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111.
We review spacetime dynamics in the presence of large-scale electromagnetic fields and then consider the effects of the magnetic component on perturbations to a spatially homogeneous and isotropic universe. Using covariant techniques, we refine and extend earlier work and provide the magnetohydrodynamic equations that describe inhomogeneous magnetic cosmologies in full general relativity. Specialising this system to perturbed Friedmann–Robertson–Walker models, we examine the effects of the field on the expansion dynamics and on the growth of density inhomogeneities, including non-adiabatic modes. We look at scalar perturbations and obtain analytic solutions for their linear evolution in the radiation, dust and inflationary eras. In the dust case we also calculate the magnetic analogue of the Jeans length. We then consider the evolution of vector perturbations and find that the magnetic presence generally reduces the decay rate of these distortions. Finally, we examine the implications of magnetic fields for the evolution of cosmological gravitational waves. 相似文献
112.
We present the first quadratic-time algorithm for the greedy triangulation of a finite planar point set, and the first linear-time algorithm for the greedy triangulation of a convex polygon. 相似文献
113.
The title compounds have been synthesized with 75% enantiomeric excess from Chiral p-tolylsulfoxides 1 and aldehydes, followed by thermal elimination of the sulfoxide. 相似文献
114.
We show that a spiral surface M in E3 is of finite type if and only if M is minimal Also, the plane is the only spiral surface in E3 whose the Gauss map G is of finite type, or satisfies the condition ΔG = ΛG, where Λ ∈ R3×3. 相似文献
115.
Grammati E. Pantziou Paul G. Spirakis Christos D. Zaroliagis 《BIT Numerical Mathematics》1992,32(2):215-236
Efficient parallel algorithms are presented, on the CREW PRAM model, for generating a succinct encoding of all pairs shortest path information in a directed planar graphG with real-valued edge costs but no negative cycles. We assume that a planar embedding ofG is given, together with a set ofq faces that cover all the vertices. Then our algorithm runs inO(log2
n) time and employsO(nq+M(q)) processors (whereM(t) is the number of processors required to multiply twot×t matrices inO(logt) time). Let us note here that wheneverq<n then our processor bound is better than the best previous one (M(n)).O(log2
n) time,n-processor algorithms are presented for various subproblems, including that of generating all pairs shortest path information in a directedouterplanar graph. Our work is based on the fundamental hammock-decomposition technique of G. Frederickson. We achieve this decomposition inO(logn log*n) parallel time by usingO(n) processors. The hammock-decomposition seems to be a fundamental operation that may help in improving efficiency of many parallel (and sequential) graph algorithms.This work was partially supported by the EEC ESPRIT Basic Research Action No. 3075 (ALCOM) and by the Ministry of Industry, Energy and Technology of Greece. 相似文献
116.
Misbah Ul Hasan Christos P. Tsonis 《Journal of polymer science. Part A, Polymer chemistry》1984,22(6):1349-1355
13C-NMR Spectroscopy has been used for the structural characterization of polybenzyls synthesized from the polymerization of benzyl chloride catalyzed by arene chromium tricarbonyls. The 13C-chemical shifts for various mono-, di-, tri-, and tetrasubstituted phenyl groups in a polybenzyl have been calculated by the use of additivity parameters and compared with their observed shifts. Although from a theroetical point of view, polysubstitution on a phenyl ring gives rise to a number of isomers, the results indicate the preferential formation of only a few of them due to steric and electronic considerations. Based on quantitative 13C-NMR data, several important structural parameters including the percentage of mono-, di-, tri-, and tetrasubstituted phenyl rings in the polybenzyl can be calculated. The extent of branching in a number of polybenzyls prepared under different experimental conditions was determined. The results indicate an increase in branching with the increase in reaction temperature. 相似文献
117.
We show that the traveling salesman problem with triangle inequality cannot be approximated with a ratio better than
when the edge lengths are allowed to be asymmetric and
when the edge lengths are symmetric, unless P=NP. The best previous lower bounds were
and
respectively. The reduction is from H?stad’s maximum satisfiability of linear equations modulo 2, and is nonconstructive.
* Supported in part by NSF ITR Grant CCR-0121555.
† Supported by NSF award CCR-0307536 and a Sloan foundation fellowship. 相似文献
118.
Christos A. Athanasiadis 《Order》2018,35(1):47-56
The symmetric group \(\mathfrak {S}_{n}\) acts naturally on the poset of injective words over the alphabet {1, 2,…,n}. The induced representation on the homology of this poset has been computed by Reiner and Webb. We generalize their result by computing the representation of \(\mathfrak {S}_{n}\) on the homology of all rank-selected subposets, in the sense of Stanley. A further generalization to the poset of r-colored injective words is given. 相似文献
119.
Simone Mastrogiovanni Christos Karathanasis Jonathan Gair Gregory Ashton Stefano Rinaldi Hsiang-Yu Huang Gergely Dálya 《Annalen der Physik》2024,536(2):2200180
Standard sirens have been the central paradigm in gravitational-wave cosmology so far. From the gravitational wave signature of compact star binaries, it is possible to measure the luminosity distance of the source directly, and if additional information on the source redshift is provided, a measurement of the cosmological expansion can be performed. This review article discusses several methodologies that have been proposed to use gravitational waves for cosmological studies. Methods that use only gravitational-wave signals and methods that use gravitational waves in conjunction with additional observations such as electromagnetic counterparts and galaxy catalogs will be discussed. The review also discusses the most recent results on gravitational-wave cosmology, starting from the binary neutron star merger GW170817 and its electromagnetic counterpart and finishing with the population of binary black holes, observed with the third Gravitational-wave Transient Catalog GWTC–3. 相似文献
120.
We study the graphs G for which their toric ideals I G are complete intersections. In particular, we prove that for a connected graph G such that I G is a complete intersection all of its blocks are bipartite except for at most two. We prove that toric ideals of graphs which are complete intersections are circuit ideals. In this case, the generators of the toric ideal correspond to even cycles of G except of at most one generator, which corresponds to two edge disjoint odd cycles joint at a vertex or with a path. We prove that the blocks of these graphs satisfy the odd cycle condition. Finally, we characterize all complete intersection toric ideals of graphs which are normal. 相似文献