85,072 research outputs found
Compact-like abelian groups without non-trivial quasi-convex null sequences
In this paper, we study precompact abelian groups G that contain no sequence
{x_n} such that {0} \cup {\pm x_n : n \in N} is infinite and quasi-convex in G,
and x_n --> 0. We characterize groups with this property in the following
classes of groups:
(a) bounded precompact abelian groups;
(b) minimal abelian groups;
(c) totally minimal abelian groups;
(d) \omega-bounded abelian groups.
We also provide examples of minimal abelian groups with this property, and
show that there exists a minimal pseudocompact abelian group with the same
property; furthermore, under Martin's Axiom, the group may be chosen to be
countably compact minimal abelian.Comment: Final versio
Truth-value semantics and functional extensions for classical logic of partial terms based on equality
We develop a bottom-up approach to truth-value semantics for classical logic
of partial terms based on equality and apply it to prove the conservativity of
the addition of partial description and partial selection functions,
independently of any strictness assumption.Comment: 15 pages, to appear in the Notre Dame Journal of Formal Logi
Interval orders and reverse mathematics
We study the reverse mathematics of interval orders. We establish the logical
strength of the implications between various definitions of the notion of
interval order. We also consider the strength of different versions of the
characterization theorem for interval orders: a partial order is an interval
order if and only if it does not contain . We also study proper
interval orders and their characterization theorem: a partial order is a proper
interval order if and only if it contains neither nor .Comment: 21 pages; to appear in Notre Dame Journal of Formal Logic; minor
changes from the previous versio
Effective dynamics of self-gravitating extended objects
We introduce an effective Lagrangian which describes the classical and
semiclassical dynamics of spherically symmetric, self-gravitating objects that
may populate the Universe at large and small (Planck) scale. These include
wormholes, black holes and inflationary bubbles. We speculate that such objects
represent some possible modes of fluctuation in the primordial spacetime foam
out of which our universe was born. Several results obtained by different
methods are encompassed and reinterpreted by our effective approach. As an
example, we discuss: i) the gravitational nucleation coefficient for a pair of
Minkowski bubbles, and ii) the nucleation coefficient of an inflationary vacuum
bubble in a Minkowski backgroundComment: 13 pages, no figures, ReVTe
Loop Quantum Mechanics and the Fractal Structure of Quantum Spacetime
We discuss the relation between string quantization based on the Schild path
integral and the Nambu-Goto path integral. The equivalence between the two
approaches at the classical level is extended to the quantum level by a
saddle--point evaluation of the corresponding path integrals. A possible
relationship between M-Theory and the quantum mechanics of string loops is
pointed out. Then, within the framework of ``loop quantum mechanics'', we
confront the difficult question as to what exactly gives rise to the structure
of spacetime. We argue that the large scale properties of the string condensate
are responsible for the effective Riemannian geometry of classical spacetime.
On the other hand, near the Planck scale the condensate ``evaporates'', and
what is left behind is a ``vacuum'' characterized by an effective fractal
geometry.Comment: 19pag. ReVTeX, 1fig. Invited paper to appear in the special issue of
{\it Chaos, Solitons and Fractals} on ``Super strings, M,F,S,...Theory''
(M.S. El Naschie and C.Castro, ed
Hausdorff dimension of a quantum string
In the path integral formulation of quantum mechanics, Feynman and Hibbs
noted that the trajectory of a particle is continuous but nowhere
differentiable. We extend this result to the quantum mechanical path of a
relativistic string and find that the ``trajectory'', in this case, is a
fractal surface with Hausdorff dimension three. Depending on the resolution of
the detecting apparatus, the extra dimension is perceived as ``fuzziness'' of
the string world-surface. We give an interpretation of this phenomenon in terms
of a new form of the uncertainty principle for strings, and study the
transition from the smooth to the fractal phase.Comment: 18 pages, non figures, ReVTeX 3.0, in print on Phys.Rev.
Robust Rotation Synchronization via Low-rank and Sparse Matrix Decomposition
This paper deals with the rotation synchronization problem, which arises in
global registration of 3D point-sets and in structure from motion. The problem
is formulated in an unprecedented way as a "low-rank and sparse" matrix
decomposition that handles both outliers and missing data. A minimization
strategy, dubbed R-GoDec, is also proposed and evaluated experimentally against
state-of-the-art algorithms on simulated and real data. The results show that
R-GoDec is the fastest among the robust algorithms.Comment: The material contained in this paper is part of a manuscript
submitted to CVI
Search for Chargino-Neutralino Associated Production at the Fermilab Tevatron Collider
We have searched in collisions at = 1.8 TeV for events
with three charged leptons and missing transverse energy. In the Minimal
Supersymmetric Standard Model, we expect trilepton events from
chargino-neutralino (\chione \chitwo) pair production, with subsequent decay
into leptons. We observe no candidate , ,
or events in 106 pb integrated
luminosity. We present limits on the sum of the branching ratios times cross
section for the four channels: \sigma_{\chione\chitwo}\cdot
BR(\chione\chitwo\to 3\ell+X) 81.5 \mgev\sp and
M_\chitwo > 82.2 \mgev\sp for , ~\mgev\sp and
M_\squark= M_\gluino.Comment: 9 pages and 3 figure
PAMELA's measurements of geomagnetic cutoff variations during solar energetic particle events
Data from the PAMELA satellite experiment were used to measure the
geomagnetic cutoff for high-energy ( 80 MeV) protons during the solar
particle events on 2006 December 13 and 14. The variations of the cutoff
latitude as a function of rigidity were studied on relatively short timescales,
corresponding to single spacecraft orbits (about 94 minutes). Estimated cutoff
values were cross-checked with those obtained by means of a trajectory tracing
approach based on dynamical empirical modeling of the Earth's magnetosphere. We
find significant variations in the cutoff latitude, with a maximum suppression
of about 6 deg for 80 MeV protons during the main phase of the storm. The
observed reduction in the geomagnetic shielding and its temporal evolution were
compared with the changes in the magnetosphere configuration, investigating the
role of IMF, solar wind and geomagnetic (Kp, Dst and Sym-H indexes) variables
and their correlation with PAMELA cutoff results.Comment: Conference: The 34th International Cosmic Ray Conference (ICRC2015),
30 July - 6 August, 2015, The Hague, The Netherlands, Volume:
PoS(ICRC2015)28
Premelting of Thin Wires
Recent work has raised considerable interest on the nature of thin metallic
wires. We have investigated the melting behavior of thin cylindrical Pb wires
with the axis along a (110) direction, using molecular dynamics and a
well-tested many-body potential. We find that---in analogy with cluster
melting---the melting temperature of a wire with radius is lower
than that of a bulk solid, , by . Surface melting
effects, with formation of a thin skin of highly diffusive atoms at the wire
surface, is observed. The diffusivity is lower where the wire surface has a
flat, local (111) orientation, and higher at (110) and (100) rounded areas. The
possible relevance to recent results on non-rupturing thin necks between an STM
tip and a warm surface is addressed.Comment: 10 pages, 4 postscript figures are appended, RevTeX, SISSA Ref.
131/94/CM/S
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