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排序方式: 共有210条查询结果,搜索用时 15 毫秒
1.
Frederico Xavier 《Journal of Geometric Analysis》1993,3(5):513-527
The helicoid and the plane are the only known complete simply connected minimal surfaces without self-intersections. In this paper we make an analytic study of this class of surfaces by first deforming them continuously into surfaces with self-intersections. Next, we study the (backward) time evolution of the set of self-intersections and see what geometric conditions must prevail in order for the self-intersections to rush off to infinity in finite time. As a result of this program it is shown that any surface of the type considered above has to satisfy at least one of five geometric possibilities. The first two of these alternatives are pathological, the third one is satisfied by the plane, and the next two are satisfied by the helicoid. 相似文献
2.
A variational method called discrete variable representation is applied to study the energy spectra of two interacting electrons in a quantum dot with a three-dimensional anisotropic harmonic confinement potential. This method, applied originally to problems in molecular physics and theoretical chemistry, is here used to solve the eigenvalue equation to relative motion between the electrons. The two-electron quantum dot spectrum is determined then with a precision of at least six digits. Moreover, the electron correlation energies for various potential confinement parameters are investigated for singlet and triplet states. When possible, the present results are compared with the available theoretical values. 相似文献
3.
4.
Cyclohexene transformation was used as test reaction to study the effect of lanthanum exchange on the catalytic behavior of HY zeolite cracking catalysts. Lanthanum has an overall positive effect increasing both acitivity and stability of HY zeolite and maximum selectivity towards isomerization products is obtained at higher conversion.
HY. , , HY. .相似文献
5.
Frederico F. Martins Dr. Ángel Sánchez-González Jose Lanuza Dr. Haralampos N. Miras Prof. Xabier Lopez Dr. Nuno A. Bandeira Dr. Adrià Gil 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(35):8977-8984
Phosphoester hydrolysis is an important chemical step in DNA repair. One archetypal molecular model of phosphoesters is para-nitrophenylphosphate (pNPP). It has been shown previously that the presence of molecular metal oxide [Mo7O24]6− may catalyse the hydrolysis of pNPP through the partial decomposition of polyoxomolybdate framework resulting in a [(PO4)2Mo5O15]6− product. Real-time monitoring of the catalytic system using electrospray ionisation mass spectrometry (ESI-MS) provided a glance into the species present in the reaction mixture and identification of potential catalytic candidates. Following up on the obtained spectrometric data, Density Functional Theory (DFT) calculations were carried out to characterise the hypothetical intermediate [Mo5O15(pNPP)2(H2O)6]6− that would be required to form under the hypothesised transformation. Surprisingly, our results point to the dimeric [Mo2O8]4− anion resulting from the decomposition of [Mo7O24]6− as the active catalytic species involved in the hydrolysis of pNPP rather than the originally assumed {Mo5O15} species. A similar study was carried out involving the same species but substituting Mo by W. The mechanism involving W species showed a higher barrier and less stable products in agreement with the non-catalytic effect found in experimental results. 相似文献
6.
Frederico Furtado Victor Ginting Felipe Pereira Michael Presho 《Transport in Porous Media》2011,90(3):927-947
A numerical method used for solving a two-phase flow problem as found in typical oil recovery is investigated in the setting
of physics-based two-level operator splitting. The governing equations involve an elliptic differential equation coupled with
a parabolic convection-dominated equation which poses a severe restriction for obtaining fully implicit numerical solutions.
Furthermore, strong heterogeneity of the porous medium over many length scales adds to the complications for effectively solving
the system. One viable approach is to split the system into three sub-systems: the elliptic, the hyperbolic, and the parabolic
equation, respectively. In doing so, we allow for the use of appropriate numerical discretization for each type of equation
and the careful exchange of information between them. We propose to use the multiscale finite volume element method (MsFVEM)
for the elliptic and parabolic equations, and a nonoscillatory difference scheme for the hyperbolic equation. Performance
of this procedure is confirmed through several numerical experiments. 相似文献
7.
The valence component of the light-front wave function of a quantum of a scalar field is studied within a bosonic model. The light-front time projection of the inhomogeneous Bethe-Salpeter equation for the vertex function in the covariant ladder approximation is given. The valence wave function and distribution are obtained from the numerical solution of the light-front equation for the vertex with different values of the time-like off-shell momentum carried by the quantum. 相似文献
8.
Silvânia Marilene de Lima Jorge Antonio Gómez Valéria P. Barros Greice de S. Vertuan Marilda das Dores Assis Carlos Frederico de Oliveira Graeff Grégoire Jean-François Demets 《Polyhedron》2010
This work presents a new oxovanadium(IV)–cucurbit[6]uril complex, which combines the catalytic properties of the metal ion with the size-excluding properties of the macrocycle cavity. In this coordination compound, the VO2+ ions are coordinated to the oxygen atoms located at the rim of the macrocycle in slightly distorted square-pyramidal configurations, which are in fact C2v symmetries. This combination results in a size-selective heterogeneous catalyst, which is able to oxidize linear alkanes like n-pentane at room temperature, but not styrene, cyclohexane or z-cyclooctene, which are too big to enter the cucurbit[6]uril cavity. The results presented here contribute to understanding the mechanism of alkane catalytic oxidation by oxovanadium(IV) complexes. 相似文献
9.
Let F:M→N be a C 1 map between Riemannian manifolds of the same dimension, M complete, N Cartan–Hadamard. We show that F is a C 1 diffeomorphism if inf x∈M |d(B ζ °F)(x)|>0 for all ζ∈N(∞) and Busemann functions B ζ . This generalizes the Cartan–Hadamard theorem and the Hadamard invertibility criterion, which requires inf x∈M ∥DF(x)?1∥?1=inf ζ∈N(∞)inf x∈M |d(B ζ °F)(x)|>0. Our proofs use a version of the shooting method for two-point boundary value problems. These ideas lead to new results about the size of the critical set of a function f∈C 2(? n ,?): a) If \(\inf_{x\in \mathbb{R}^{n}}|\operatorname{Hess} f(x)v|>0\) for all v≠0 then the function f has precisely one critical point. (b) If g∈C 2(? n ,?) is the C 1 local uniform limit of functions as in a), and \(\operatorname{Hess} g(x)\) is nowhere singular, then g has at most one critical point. The totality of functions described in (b) properly contains the class consisting of all C 2 strictly convex functions defined on ? n . 相似文献
10.
Frederico Caeiro M. Ivette Gomes Björn Vandewalle 《Methodology and Computing in Applied Probability》2014,16(1):1-29
In this paper, for heavy-tailed models and through the use of probability weighted moments based on the largest observations, we deal essentially with the semi-parametric estimation of the Value-at-Risk at a level p, the size of the loss occurred with a small probability p, as well as the dual problem of estimation of the probability of exceedance of a high level x. These estimation procedures depend crucially on the estimation of the extreme value index, the primary parameter in Statistics of Extremes, also done on the basis of the same weighted moments. Under regular variation conditions on the right-tail of the underlying distribution function F, we prove the consistency and asymptotic normality of the estimators under consideration in this paper, through the usual link of their asymptotic behaviour to the one of the extreme value index estimator they are based on. The performance of these estimators, for finite samples, is illustrated through Monte-Carlo simulations. An adaptive choice of thresholds is put forward. Applications to a real data set in the field of insurance as well as to simulated data are also provided. 相似文献