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Sia dato uno spazio topologicoE con azione di un monoide topologicoH e siaE→B una funzione continue che, su ogni apertoU di una partizione dell'unità diB, sia, a meno di omotopia, la proiezioneU×H→U (ovvero una fibrazione numerabile). Un classico risultato di A. Dold e R. Lashof (1959) permette di costruire, a partire
daE→B, una funzione continuaE
∞→B∞, conE
∞ debolmente contraibile e munito di azione diH: laH-fibrazione universale associata daH. Tale funzione, in generale, non è purtroppo numerabile e quindi non si presta alla classificazione delleH-fibrazioni numerabili. Successivamente (1971), M. Fuchs ha trovato un modo di modificare la costruzione di Dold-Lashof per
recuperare la numerabilità. La costruzione di Dold-Lashof-Fuchs è, da allora, uno dei principali strumenti nella teoria degli
spazi classificanti di monoidi topologici, anche se vi è un uso di topologie alquanto complesse e quindi poco maneggevoli.
In questo lavoro ci proponiamo di mostrare come, lavorando nella categoria conveniente deik-spazi, sia possibile descrivere la costruzione di Dold-Lashof-Fuchs in modo estremamente semplificato ed adattarla anche
alla classificazione delleF-fibrazioni numerabili.
Conferenza tenuta da R. Piccinini il 15 maggio 1995 相似文献
Conferenza tenuta da R. Piccinini il 15 maggio 1995 相似文献
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M.?SchwambEmail author S.?Boffi C.?Giusti F.?D.?Pacati 《The European Physical Journal A - Hadrons and Nuclei》2004,20(2):233-244
The influence of the mutual interaction between the two outgoing nucleons (NN-FSI) in electro- and photoinduced two-nucleon knockout from 16O has been investigated perturbatively. It turns out that the effect of NN-FSI depends on the kinematics and on the type of reaction considered. The effect is generally larger in pp- than in pn-knockout and in electroinduced than in photoinduced reactions. In superparallel kinematics NN-FSI leads in the (e,epp) channel to a strong increase of the cross-section, that is mainly due to a strong enhancement of the
-current contribution. In pn-emission, however, this effect is partially cancelled by a destructive interference with the seagull current. For photoreactions NN-FSI is considerably reduced in superparallel kinematics and can be practically negligible in specific kinematics.Received: 1 July 2003, Published online: 10 February 2004PACS:
13.75.Cs Nucleon-nucleon interactions (including antinucleons, deuterons, etc.) - 21.60.-n Nuclear structure models and methods - 25.30.Fj Inelastic electron scattering to continuum 相似文献
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C. Giusti F. D. Pacati M. Schwamb S. Boffi 《The European Physical Journal A - Hadrons and Nuclei》2007,33(1):29-38
Results for the cross-sections of the exclusive 16O(e, e'pn)14N and 16O(γ, pn)14N knockout reactions are presented and discussed in different kinematics. In comparison with earlier work, a complete treatment
of the center-of-mass (CM) effects in the nuclear one-body current is considered in connection with the problem of the lack
of orthogonality between initial bound and final scattering states. The effects due to CM and orthogonalization are investigated
in combination with different treatments of correlations in the two-nucleon overlap function and for different parametrizations
of the two-body currents. The CM effects lead in super-parallel kinematics to a dramatic increase of the 16O(e, e'pn) cross-section to the 12
+ excited state (3.95MeV) of 14N . In all the situations considered the results are very sensitive to the treatment of correlations. A crucial role is played
by tensor correlations, but also the contribution of long-range correlations is important. 相似文献
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M. Schwamb S. Boffi C. Giusti F.D. Pacati 《The European Physical Journal A - Hadrons and Nuclei》2003,17(1):7-10
The influence of the mutual interaction between the two outgoing nucleons (NN-FSI) in the 16O(e, e'pp) reaction has been investigated. Results for various kinematics are discussed. In general, the effect of NN-FSI depends on
kinematics and the chosen final state in the excitation spectrum of 14C.
Received: 19 December 2002 / Accepted: 6 February 2003 / Published online: 15 April 2003 相似文献
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Computational fluid dynamics, where simulations require largecomputation times, is one of the areas of application of highperformance computing. Schemes such as the SIMPLE (semi-implicitmethod for pressure-linked equations) algorithm are often usedto solve the discrete Navier-Stokes equations. Generally theseschemes take a short time per iteration but require a largenumber of iterations. For simple geometries (or coarser grids)the overall CPU time is small. However, for finer grids or morecomplex geometries the increase in the number of iterationsmay be a drawback and the decoupling of the differential equationsinvolved implies a slow convergence of rotationally dominatedproblems that can be very time consuming for realistic applications.So we analyze here another approach, DIRECTO, that solves theequations in a coupled way. With recent advances in hardwaretechnology and software design, it became possible to solvecoupled Navier-Stokes systems, which are more robust but implyincreasing computational requirements (both in terms of memoryand CPU time). Two approaches are described here (band blockLU factorization and preconditioned GMRES) for the linear solverrequired by the DIRECTO algorithm that solves the fluid flowequations as a coupled system. Comparisons of the effectivenessof incomplete factorization preconditioners applied to the GMRES(generalized minimum residual) method are shown. Some numericalresults are presented showing that it is possible to minimizeconsiderably the CPU time of the coupled approach so that itcan be faster than the decoupled one. 相似文献