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Search Results: 1 - 10 of 462053 matches for " A. Karasev "
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Topological model categories generated by finite complexes
A. Chigogidze,A. Karasev
Mathematics , 2002,
Abstract: Our main result states that for each finite complex L the category ${\bf TOP}$ of topological spaces possesses a model category structure (in the sense of Quillen) whose weak equivalences are precisely maps which induce isomorphisms of all [L]-homotopy groups. The concept of [L]-homotopy has earlier been introduced by the first author and is based on Dranishnikov's notion of extension dimension. As a corollary we obtain an algebraic characterization of [L]-homotopy equivalences between [L]-complexes. This result extends two classical theorems of J. H. C. Whitehead. One of them -- describing homotopy equivalences between CW-complexes as maps inducing isomorphisms of all homotopy groups -- is obtained by letting $L = \{{\rm point}\}$. The other -- describing n-homomotopy equivalences between at most $(n+1)$-dimensional CW-complexes as maps inducing isomorophisms of k-dimensional homotopy groups with $k \leq n$ -- by letting $L = S^{n+1}$, $n \geq 0$.
On two problems in extension theory
A. V. Karasev
Mathematics , 2003,
Abstract: In this note we introduce the concept of a quasi-finite complex. Next, we show that for a given countable and locally finite CW complex L the following conditions are equivalent: (i) L is quasi-finite. (ii) There exists a [L]-invertible mapping of a metrizable compactum X with e-dim X = [L] onto the Hilbert cube. Finally, we construct an example of a quasi-finite complex L such that its extension type [L] does not contain a finitely dominated complex.
On [L]-homotopy groups
A. V. Karasev
Mathematics , 2000,
Abstract: Some properties of [L]-homotopy group for finite complex L are investigated. It is proved that for complex L whose extension type lying between Sn and Sn+1 n-th [L]-homotopy group of Sn is isomorphic to Z.
Continuous selections with respect to extension dimension
A. V. Karasev
Mathematics , 1999,
Abstract: The paper is devoted to generalization of well-known Michael's Selection theorem on the case of extension dimension.
Approximations and selections of multivalued mappings of finite-dimensional spaces
N. Brodsky,A. Chigogidze,A. Karasev
Mathematics , 2001,
Abstract: We prove extension-dimensional versions of finite dimensional selection and approximation theorems. As applications, we obtain several results on extension dimension.
Topological semigroups and universal spaces related to extension dimension
A. Chigogidze,A. Karasev,M. Zarichnyi
Mathematics , 2001,
Abstract: It is proved that there is no structure of left (right) cancelative semigroup on $[L]$-dimensional universal space for the class of separable compact spaces of extensional dimension $\le [L]$. Besides, we note that the homeomorphism group of $[L]$-dimensional space whose nonempty open sets are universal for the class of separable compact spaces of extensional dimension $\le [L]$ is totally disconnected.
Real rank and squaring mapping for unital C*-algebras
A. Chigogidze,A. Karasev,M. Rordam
Mathematics , 2002,
Abstract: It is proved that if X is a compact Hausdorff space of Lebesgue dimension $\dim(X)$, then the squaring mapping $\alpha_{m} \colon (C(X)_{\mathrm{sa}})^{m} \to C(X)_{+}$, defined by $\alpha_{m}(f_{1},..., f_{m}) = \sum_{i=1}^{m} f_{i}^{2}$, is open if and only if $m -1 \ge \dim(X)$. Hence the Lebesgue dimension of X can be detected from openness of the squaring maps $\alpha_m$. In the case m=1 it is proved that the map $x \mapsto x^2$, from the self-adjoint elements of a unital $C^{\ast}$-algebra A into its positive elements, is open if and only if A is isomorphic to C(X) for some compact Hausdorff space X with $\dim(X)=0$.
AX J1749.1-2733 and AX J1749.2-2725 - the close pair of X-ray pulsars behind the Galactic Center: an optical identification
D. Karasev,A. Lutovinov,R. Burenin
Physics , 2010, DOI: 10.1111/j.1745-3933.2010.00949.x
Abstract: Two faint X-ray pulsars, AX J1749.2-2725 and AX J1749.1-2733, located in the direction to the Galactic Center, were studied in detail using data of INTEGRAL, XMM-Newton and Chandra observatories in X-rays, the SOFI/NTT instrument in infrared and the RTT150 telescope in optics. X-ray positions of both sources were determined with the uncertainty better than ~1 arcsec, that allowed us to identify their infrared counterparts. From the subsequent analysis of infrared and optical data we conclude that counterparts of both pulsars are likely massive stars of B0-B3 classes located behind the Galactic Center at distances of 12-20 kpc, depending on the type, probably in further parts of galactic spiral arms. In addition, we investigated the extinction law towards the galactic bulge and found that it is significantly different from standard one.
Quantum Magnetic Algebra and Magnetic Curvature
M. V. Karasev,T. A. Osborn
Mathematics , 2003, DOI: 10.1088/0305-4470/37/6/025
Abstract: The symplectic geometry of the phase space associated with a charged particle is determined by the addition of the Faraday 2-form to the standard structure on the Euclidean phase space. In this paper we describe the corresponding algebra of Weyl-symmetrized functions in coordinate and momentum operators satisfying nonlinear commutation relations. The multiplication in this algebra generates an associative product of functions on the phase space. This product is given by an integral kernel whose phase is the symplectic area of a groupoid-consistent membrane. A symplectic phase space connection with non-trivial curvature is extracted from the magnetic reflections associated with the Stratonovich quantizer. Zero and constant curvature cases are considered as examples. The quantization with both static and time dependent electromagnetic fields is obtained. The expansion of the product by the deformation parameter, written in the covariant form, is compared with the known deformation quantization formulas.
Symplectic areas, quantization, and dynamics in electromagnetic fields
M. V. Karasev,T. A. Osborn
Mathematics , 2000, DOI: 10.1063/1.1426688
Abstract: A gauge invariant quantization in a closed integral form is developed over a linear phase space endowed with an inhomogeneous Faraday electromagnetic tensor. An analog of the Groenewold product formula (corresponding to Weyl ordering) is obtained via a membrane magnetic area, and extended to the product of N symbols. The problem of ordering in quantization is related to different configurations of membranes: a choice of configuration determines a phase factor that fixes the ordering and controls a symplectic groupoid structure on the secondary phase space. A gauge invariant solution of the quantum evolution problem for a charged particle in an electromagnetic field is represented in an exact continual form and in the semiclassical approximation via the area of dynamical membranes.
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