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991.
Bornheim A Lipeles E Pappas SP Shapiro A Sun WM Weinstein AJ Masek G Paar HP Mahapatra R Morrison RJ Briere RA Chen GP Ferguson T Tatishvili G Vogel H Adam NE Alexander JP Bebek C Berkelman K Blanc F Boisvert V Cassel DG Drell PS Duboscq JE Ecklund KM Ehrlich R Gibbons L Gittelman B Gray SW Hartill DL Heltsley BK Hsu L Jones CD Kandaswamy J Kreinick DL Magerkurth A Mahlke-Krüger H Meyer TO Mistry NB Nordberg E Palmer M Patterson JR Peterson D Pivarski J Riley D Sadoff AJ Schwarthoff H 《Physical review letters》2002,88(23):231803
We report a new measurement of the Cabibbo-Kobayashi-Maskawa parameter Vub made with a sample of 9.7 x 10(6) BB- events collected with the CLEO II detector. Using heavy quark theory, we combine the observed yield of leptons from semileptonic B decay in the end-point momentum interval 2.2-2.6 GeV/c with recent CLEO II data on B-->X(s)gamma to find Vub = (4.08+/-0.34+/-0.44+/-0.16+/-0.24)x10(-3), where the first two uncertainties are experimental and the last two are from theory. 相似文献
992.
Alexander Moretó 《Algebras and Representation Theory》2007,10(4):333-338
Given a group G, Γ(G) is the graph whose vertices are the primes that divide the degree of some irreducible character and two vertices p and q are joined by an edge if pq divides the degree of some irreducible character of G. By a definition of Lewis, a graph Γ has bounded Fitting height if the Fitting height of any solvable group G with Γ(G)=Γ is bounded (in terms of Γ). In this note, we prove that there exists a universal constant C such that if Γ has bounded Fitting height and Γ(G)=Γ then h(G)≤C. This solves a problem raised by Lewis.
Research supported by the Spanish Ministerio de Educación y Ciencia, MTM2004-06067-C02-01 and MTM2004-04665, the FEDER and
Programa Ramón y Cajal. 相似文献
993.
Michael Grouchko Alexander Kamyshny Keren Ben-Ami Shlomo Magdassi 《Journal of nanoparticle research》2009,11(3):713-716
Synthesis of well dispersed copper nanoparticles was achieved by reduction of copper nitrate in aqueous solution using hydrazine
monohydrate as a reducer in the presence of preformed silver nanoparticles as catalysts. It has been demonstrated that addition
of silver nanoparticles to the reaction mixture leads to formation of aqueous dispersion of copper nanoparticles and also
results in a drastic reduction in reaction time compared to procedures reported in the literature. The absorption spectrum
of the dispersions, HR-TEM and STEM images and XRD pattern indicate the formation of copper nanoparticles with particle size
in the range of 5–50 nm. 相似文献
994.
Let X be an anisotropic projective quadric over a field F of characteristic not 2. The essential dimension dimes(X) of X, as defined by Oleg Izhboldin, is dimes(X)=dim(X)-i(X) +1, where i(X) is the first Witt index of X (i.e., the Witt index of X over its function field).Let Y be a complete (possibly singular) algebraic variety over F with all closed points of even degree and such that Y has a closed point of odd degree over F(X). Our main theorem states that dimes(X)dim(Y) and that in the case dimes(X)=dim(Y) the quadric X is isotropic over F(Y).Applying the main theorem to a projective quadric Y, we get a proof of Izhboldins conjecture stated as follows: if an anisotropic quadric Y becomes isotropic over F(X), then dimes(X)dimes(Y), and the equality holds if and only if X is isotropic over F(Y). We also solve Knebuschs problem by proving that the smallest transcendence degree of a generic splitting field of a quadric X is equal to dimes(X). To the memory of Oleg Izhboldin 相似文献
995.
In this paper, we try to answer the following question: given a modular tensor category ? with an action of a compact group
G, is it possible to describe in a suitable sense the “quotient” category ?/G? We give a full answer in the case when ?=?ℯ? is the category of vector spaces; in this case, ?ℯ?/G turns out to be the category of representation of Drinfeld's double D(G). This should be considered as the category theory analog of the topological identity {pt}/G=BG.
This implies a conjecture of Dijkgraaf, Vafa, E. Verlinde and H. Verlinde regarding so-called orbifold conformal field theories:
if ? is a vertex operator algebra which has a unique irreducible module, ? itself, and G is a compact group of automorphisms of ?, and some not too restrictive technical conditions are satisfied, then G is finite, and the category of representations of the algebra of invariants, ?
G
, is equivalent as a tensor category to the category of representations of Drinfeld's double D(G). We also get some partial results in the non-holomorphic case, i.e. when ? has more than one simple module.
Received: 27 August 2001 / Accepted: 1 March 2002 相似文献
996.
Spectral properties and emission efficiencies of GdVO4 phosphors 总被引:2,自引:0,他引:2
M. Anitha P. Ramakrishnan A. Chatterjee G. Alexander H. Singh 《Applied Physics A: Materials Science & Processing》2002,74(2):153-162
GdVO4 with activators Eu, Dy, Sm and Bi has been synthesised by a solid-state reaction. GdVO4:Eu3+ (3%) yielded the highest quantum efficiency of 95%. Interesting energy-transfer properties have been revealed in the mixed-activator
phosphor (GdVO4:Eu3+, Sm3+) when excited in the 4f shell of Sm3+ at 408 nm. Bismuth-activated GdVO4 gives rise to a broad-band emission peaking at 525 nm in comparison to YVO4:Bi3+, which gives an emission peak at 570 nm under UV excitation. The quantum efficiency of GdVO4:Bi3+ increases gradually with bismuth concentration and reaches a maximum of 80% for a bismuth concentration of ≈0.5%. There is
a shift in the excitation band of GdVO4:Bi3+ towards longer wavelengths with increasing concentration of bismuth, which can lead to energy transfer from bismuth to europium
in a phosphor with both these activators. Heat treatment of GdVO4:Bi3+ at 1500 °C for 3–3.5 h resulted in a large percentage of bismuth being lost from the lattice as evaluated by X-ray fluorescence.
However, if a large percentage of bismuth (of the order of 3% or more) is initially added, a sufficient quantity of bismuth
can still be retained after heat treatment, which can lead to the development of ceramic scintillators for X-ray tomographic
applications. Addition of 3–5% boron gives a white GdVO4 phosphor without any chemical treatment.
Received: 27 Feruary 2001 / Accepted: 1 August 2001 / Published online: 30 October 2001 相似文献
997.
Giving a generalization of Berkes and Horváth (2003), we consider the Euclidean norm of vector-valued stochastic processes, which can be approximated with a vector-valued Wiener process having a linear drift. The suprema of the Euclidean norm of the processes are not far away from the norm of the processes at the right most point. We also obtain an approximation for the supremum of the weighted Euclidean norm with a Wiener process. 相似文献
998.
Alexander Goldenshluger 《Journal of multivariate analysis》2003,84(1):40-60
The problem of optimal prediction in the stochastic linear regression model with infinitely many parameters is considered. We suggest a prediction method that outperforms asymptotically the ordinary least squares predictor. Moreover, if the random errors are Gaussian, the method is asymptotically minimax over ellipsoids in ?2. The method is based on a regularized least squares estimator with weights of the Pinsker filter. We also consider the case of dynamic linear regression, which is important in the context of transfer function modeling. 相似文献
999.
In 1991, one of the authors showed the existence of quadratic transformations between the Painlevé VI equations with local monodromy differences (1/2, a, b, ±1/2) and (a, a, b, b). In the present paper we give concise forms of these transformations. They are related to the quadratic transformations obtained by Manin and Ramani–Grammaticos–Tamizhmani via Okamoto transformations. To avoid cumbersome expressions with differentiation, we use contiguous relations instead of the Okamoto transformations. The 1991 transformation is particularly important as it can be realized as a quadratic‐pull back transformation of isomonodromic Fuchsian equations. The new formulas are illustrated by derivation of explicit expressions for several complicated algebraic Painlevé VI functions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
1000.
Attempts were made to grow CeO2 and ThO2 single crystals doped with transition metal ions. Only Fe3+ and Mn2+ could be detected by the EPR technique. The EPR spectrum of Fe3+ in CeO2 exhibits the well-known fine structure in cubic fields. The parameters areg=2.0044(1) anda=15.6(1)·10?4 cm?1. The hyperfine constantA for57Fe in hexahedral coordination was found to be 8.9(1)·10?4 cm?1. The EPR spectrum of Mn2+ in CeO2 reveals two cubic Mn2+ centers. The parameters for center 1 areg=1.9999(1) andA=86.9(1)·10?4 cm?1 and for center 2g=1.9984(1) andA=87.0(1)·10?4 cm?1. Heating the Mn doped CeO2 samples in hydrogen, the Mn2+ centers transform from cubic into trigonal centers with approximate values ofg=1.9988(2),A=84.5(6)·10?4 cm?1 andD=203(1)·10?4 cm?1. The two observed Mn2+ centers in ThO2 exhibita priori axial symmetry with approximate values ofg=2.0006(2),A=88.9(4)·10?4 cm?1 andD=33(3)·10?4 cm?1. 相似文献