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11.
Gabriel Picavet 《Journal of Pure and Applied Algebra》2002,174(2):163-185
We define and study algebraically flat algebras in order to have a better understanding of algebraically projective algebras of finite type (the projective algebras of literature). A close examination of the differential properties of these algebras leads to our main structure theorem. As a corollary, we get that an algebraically projective algebra of finite type over a field is either a polynomial ring or the affine algebra of a complete intersection. 相似文献
12.
Using recursive variants of Baire notions of nowhere dense and meagre sets we study the topological size of speedable and infinitely often speedable functions in a machine-independent framework. We show that the set of speedable functions is not “small” whereas the set of infinitely often speedable functions is “large”. In this way we offer partial answers to a question in [4]. 相似文献
13.
14.
In many Italian archaeological sites dated between the sixth and third centuries BC, unworked lumps of Cu-based materials are sometimes found, the so called Aes Rude, which according to archaeological considerations were appreciated as currency, as a medium of exchange and as a form of saving. The microchemical investigation of these ancient artefacts discloses their nature as apparently not usable for any functional applications or possible use. Indeed, Aes Rude resemble ordinary copper material, but microchemical results indicate that they are constituted by highly ferruginous leaded copper, making them useless for producing other metal objects by means of casting or hot and cold working. Notwithstanding this intrinsic negative feature, the production of these intractable Cu-based alloys was deliberately carried out to maximise the process yield in terms of produced metal from an impure and unselected metal ore by tailoring the smelting process parameters. With these considerations in mind, the microchemical investigation of these ancient iron–copper alloys gives evidence of the passage from the acceptance of an artefact value based on its true nature or potential use to the acceptance of the value based only on its appearance or form irrespective of its present or future use. This information could contribute to a better understanding of the evolution of human thought and economic and social interactions. 相似文献
15.
We show that if a closed manifold M admits an ℱ-structure (not necessarily polarized, possibly of rank zero) then its minimal entropy vanishes. In particular,
this is the case if M admits a non-trivial S
1-action. As a corollary we obtain that the simplicial volume of a manifold admitting an ℱ-structure is zero.?We also show
that if M admits an ℱ-structure then it collapses with curvature bounded from below. This in turn implies that M collapses with bounded scalar curvature or, equivalently, its Yamabe invariant is non-negative.?We show that ℱ-structures
of rank zero appear rather frequently: every compact complex elliptic surface admits one as well as any simply connected closed
5-manifold.?We use these results to study the minimal entropy problem. We show the following two theorems: suppose that M is a closed manifold obtained by taking connected sums of copies of S
4, ℂP
2,
2,S
2×S
2and the K3 surface. Then M has zero minimal entropy. Moreover, M admits a smooth Riemannian metric with zero topological entropy if and only if M is diffeomorphic to S
4,ℂP
2,S
2×S
2,ℂP
2#
2 or ℂP
2# ℂP
2. Finally, suppose that M is a closed simply connected 5-manifold. Then M has zero minimal entropy. Moreover, M admits a smooth Riemannian metric with zero topological entropy if and only if M is diffeomorphic to S
5,S
3×S
2, then on trivial S
3-bundle over S
2 or the Wu-manifold SU(3)/SO(3).
Oblatum 13-III-2002 & 12-VIII-2002?Published online: 8 November 2002
G.P. Paternain was partially supported by CIMAT, Guanajuato, México.?J. Petean is supported by grant 37558-E of CONACYT. 相似文献
16.
In this paper we compute the number of curves of genus 2 defined over a finite field k of odd characteristic up to isomorphisms defined over k; the even characteristic case is treated in an ongoing work (G. Cardona, E. Nart, J. Pujolàs, Curves of genus 2 over field of even characteristic, 2003, submitted for publication). To this end, we first give a parametrization of all points in
, the moduli variety that classifies genus 2 curves up to isomorphism, defined over an arbitrary perfect field (of zero or odd characteristic) and corresponding to curves with non-trivial reduced group of automorphisms; we also give an explicit representative defined over that field for each of these points. Then, we use cohomological methods to compute the number of k-isomorphism classes for each point in
. 相似文献
17.
In this paper we study the 2-dimension of a finite poset from the topological point of view. We use homotopy theory of finite
topological spaces and the concept of a beat point to improve the classical results on 2-dimension, giving a more complete answer to the problem of all possible 2-dimensions
of an n-point poset.
相似文献
18.
Gabriel M. Rebeiz Curtis C. Ling David B. Rutledge 《International Journal of Infrared and Millimeter Waves》1989,10(8):931-936
An accurate monolithic power meter has been developed for millimeter-wave applications. The detector is a large-area Bismuth bolometer, integrated on a fused-Quartz substrate. It simply measures the temperature change caused by the absorption of millimeter-wave radiation. The power meter is simple to fabricate, inexpensive, and can be easily calibrated using a low-frequency network. The measured responsivity for a 50 bolometer, with an area of 1×1cm, at a bias of 1V. and a video modulation of 100Hz, is 1mV/W. The noise spectrum exhibits a 1/f rolloff till 1KHz, and is limited by the Johnson noise for higher frequencies. The NEP of the detector is 3WHz–1/2 at a video modulation of 1KHz. It is possible to decrease the current NEP by fabricating bolometers with higher responsivities. Possible application areas are absolute power calibration and localized power density measurements at millimeter and submillimeter wavelengths. 相似文献
19.
After administration of [3H] cortisol, the specific radioactivity of the urinary cortisol metabolites is determined by chemical conversion of the 17-hydroxycorticosteroid conjugates to 17-ketosteroids, extraction with Sep-Pak C18 cartridges, derivatisation with 2,4-dinitrophenylhydrazine and isocratic reversed-phase high-performance liquid chromatography. This allows on-line measurement of ultra-violet absorbance; fractions are collected and 3H is measured by scintillation counting. Alternatively extraction and chromatography are done after enzymic hydrolysis of the steroid conjugates, and the major metabolites are determined colorimetrically in the fractions. The results from the two methods were consistent with each other and with published data. 相似文献
20.