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51.
We study graphs defined on families of finite sets of natural numbers and their chromatic properties. Of particular interest are graphs for which the edge relation is given by the shift. We show that when considering shift graphs with infinite chromatic number, one can center attention on graphs defined on precompact thin families. We define a quasi-order relation on the collection of uniform families defined in terms of homomorphisms between their corresponding shift graphs, and show that there are descending -sequences. Specker graphs are also considered and their relation with shift graphs is established. We characterize the family of Specker graphs which contain a homomorphic image of a shift graph. 相似文献
52.
Several relevant geotechnical works, such as railway and road embankments, offshore foundations and vibrating machine foundations, are affected by the progressive accumulation of irreversible settlements. These latter represent the macroscopic evidence of the progressive rearrangement of particles under cycling loading, which is commonly referred to, in the literature, as ratcheting. This phenomenon is well known, but it is quite difficult to describe it by means of an appropriate constitutive model. As a consequence, the evaluation of durability of the aforementioned structures remains an open problem. In this article, the phenomenon will be approached by employing a Distinct Element model capable of describing the evolution of the microstructure induced by cyclic mechanical perturbations. Several analyses are performed in order to stress the influence of both the stress level and loading history on the mechanical response of a numerical model of a sand specimen. The numerical analyses are intended to provide an experimental background for conceiving a simplified macro approach based on generalised plasticity theory. In particular by means of probe test the plastic potential and the hardening parameters will be defined as a function of the current stress state and loading history. 相似文献
53.
We investigate the class of spherically symmetric, anisotropic (principal stresses unequal) fluids admitting the existence of a vector field, assumed to be orthogonal to the four velocity of the fluid, and with respect to which the Lie derivative of the metric tensor is proportional to the shear tensor. The physical variables, as well as the evolution equation of the bounday surface, are found for different models. 相似文献
54.
55.
B. Fornal F. Gramegna G. Prete R. Burch G. D'Erasmo E. M. Fiore L. Fiore A. Pantaleo V. Paticchio G. Viesti P. Blasi N. Gelli F. Lucarelli M. Anghinolfi P. Corvisiero M. Taiuti A. Zucchiatti P. F. Bortignon D. Fabris G. Nebbia J. A. Ruiz M. Gonin J. B. Natowitz 《Zeitschrift für Physik A Hadrons and Nuclei》1991,340(1):59-62
Continuumγ- ray spectra from the decay of59Cu formed at an excitation energy of 100 MeV and angular momenta up to 43? by means of the reaction 190 MeV32S +27Al have been measured and analyzed. The parameters of the Giant Dipole Resonance (GDR) have been extracted using the statistical model. The derived GDR width confirms the sizeable broadening of this resonance in59Cu already reported in our earlier investigation at 77 MeV excitation energy (Jcrit=38?). Estimates of the GDR width have been performed in the adiabatic approximation. Predicted values account qualitatively for the experimental data of59Cu as well as of the heavier isotope63Cu, in which the broadening was not seen up to 77MeV excitation (Jcrit=35?). The present analysis demonstrates the strong sensitivity of the GDR to spin effects in this mass region. 相似文献
56.
G. Viesti M. Lunardon D. Bazzacco D. Fabris S. Lunardi N. H. Medina G. Nebbia C. Rossi Alvarez G. de Angelis M. Cinausero M. De Poli E. Farnea E. Fioretto G. Prete G. Maron D. R. Napoli 《Acta Physica Hungarica A》1995,2(3-4):255-268
Experimental data collected so far demonstrate that the population of superdeformed (SD) structures in nuclei with mass A~150 is influenced by the asymmetry in the entrance channel of the reaction. Furthermore, evidences for the population of hyperdeformed (HD) structures in the same mass region have been obtained only when charged particles are emitted in the evaporation chain. Dynamical model calculations show that in the reactions used to populate SD bands the equilibration times of the compound nuclei are not very sensitive on the mass asymmetry of the entrance channel. A qualitative understanding of both phenomena (SD and HD) is obtained assuming a link between the population of evaporation residues at very high spin and the dynamical hindrance of fission. Recent experimental results obtained in this field at the GASP spectrometer are presented for the case of the152Dy nucleus populated in the37Cl+120Sn reaction. Proton and alpha particle spectra in coincidence with SD structures have been obtained. A sizeable difference is evidenced between alpha particle spectra in coincidence with SD and normally deformed structures. 相似文献
57.
It is shown that close links exist between two important types of polarized partition relations of higher dimensions. Project supported by a South-South Fellowship of the Third World Academy of Sciences and by the National Natural Science Foundation of China (Grant No. 19671061). 相似文献
58.
59.
60.
We consider several kinds of partition relations on the set of real numbers and its powers, as well as their parameterizations with the set of all infinite sets of natural numbers, and show that they hold in some models of set theory. The proofs use generic absoluteness,
that is, absoluteness under the required forcing extensions. We show that Solovay models are absolute under those forcing
extensions, which yields, for instance, that in these models for every well ordered partition of there is a sequence of perfect sets whose product lies in one piece of the partition. Moreover, for every finite partition
of there is and a sequence of perfect sets such that the product lies in one piece of the partition, where is the set of all infinite subsets of X. The proofs yield the same results for Borel partitions in ZFC, and for more complex partitions in any model satisfying a certain degree of generic absoluteness.
This work was supported by the research projects MTM 2005-01025 of the Spanish Ministry of Science and Education and 2005SGR-00738
of the Generalitat de Catalunya. A substantial part of the work was carried out while the second-named author was ICREA Visiting
Professor at the Centre de Recerca Matemàtica in Bellaterra (Barcelona), and also during the first-named author’s stays at
the Instituto Venezolano de Investigaciones Científicas and the California Institute of Technology. The authors gratefully
acknowledge the support provided by these institutions. 相似文献