7,961 research outputs found
Logic Programming and Logarithmic Space
We present an algebraic view on logic programming, related to proof theory
and more specifically linear logic and geometry of interaction. Within this
construction, a characterization of logspace (deterministic and
non-deterministic) computation is given via a synctactic restriction, using an
encoding of words that derives from proof theory.
We show that the acceptance of a word by an observation (the counterpart of a
program in the encoding) can be decided within logarithmic space, by reducing
this problem to the acyclicity of a graph. We show moreover that observations
are as expressive as two-ways multi-heads finite automata, a kind of pointer
machines that is a standard model of logarithmic space computation
Heisenberg exchange in magnetic monoxides
The superexchange intertacion in transition-metal oxides, proposed initially
by Anderson in 1950, is treated using contemporary tight-binding theory and
existing parameters. We find also a direct exchange for nearest-neighbor metal
ions, larger by a factor of order five than the superexchange. This direct
exchange arises from Vddm coupling, rather than overlap of atomic charge
densities, a small overlap exchange contribution which we also estimate. For
FeO and CoO there is also an important negative contribution, related to Stoner
ferromagnetism, from the partially filled minority-spin band which broadens
when ionic spins are aligned. The corresponding J1 and J2 parameters are
calculated for MnO, FeO, CoO, and NiO. They give good accounts of the Neel and
the Curie-Weiss temperatures, show appropriate trends, and give a reasonable
account of their volume dependences. For MnO the predicted value for the
magnetic susceptibility at the Neel temperature and the crystal distortion
arising from the antiferromagnetic transition were reasonably well given.
Application to CuO2 planes in the cuprates gives J=1220oK, compared to an
experimental 1500oK, and for LiCrO2 gives J1=4 50oK compared to an experimental
230oK.Comment: 21 pages, 1 figure, submitted to Phys. Rev. B 1/19/07. Realized
J=4V^2/U applies generally, as opposed to J=2V^2/U from one-electron theory
(1/28 revision
Space Velocities of Southern Globular Clusters. IV. First Results for Inner-Galaxy Clusters
We have measured the absolute proper motions of four low-latitude,
inner-Galaxy globular clusters. These clusters are: NGC 6266 (M62), NGC 6304,
NGC 6316 and NGC 6723. The proper motions are on the Hipparcos system, as no
background extragalactic objects are found in these high-extinction regions.
The proper-motion uncertainties range between 0.3 and 0.6 mas/yr.
We discuss the kinematics of these clusters and of three additional bulge
clusters -- NGC 6522, NGC 6528 and NFC 6553 -- whose proper motions with
respect to bulge stars had been determined previously. We find that all of the
clusters have velocities that confine them to the bulge region. Of the three
metal poor clusters ([Fe/H] < -1.0), NGC 6522, and NGC 6723 have kinematics
consistent with halo membership. The third cluster, NGC 6266 however, appears
to belong to a rotationally-supported system. Of the four metal rich clusters
([Fe/H] >= -1.0), NGC 6304 and NGC 6553 also have kinematics consistent with
membership to a rotationally-supported system. NGC 6528 has kinematics,
metallicity and mass that argue in favor of a genuine Milky-Way bar cluster.
NGC 6316's kinematics indicate membership to a hotter system than the bar.Comment: 4 figures, 5 tables; accepted for publication in A
Active Brownian Motion Tunable by Light
Active Brownian particles are capable of taking up energy from their
environment and converting it into directed motion; examples range from
chemotactic cells and bacteria to artificial micro-swimmers. We have recently
demonstrated that Janus particles, i.e. gold-capped colloidal spheres,
suspended in a critical binary liquid mixture perform active Brownian motion
when illuminated by light. In this article, we investigate in some more details
their swimming mechanism leading to active Brownian motion. We show that the
illumination-borne heating induces a local asymmetric demixing of the binary
mixture generating a spatial chemical concentration gradient, which is
responsible for the particle's self-diffusiophoretic motion. We study this
effect as a function of the functionalization of the gold cap, the particle
size and the illumination intensity: the functionalization determines what
component of the binary mixture is preferentially adsorbed at the cap and the
swimming direction (towards or away from the cap); the particle size determines
the rotational diffusion and, therefore, the random reorientation of the
particle; and the intensity tunes the strength of the heating and, therefore,
of the motion. Finally, we harness this dependence of the swimming strength on
the illumination intensity to investigate the behaviour of a micro-swimmer in a
spatial light gradient, where its swimming properties are space-dependent
Chimpanzees produce diverse vocal sequences with ordered and recombinatorial properties
The origins of human language remains a major question in evolutionary science. Unique to human language is the capacity to flexibly recombine a limited sound set into words and hierarchical sequences, generating endlessly new sentences. In contrast, sequence production of other animals appears limited, stunting meaning generation potential. However, studies have rarely quantified flexibility and structure of vocal sequence production across the whole repertoire. Here, we used such an approach to examine the structure of vocal sequences in chimpanzees, known to combine calls used singly into longer sequences. Focusing on the structure of vocal sequences, we analysed 4826 recordings of 46 wild adult chimpanzees from TaĂŻ National Park. Chimpanzees produced 390 unique vocal sequences. Most vocal units emitted singly were also emitted in two-unit sequences (bigrams), which in turn were embedded into three-unit sequences (trigrams). Bigrams showed positional and transitional regularities within trigrams with certain bigrams predictably occurring in either head or tail positions in trigrams, and predictably co-occurring with specific other units. From a purely structural perspective, the capacity to organize single units into structured sequences offers a versatile system potentially suitable for expansive meaning generation. Further research must show to what extent these structural sequences signal predictable meanings
The Southern Proper Motion Program IV. The SPM4 Catalog
We present the fourth installment of the Yale/San Juan Southern Proper Motion
Catalog, SPM4. The SPM4 contains absolute proper motions, celestial
coordinates, and (B,V) photometry for over 103 million stars and galaxies
between the south celestial pole and -20 deg declination. The catalog is
roughly complete to V=17.5 and is based on photographic and CCD observations
taken with the Yale Southern Observatory's double-astrograph at Cesco
Observatory in El Leoncito, Argentina. The proper-motion precision, for
well-measured stars, is estimated to be 2 to 3 mas/yr, depending on the type of
second-epoch material. At the bright end, proper motions are on the
International Celestial Reference System by way of Hipparcos Catalog stars,
while the faint end is anchored to the inertial system using external galaxies.
Systematic uncertainties in the absolute proper motions are on the order of 1
mas/yr.Comment: 34 pages, 8 figures, 3 tables; accepted for publication in AJ; note -
modified author list and acknowledgements sectio
Thymic epithelial tumours: ESMO Clinical Practice Guidelines for diagnosis, treatment and follow-upâ€
- …