781 research outputs found
Cotilting bimodules and their dualities
No full textA right R-module UR is said to be cotilting if Cog(UR)=⊥UR, where ⊥UR=KerExt1R(,U). So cotilting modules generalize injective cogenerators. If U is cotilting, then T=(KerHom(−,U),Cog(U)) is a torsion theory, the so-called cotilting torsion theory. A bimodule SUR is cotilting if SUR is faithfully balanced and both UR and SU are cotilting modules. So cotilting bimodules generalize Morita bimodules. The main topic of the paper is the study of dualities induced by cotilting bimodules, as a generalization of the classical theory of Morita dualities.
Let SUR be a cotilting bimodule and put Δ=Hom(,U) and Γ=Ext1(,U). Denote by Y the class of all U-reflexive modules, by C the class of all modules of the form K/L where K,L∈Y, and by X the class of all T-torsion modules from C. These classes are studied in the first part of the paper, in order to prove their closure properties, and show that they are sufficiently large. For example, by Proposition 5, C contains all finitely presented modules.
The main result of the paper is the following "cotilting theorem'': If U is a cotilting bimodule, then Δ and Γ realize a duality between the classes Y and X, respectively. Moreover, there is a natural morphism γM:Γ2(M)→M such that the sequence 0→Γ2(M)@>γM>>M@>δM>>Δ2(M)@>>>0 is exact for all M∈C, where δM is the evaluation map.
In the case of Morita dualities, Müller proved that U-reflexive modules coincide with the linearly compact ones. Inspired by this result, the author studies U-torsionless linearly compact (U-tl.l.c.) modules in the case when U is a cotilting module. In Proposition 10, he proves that if SUR is a cotilting bimodule then any U-tl.l.c. bimodule is U-reflexive, and asks whether the converse is true. (By a recent example of D'Este, this is not true in general.)
Finally, the author introduces abstract "duality conditions'' for a pair of torsion theories. These are necessary conditions for the pair to be cogenerated by a cotilting bimodule. The conditions are not sufficient in general. Nevertheless, by Proposition 13, they ensure uniqueness of the representing bimodule
PROVE DI SEMINA DI FAGGIO (Fagus sylvatica L.) IN UNA PECCETA SUL M. BALDO (VR)
No full textSi è provata la semina di faggio in una pecceta di origine artificiale sul M. Baldo (VR), ricorrendo a modalità operative diverse (semina a buchette o a spaglio, impiego di seme fresco o conservato) e in differenti condizioni di copertura, realizzate intervenendo in modo diverso sul soprassuolo di conifere: copertura piena (nessun intervento), ridotta (diradamento) o assente (taglio raso). Nell'ambito della modalità di semina risultata più efficace (a buchette, con seme fresco) le nascite si sono verificate con successo indipendentemente dal grado di copertura. Tuttavia sia la sopravvivenza dei semenzali già al termine della prima stagione vegetativa, che le dimensioni raggiunte da questi nei primi due anni di vita hanno risentito significativamente del grado di illuminazione disponibile
Tilting modules and -modules
No full textC. Menini and A. Orsatti [Rend. Sem. Mat. Univ. Padova 82 (1989), 203--231 (1990); MR1049594 (91h:16026)] introduced ∗-modules in order to characterize equivalences between certain full subcategories of module categories over two rings. If one restricts the study to the case of finite-dimensional algebras over a field k, it was shown by G. D'Este and the reviewer [Rend. Sem. Mat. Univ. Padova 83 (1990), 77--80; MR1066430 (91i:16027)] that faithful ∗-modules are tilting modules in the sense of the reviewer and C. M. Ringel [Trans. Amer. Math. Soc. 274 (1982), no. 2, 399--443; MR0675063 (84d:16027)]. The paper under review now generalizes this characterization to arbitrary rings using the natural generalizations for tilting modules in this case
Rotational properties of strange stars
No full textIn this paper, we present results from an investigation of the rotational properties of strange stars, using models with a canonical value of the bag constant. The changes in structure resulting from uniform rotation have been calculated within the slow rotation regime and the minimum rotation periods consistent with stability to nonaxisymmetric perturbations have also been calculated. The minimum period is found to be set by the onset of instability in either the m = 2 or m = 3 mode. The first of these modes, which is probably inaccessible to standard neutron stars, may be the critical one for old strange stars spun up by accretion and this could be of importance in giving an observational test for distinguishing between strange stars and standard neutron stars
Una visita andrologica. Una prospettiva centrata sulla malattia e una prospettiva centrata sul paziente.
No full text
Low metallicity and ultra-luminous X-ray sources in the Cartwheel galaxy
Get PDFObservations of turbulent velocity dispersions in the H i component of galactic discs show a characteristic floor in galaxies with low star formation rates and within individual galaxies the dispersion profiles decline with radius. We carry out several high-resolution adaptive mesh simulations of gaseous discs embedded within dark matter haloes to explore the roles of cooling, star formation, feedback, shearing motions and baryon fraction in driving turbulent motions. In all simulations the disc slowly cools until gravitational and thermal instabilities give rise to a multiphase medium in which a large population of dense self-gravitating cold clouds are embedded within a warm gaseous phase that forms through shock heating. The diffuse gas is highly turbulent and is an outcome of large-scale driving of global non-axisymmetric modes as well as cloud–cloud tidal interactions and merging. At low star formation rates these processes alone can explain the observed H i velocity dispersion profiles and the characteristic value of ∼10 km s−1 observed within a wide range of disc galaxies. Supernovae feedback creates a significant hot gaseous phase and is an important driver of turbulence in galaxies with a star formation rate per unit area ≳10−3 M⊙ yr−1 kpc−2
Le conversioni dei cedui nel Veneto. Risultati di alcune prove sperimentali nel primo decennio
No full textMillisecond pulsars around intermediate-mass black holes in globular clusters
Get PDFGlobular clusters (GCs) are expected to be breeding grounds for the formation of single or binary intermediate-mass black holes (IMBHs) of ≳100 M⊙, but a clear signature of their existence is still missing. In this context, we study the process of dynamical capture of a millisecond pulsar (MSP) by a single or binary IMBH, simulating various types of single-binary and binary-binary encounters. It is found that [IMBH, MSP] binaries form over cosmic time in a cluster, at rates ≲10−11 yr−1, via encounters of wide-orbit binary MSPs off the single IMBH, and at a lower pace, via interactions of (binary or single) MSPs with the IMBH orbited by a typical cluster star. The formation of an [IMBH, MSP] system is strongly inhibited if the IMBH is orbited by a stellar mass black hole (BH): in this case, the only viable path is through the formation of a rare stable hierarchical triplet with the MSP orbiting exterior to the [IMBH, BH] binary. The [IMBH, MSP] binaries that form are relatively short-lived, ≲108−109 yr, since their orbits decay via emission of gravitational waves. The detection of an [IMBH, MSP] system has a low probability of occurrence, when inferred from the current sample of MSPs in GCs. If next-generation radio telescopes, like Square Kilometre Array (SKA), will detect an order of magnitude larger population of MSP in GCs, at least one [IMBH, MSP] is expected. Therefore, a complete search for low-luminosity MSPs in the GCs of the Milky Way with SKA will have the potential of testing the hypothesis that IMBHs of the order of 100 M⊙ are commonly hosted in GCs. The discovery will unambiguously prove that BHs exist in the still uncharted interval of masses around ≳100
Equivalences between projective and injective modules and Morita duality for Artinian rings
No full textGiven two rings A and R, we study the equivalences between all projective right A-modules and all injective right R-modules. We prove that such equivalences exist if and only if A and R are Artinian with a Morita duality. This naturally generalizes a well-known result on quasi-Frobenius rings
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