72,999 research outputs found
On the uniform one-dimensional fragment
The uniform one-dimensional fragment of first-order logic, U1, is a recently
introduced formalism that extends two-variable logic in a natural way to
contexts with relations of all arities. We survey properties of U1 and
investigate its relationship to description logics designed to accommodate
higher arity relations, with particular attention given to DLR_reg. We also
define a description logic version of a variant of U1 and prove a range of new
results concerning the expressivity of U1 and related logics
One-dimensional fragment of first-order logic
We introduce a novel decidable fragment of first-order logic. The fragment is
one-dimensional in the sense that quantification is limited to applications of
blocks of existential (universal) quantifiers such that at most one variable
remains free in the quantified formula. The fragment is closed under Boolean
operations, but additional restrictions (called uniformity conditions) apply to
combinations of atomic formulae with two or more variables. We argue that the
notions of one-dimensionality and uniformity together offer a novel perspective
on the robust decidability of modal logics. We also establish that minor
modifications to the restrictions of the syntax of the one-dimensional fragment
lead to undecidable formalisms. Namely, the two-dimensional and non-uniform
one-dimensional fragments are shown undecidable. Finally, we prove that with
regard to expressivity, the one-dimensional fragment is incomparable with both
the guarded negation fragment and two-variable logic with counting. Our proof
of the decidability of the one-dimensional fragment is based on a technique
involving a direct reduction to the monadic class of first-order logic. The
novel technique is itself of an independent mathematical interest
On the uniform one-dimensional fragment over ordered models
The uniform one-dimensional fragment U1 is a recently introduced extension of the two-variable fragment FO2. The logic U1 enables the use of relation symbols of all arities and thereby extends the scope of applications of FO2. In this thesis we show that the satisfiability and finite satisfiability problems of U1 over linearly ordered models are NExpTime-complete. The corresponding problems for FO2 are likewise NExpTime-complete, so the transition from FO2 to U1 in the ordered realm causes no increase in complexity. To contrast our results, we also establish that U1 with an unrestricted use of two built-in linear orders is undecidable
Critical scaling in standard biased random walks
The spatial coverage produced by a single discrete-time random walk, with
asymmetric jump probability and non-uniform steps, moving on an
infinite one-dimensional lattice is investigated. Analytical calculations are
complemented with Monte Carlo simulations. We show that, for appropriate step
sizes, the model displays a critical phenomenon, at . Its scaling
properties as well as the main features of the fragmented coverage occurring in
the vicinity of the critical point are shown. In particular, in the limit , the distribution of fragment lengths is scale-free, with nontrivial
exponents. Moreover, the spatial distribution of cracks (unvisited sites)
defines a fractal set over the spanned interval. Thus, from the perspective of
the covered territory, a very rich critical phenomenology is revealed in a
simple one-dimensional standard model.Comment: 4 pages, 4 figure
Collapse of Primordial Filamentary Clouds under Far-Ultraviolet Radiation
Collapse and fragmentation of primordial filamentary clouds under isotropic
dissociation radiation is investigated with one-dimensional hydrodynamical
calculations. We investigate the effect of dissociation photon on the
filamentary clouds with calculating non-equilibrium chemical reactions. With
the external radiation assumed to turn on when the filamentary cloud forms, the
filamentary cloud with low initial density ()
suffers photodissociation of hydrogen molecules. In such a case, since main
coolant is lost, temperature increases adiabatically enough to suppress
collapse. As a result, the filamentary cloud fragments into very massive clouds
(). On the other hand, the evolution of the filamentary
clouds with high initial density () is hardly
affected by the external radiation. This is because the filamentary cloud with
high initial density shields itself from the external radiation. It is found
that the external radiation increases fragment mass. This result is consistent
with previous results with one-zone models. It is also found that fragment mass
decreases owing to the external dissociation radiation in the case with
sufficiently large line mass.Comment: 26 pages, 15 figures, accepted by PAS
The fragmentation of expanding shells II: Thickness matters
We study analytically the development of gravitational instability in an
expanding shell having finite thickness. We consider three models for the
radial density profile of the shell: (i) an analytic uniform-density model,
(ii) a semi-analytic model obtained by numerical solution of the hydrostatic
equilibrium equation, and (iii) a 3D hydrodynamic simulation. We show that all
three profiles are in close agreement, and this allows us to use the first
model to describe fragments in the radial direction of the shell. We then use
non-linear equations describing the time-evolution of a uniform oblate spheroid
to derive the growth rates of shell fragments having different sizes. This
yields a dispersion relation which depends on the shell thickness, and hence on
the pressure confining the shell. We compare this dispersion relation with the
dispersion relation obtained using the standard thin-shell analysis, and show
that, if the confining pressure is low, only large fragments are unstable. On
the other hand, if the confining pressure is high, fragments smaller than
predicted by the thin-shell analysis become unstable. Finally, we compare the
new dispersion relation with the results of 3D hydrodynamic simulations, and
show that the two are in good agreement.Comment: 9 pages, 9 figures, accepted by MNRA
Analysis of simple 2-D and 3-D metal structures subjected to fragment impact
Theoretical methods were developed for predicting the large-deflection elastic-plastic transient structural responses of metal containment or deflector (C/D) structures to cope with rotor burst fragment impact attack. For two-dimensional C/D structures both, finite element and finite difference analysis methods were employed to analyze structural response produced by either prescribed transient loads or fragment impact. For the latter category, two time-wise step-by-step analysis procedures were devised to predict the structural responses resulting from a succession of fragment impacts: the collision force method (CFM) which utilizes an approximate prediction of the force applied to the attacked structure during fragment impact, and the collision imparted velocity method (CIVM) in which the impact-induced velocity increment acquired by a region of the impacted structure near the impact point is computed. The merits and limitations of these approaches are discussed. For the analysis of 3-d responses of C/D structures, only the CIVM approach was investigated
Recursive tilings and space-filling curves with little fragmentation
This paper defines the Arrwwid number of a recursive tiling (or space-filling
curve) as the smallest number w such that any ball Q can be covered by w tiles
(or curve sections) with total volume O(vol(Q)). Recursive tilings and
space-filling curves with low Arrwwid numbers can be applied to optimise disk,
memory or server access patterns when processing sets of points in
d-dimensional space. This paper presents recursive tilings and space-filling
curves with optimal Arrwwid numbers. For d >= 3, we see that regular cube
tilings and space-filling curves cannot have optimal Arrwwid number, and we see
how to construct alternatives with better Arrwwid numbers.Comment: Manuscript accompanying abstract in EuroCG 2010, including full
proofs, 20 figures, references, discussion et
Collapse and Fragmentation of Magnetized Cylindrical Clouds
Gravitational collapse of the cylindrical elongated cloud is studied by
numerical magnetohydrodynamical simulations. In the infinitely long cloud in
hydrostatic configuration, small perturbations grow by the gravitational
instability. The most unstable mode indicated by a linear perturbation theory
grows selectively even from a white noise. The growth rate agrees with that
calculated by the linear theory. First, the density-enhanced region has an
elongated shape, i.e., prolate spheroidal shape. As the collapse proceeds, the
high-density fragment begins to contract mainly along the symmetry axis.
Finally, a spherical core is formed in the non-magnetized cloud. In contrast,
an oblate spheroidal dense disk is formed in a cloud in which the magnetic
pressure is nearly equal to the thermal one. The radial size of the disk
becomes proportional to the initial characteristic density scale-height in the
r-direction. As the collapse proceeds, a slowly contracting dense part is
formed (approximately < 10% in mass) inside of the fast contracting disk. And
this is separated from other part of the disk whose inflow velocity is
accelerated as reaching the center of the core. From arguments on the Jeans
mass and the magnetic critical mass, it is concluded that the fragments formed
in a cylindrical elongated cloud can not be supported against the self- gravity
and it will eventually collapse.Comment: 20 pages, figures available upon request, LaTeX, NIGAST040
Krausz dimension and its generalizations in special graph classes
A {\it krausz -partition} of a graph is the partition of into
cliques, such that any vertex belongs to at most cliques and any two
cliques have at most vertices in common. The {\it -krausz} dimension
of the graph is the minimum number such that has a
krausz -partition. 1-krausz dimension is known and studied krausz
dimension of graph .
In this paper we prove, that the problem is polynomially
solvable for chordal graphs, thus partially solving the problem of P. Hlineny
and J. Kratochvil. We show, that the problem of finding -krausz dimension is
NP-hard for every , even if restricted to (1,2)-colorable graphs, but
the problem is polynomially solvable for -polar
graphs for every fixed
- …