?I, and (iii) every t-subset of X is contained in precisely λ blocks. The purpose of this paper is to investigate the existence of 3-designs with 3?k?v?32 and λ>0.Wilson has shown that there exists a constant N(t, k, v) such that designs Bt[k,λ;v] exist provided λ>N(t,k,v) and λ satisfies the trivial necessary conditions. We show that N(3,k,v)=0 for most of the cases under consideration and we give a numerical upper bound on N(3, k, v) for all 3?k?v?32. We give explicit constructions for all the designs needed. 相似文献
6.
In the kernel clustering problem we are given a (large) n × n symmetric positive semidefinite matrix A = (aij) with \begin{align*}\sum_{i=1}^n\sum_{j=1}^n a_{ij}=0\end{align*} and a (small) k × k symmetric positive semidefinite matrix B = (bij). The goal is to find a partition {S1,…,Sk} of {1,…n} which maximizes \begin{align*}\sum_{i=1}^k\sum_{j=1}^k \left(\sum_{(p,q)\in S_i\times S_j}a_{pq}\right)b_{ij}\end{align*}. We design a polynomial time approximation algorithm that achieves an approximation ratio of \begin{align*}\frac{R(B)^2}{C(B)}\end{align*}, where R(B) and C(B) are geometric parameters that depend only on the matrix B, defined as follows: if bij = 〈vi,vj〉 is the Gram matrix representation of B for some \begin{align*}v_1,\ldots,v_k\in \mathbb{R}^k\end{align*} then R(B) is the minimum radius of a Euclidean ball containing the points {v1,…,vk}. The parameter C(B) is defined as the maximum over all measurable partitions {A1,…,Ak} of \begin{align*}\mathbb{R}^{k-1}\end{align*} of the quantity \begin{align*}\sum_{i=1}^k\sum_{j=1}^k b_{ij}\langle z_i,z_j\rangle\end{align*}, where for i∈{1,…,k} the vector \begin{align*}z_i\in \mathbb{R}^{k-1}\end{align*} is the Gaussian moment of Ai, i.e., \begin{align*}z_i=\frac{1}{(2\pi)^{(k-1)/2}}\int_{A_i}xe^{-\|x\|_2^2/2}dx\end{align*}. We also show that for every ε > 0, achieving an approximation guarantee of \begin{align*}(1-\varepsilon)\frac{R(B)^2}{C(B)}\end{align*} is Unique Games hard. © 2012 Wiley Periodicals, Inc. Random Struct. Alg., 2013 相似文献
7.
In this article we establish necessary and sufficient conditions for the existence and the expressions of the general real solutions to the classical system of quaternion matrix equations A 1 XB 1 = C 1, A 2 XB 2 = C 2. Moreover, formulas of the maximal and minimal ranks of four real matrices X 1, X 2, X 3, and X 4 in solution X = X 1 + X 2 i + X 3 j + X 4 k to the system mentioned above are derived. As applications, we give necessary and sufficient conditions for the quaternion matrix equations A 1 XB 1 = C 1, A 2 XB 2 = C 2, A 3 XB 3 = C 3 to have common real solutions. In addition, the maximal and minimal ranks of four real matrices E, F, G, and H in the common generalized inverse of A 1 + B 1 i + C 1 j + D 1 k and A 2 + B 2 i + C 2 j + D 2 k, which can be expressed as E + Fi + Gj + Hk are also presented. 相似文献
8.
Let X be a Banach space with closed unit ball B. Given k
, X is said to be k-β, respectively, (k + 1)-nearly uniformly convex ((k + 1)-NUC), if for every ε > 0 there exists δ, 0 < δ < 1, so that for every x B and every ε-separated sequence (xn) B there are indices (ni)ki = 1, respectively, (ni)k + 1i = 1, such that (1/(k + 1))||x + ∑ki = 1 xni|| ≤ 1 − δ, respectively, (1/(k + 1))||∑k + 1i = 1 xni|| ≤ 1 − δ. It is shown that a Banach space constructed by Schachermayer is 2-β, but is not isomorphic to any 2-NUC Banach space. Modifying this example, we also show that there is a 2-NUC Banach space which cannot be equivalently renormed to be 1-β. 相似文献
9.
An optimal bound on the tail distribution of the number of recurrences of an event in product spaces
Let X
1
,X
2
,... be independent random variables and a a positive real number. For the sake of illustration, suppose A is the event that |X
i+1
+...+X
j
|≥a for some integers 0≤i<j<∞. For each k≥2 we upper-bound the probability that A occurs k or more times, i.e. that A occurs on k or more disjoint intervals, in terms of P(A), the probability that A occurs at least once.
More generally, let X=(X
1
,X
2
,...)Ω=Π
j
≥1Ω
j
be a random element in a product probability space (Ω,ℬ,P=⊗
j
≥1
P
j
). We are interested in events AB that are (at most contable) unions of finite-dimensional cylinders. We term such sets sequentially searchable. Let L(A) denote the (random) number of disjoint intervals (i,j] such that the value of X
(i,j]
=(X
i+1
,...,X
j
) ensures that XA. By definition, for sequentially searchable A, P(A)≡P(L(A)≥1)=P(𝒩−ln
(P(Ac))
≥1), where 𝒩γ denotes a Poisson random variable with some parameter γ>0. Without further assumptions we prove that, if 0<P(A)<1, then P(L(A)≥k)<P(𝒩−ln
(P(Ac))
≥k) for all integers k≥2.
An application to sums of independent Banach space random elements in l
∞
is given showing how to extend our theorem to situations having dependent components.
Received: 8 June 2001 / Revised version: 30 October 2002 Published online: 15 April 2003
RID="*"
ID="*" Supported by NSF Grant DMS-99-72417.
RID="†"
ID="†" Supported by the Swedish Research Council.
Mathematics Subject Classification (2000): Primary 60E15, 60G50
Key words or phrases: Tail probability inequalities – Hoffmann-Jo rgensen inequality – Poisson bounds – Number of event recurrences – Number of
entrance times – Product spaces 相似文献
10.
L. Yu. Glebskii 《Mathematical Notes》1999,65(1):31-40
Theorems are proved establishing a relationship between the spectra of the linear operators of the formA+Ωg
iBigi
−1 andA+B
i, whereg
i∈G, andG is a group acting by linear isometric operators. It is assumed that the closed operatorsA andB
i possess the following property: ‖B
iA−1gBjA−1‖→0 asd(e,g)→∞. Hered is a left-invariant metric onG ande is the unit ofG. Moreover, the operatorA is invariant with respect to the action of the groupG. These theorems are applied to the proof of the existence of multicontour solutions of dynamical systems on lattices.
Translated fromMatematicheskie Zametki, Vol. 65, No. 1, pp. 37–47, January, 1999. 相似文献
11.
Wenliang Zhang 《Advances in Mathematics》2011,(1):575
Let X be a projective scheme over a field k and let A be the local ring at the vertex of the affine cone of X under some embedding . We prove that, when char(k)>0, the Lyubeznik numbers λi,j(A) are intrinsic numerical invariants of X, i.e., λi,j(A) depend only on X, but not on the embedding. 相似文献
12.
The chaos caused by a strong-mixing preserving transformation is discussed and it is shown that for a topological spaceX satisfying the second axiom of countability and for an outer measurem onX satisfying the conditions: (i) every non-empty open set ofX ism-measurable with positivem-measure; (ii) the restriction ofm on Borel σ-algebra ℬ(X) ofX is a probability measure, and (iii) for everyY⊂X there exists a Borel setB⊂ℬ(X) such thatB⊃Y andm(B) =m(Y), iff:X→X is a strong-mixing measure-preserving transformation of the probability space (X, ℬ(X),m), and if {m}, is a strictly increasing sequence of positive integers, then there exists a subsetC⊂X withm (C) = 1, finitely chaotic with respect to the sequence {m
i}, i.e. for any finite subsetA ofC and for any mapF:A→X there is a subsequencer
i such that limi→∞
f
r
i(a) =F(a) for anya ∈A. There are some applications to maps of one dimension.
the National Natural Science Foundation of China. 相似文献
13.
Thomas J. Laffey 《Linear algebra and its applications》1977,16(3):189-201
Let be the complex algebra generated by a pair of n × n Hermitian matrices A, B. A recent result of Watters states that A, B are simultaneously unitarily quasidiagonalizable [i.e., A and B are simultaneously unitarily similar to direct sums C1⊕…⊕Ct,D1⊕…⊕Dt for some t, where Ci, Di are ki × ki and ki?2(1?i?t)] if and only if [p(A, B), A]2 and [p(A, B), B]2 belong to the center of for all polynomials p(x, y) in the noncommuting variables x, y. In this paper, we obtain a finite set of conditions which works. In particular we show that if A, B are positive semidefinite, then A, B are simultaneously quasidiagonalizable if (and only if) [A, B]2, [A2, B]2 and [A, B2]2 commute with A, B. 相似文献
14.
Let G be a graph of order n, and n = Σki=1 ai be a partition of n with ai ≥ 2. In this article we show that if the minimum degree of G is at least 3k−2, then for any distinct k vertices v1,…, vk of G, the vertex set V(G) can be decomposed into k disjoint subsets A1,…, Ak so that |Ai| = ai,viisAi is an element of Ai and “the subgraph induced by Ai contains no isolated vertices” for all i, 1 ≥ i ≥ k. Here, the bound on the minimum degree is sharp. © 1997 John Wiley & Sons, Inc. 相似文献
15.
Iosef Pinelis 《Annals of Combinatorics》2002,6(1):103-106
Let (Ai) i ? I (A_i) _{i \in I} and (Bi) i ? I (B_i) _ {i \in I} be two (possibly infinite) families of finite sets. Let cl(P) denote the closure of the set P : = { (Ai, Bi ): i ? I } P := \{ ({A_i}, {B_i} ): i \in I \} of the pairs with respect to the componentwise union and intersection operations. Then there exists an injective map èi ? I Ai ? èi ? I Bi {\displaystyle \bigcup _ {i \in I}} A_i \rightarrow {\displaystyle \bigcup _ {i \in I }} B_i such that f (Ai) í Bi f (A_i) \subseteq B_i for every i if, and only if, card (A) £ (A) \leq card (B) for every pair (A, B) ? cl (P) (A, B) \in cl (P) . 相似文献
16.
Sam Gutmann 《Journal of multivariate analysis》1978,8(4):573-578
Let (X1, X2,…, Xk, Y1, Y2,…, Yk) be multivariate normal and define a matrix C by Cij = cov(Xi, Yj). If (i)
(X1,…, Xk) =
(Y1,…, Yk) and (ii) C is symmetric positive definite, then 0 < varf(X1,…, Xk) < ∞ corr(f(X1,…, Xk),f(Y1,…, Yk)) > 0. Condition (i) is necessary for the conclusion. The sufficiency of (i) and (ii) follows from an infinite-dimensional version, which can also be applied to a pair of jointly normal Brownian motions. 相似文献
17.
Let (T, M) be a complete local domain containing the integers. Let p 1 ? p 2 ? ··· ? p n be a chain of nonmaximal prime ideals of T such that T p n is a regular local ring. We construct a chain of excellent local domains A n ? A n?1 ? ··· ? A 1 such that for each 1 ≤ i ≤ n, the completion of A i is T, the generic formal fiber of A i is local with maximal ideal p i , and if I is a nonzero ideal of A i then A i /I is complete. We then show that if Q is a nonmaximal prime ideal of T and 1 ≤ h = ht T Q, then there is a chain of excellent local domains B 0 ? B 1 ? ··· ? B h ? T such that for every i = 0, 1, 2,…, h we have ht(Q ∩ B i ) = i, the completion of B i is isomorphic to T[[X 1, X 2,…, X i ]] where the X j 's are indeterminants, and the formal fiber of Q ∩ B i is local. 相似文献
18.
In the paper, the split quaternion matrix equation AXAη*=B is considered, where the operator Aη* is the η-conjugate transpose of A, where η∈{i,j,k}. We propose some new real representations, which well exploited the special structures of the original matrices. By using this method, we obtain the necessary and sufficient conditions for AXAη*=B to have X=±Xη* solutions and derive the general expressions of solutions when it is consistent. In addition, we also derive the general expressions of the least squares X=±Xη* solutions to it in case that this matrix equation is not consistent. 相似文献
19.
Let K
0(Var
k
) be the Grothendieck ring of algebraic varieties over a field k. Let X, Y be two algebraic varieties over k which are piecewise isomorphic (i.e. X and Y admit finite partitions X
1, ..., X
n
, Y
1, ..., Y
n
into locally closed subvarieties such that X
i
is isomorphic to Y
i
for all i ≤ n), then [X] = [Y] in K
0(Var
k
). Larsen and Lunts ask whether the converse is true. For characteristic zero and algebraically closed field k, we answer positively this question when dim X ≤ 1 or X is a smooth connected projective surface or if X contains only finitely many rational curves. 相似文献