275 research outputs found
Lower Bounds and Approximation Algorithms for Search Space Sizes in Contraction Hierarchies
Contraction hierarchies (CH) is a prominent preprocessing-based technique that accelerates the computation of shortest paths in road networks by reducing the search space size of a bidirectional Dijkstra run. To explain the practical success of CH, several theoretical upper bounds for the maximum search space size were derived in previous work. For example, it was shown that in minor-closed graph families search space sizes in ?(?n) can be achieved (with n denoting the number of nodes in the graph), and search space sizes in ?(h log D) in graphs of highway dimension h and diameter D. In this paper, we primarily focus on lower bounds. We prove that the average search space size in a so called weak CH is in ?(b_?) for ? ? 2/3 where b_? is the size of a smallest ?-balanced node separator. This discovery allows us to describe the first approximation algorithm for the average search space size. Our new lower bound also shows that the ?(?n) bound for minor-closed graph families is tight. Furthermore, we deeper investigate the relationship of CH and the highway dimension and skeleton dimension of the graph, and prove new lower bound and incomparability results. Finally, we discuss how lower bounds for strong CH can be obtained from solving a HittingSet problem defined on a set of carefully chosen subgraphs of the input network
Functional characterization of a novel arachidonic acid 12S-lipoxygenase in the halotolerant bacterium Myxococcus fulvus exhibiting complex social living patterns
Lipoxygenases are lipid peroxidizing enzymes, which frequently occur in higher plants and mammals. These enzymes are also expressed in lower multicellular organisms but here they are not widely distributed. In bacteria, lipoxygenases rarely occur and evaluation of the currently available bacterial genomes suggested that <0.5% of all sequenced bacterial species carry putative lipoxygenase genes. We recently rescreened the public bacterial genome databases for lipoxygenaseâlike sequences and identified two novel lipoxygenase isoforms (MFâLOX1 and MFâLOX2) in the halotolerant Myxococcus fulvus. Both enzymes share a low degree of amino acid conservation with wellâcharacterized eukaryotic lipoxygenase isoforms but they involve the catalytically essential iron cluster. Here, we cloned the MFâLOX1 cDNA, expressed the corresponding enzyme as Nâterminal hexaâhisâtag fusion protein, purified the recombinant enzyme to electrophoretic homogeneity, and characterized it with respect to its proteinâchemical and enzymatic properties. We found that M. fulvus expresses a catalytically active intracellular lipoxygenase that converts arachidonic acid and other polyunsaturated fatty acids enantioselectively to the corresponding nâ9 hydroperoxy derivatives. The enzyme prefers C20â and C22âpolyenoic fatty acids but does not exhibit significant membrane oxygenase activity. The possible biological relevance of MFâLOX1 will be discussed in the context of the suggested concepts of other bacterial lipoxygenases
Half-Metallic Ferromagnetism in the Heusler Compound CoFeSi revealed by Resistivity, Magnetoresistance, and Anomalous Hall Effect measurements
We present electrical transport data for single-crystalline CoFeSi which
provide clear-cut evidence that this Heusler compound is truly a half-metallic
ferromagnet, i.e. it possesses perfect spin-polarization. More specifically,
the temperature dependence of is governed by electron scattering off
magnons which are thermally excited over a sizeable gap
() separating the electronic majority states at the Fermi level
from the unoccupied minority states. As a consequence, electron-magnon
scattering is only relevant at but freezes out at lower
temperatures, i.e., the spin-polarization of the electrons at the Fermi level
remains practically perfect for . The gapped magnon population
has a decisive influence on the magnetoresistance and the anomalous Hall effect
(AHE): i) The magnetoresistance changes its sign at , ii) the
anomalous Hall coefficient is strongly temperature dependent at and compatible with Berry phase related and/or side-jump electronic
deflection, whereas it is practically temperature-independent at lower
temperatures
Low-temperature properties of single-crystal CrB
We report the low-temperature properties of B-enriched single-crystal
CrB as prepared from high-purity Cr and B powder by a solid-state
reaction and optical float zoning. The electrical resistivity, ,
Hall effect, , and specific heat, , are characteristic of an
exchange-enhanced Fermi liquid ground state, which develops a slightly
anisotropic spin gap below . This observation is corroborated by the absence of a Curie dependence in
the magnetization for reported in the literature. Comparison of
with , where we infer lattice contributions from
measurements of VB, reveals strong antiferromagnetic spin fluctuations with
a characteristic spin fluctuation temperature
in the paramagnetic state, followed by a pronounced second-order mean-field
transition at , and unusual excitations around . The pronounced anisotropy of above is
characteristic of an easy-plane anisotropy of the spin fluctuations consistent
with the magnetization. The ratio of the Curie-Weiss to the Nel
temperatures, , inferred from the
magnetization, implies strong geometric frustration. All physical properties
are remarkably invariant under applied magnetic fields up to ,
the highest field studied. In contrast to earlier suggestions of local-moment
magnetism our study identifies CrB as a weak itinerant antiferromagnet
par excellence with strong geometric frustration.Comment: 15 pages, 9 figure
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