920 research outputs found
Intermittent accreting millisecond pulsars: light houses with broken lamps?
Intermittent accreting millisecond X-ray pulsars are an exciting new type of
sources. Their pulsations appear and disappear either on timescales of hundreds
of seconds or on timescales of days. The study of these sources add new
observational constraints to present models that explain the presence or not of
pulsations in neutron star LMXBs. In this paper we present preliminary results
on spectral and aperiodic variability studies of all intermittent AMSPs, with a
particular focus on the comparison between pulsating and non pulsating periods.Comment: 4 pages, 2 figures; to appear in the proceedings of the workshop "A
Decade of Accreting Millisecond X-ray Pulsars", Amsterdam, April 2008, eds.
R. Wijnands et al. (AIP Conf. Proc.
Towards the deformation quantization of linearized gravity
We present a first attempt to apply the approach of deformation quantization
to linearized Einstein's equations. We use the analogy with Maxwell equations
to derive the field equations of linearized gravity from a modified Maxwell
Lagrangian which allows the construction of a Hamiltonian in the standard way.
The deformation quantization procedure for free fields is applied to this
Hamiltonian. As a result we obtain the complete set of quantum states and its
discrete spectrum.Comment: 13 pages, no figures **preliminary entry **
A New Symmetric Expression of Weyl Ordering
For the creation operator \adag and the annihilation operator of a
harmonic oscillator, we consider Weyl ordering expression of (\adag a)^n and
obtain a new symmetric expression of Weyl ordering w.r.t. \adag a \equiv N
and a\adag =N+1 where is the number operator. Moreover, we interpret
intertwining formulas of various orderings in view of the difference theory.
Then we find that the noncommutative parameter corresponds to the increment of
the difference operator w.r.t. variable . Therefore, quantum
(noncommutative) calculations of harmonic oscillators are done by classical
(commutative) ones of the number operator by using the difference theory. As a
by-product, nontrivial relations including the Stirling number of the first
kind are also obtained.Comment: 15 pages, Latex2e, the title before replacement is "Orderings of
Operators in Quantum Physics", new proofs by using a difference operator
added, some references added, to appear in Modern Physics Letters
Meadows and the equational specification of division
The rational, real and complex numbers with their standard operations,
including division, are partial algebras specified by the axiomatic concept of
a field. Since the class of fields cannot be defined by equations, the theory
of equational specifications of data types cannot use field theory in
applications to number systems based upon rational, real and complex numbers.
We study a new axiomatic concept for number systems with division that uses
only equations: a meadow is a commutative ring with a total inverse operator
satisfying two equations which imply that the inverse of zero is zero. All
fields and products of fields can be viewed as meadows. After reviewing
alternate axioms for inverse, we start the development of a theory of meadows.
We give a general representation theorem for meadows and find, as a corollary,
that the conditional equational theory of meadows coincides with the
conditional equational theory of zero totalized fields. We also prove
representation results for meadows of finite characteristic
Partially-commutative context-free languages
The paper is about a class of languages that extends context-free languages
(CFL) and is stable under shuffle. Specifically, we investigate the class of
partially-commutative context-free languages (PCCFL), where non-terminal
symbols are commutative according to a binary independence relation, very much
like in trace theory. The class has been recently proposed as a robust class
subsuming CFL and commutative CFL. This paper surveys properties of PCCFL. We
identify a natural corresponding automaton model: stateless multi-pushdown
automata. We show stability of the class under natural operations, including
homomorphic images and shuffle. Finally, we relate expressiveness of PCCFL to
two other relevant classes: CFL extended with shuffle and trace-closures of
CFL. Among technical contributions of the paper are pumping lemmas, as an
elegant completion of known pumping properties of regular languages, CFL and
commutative CFL.Comment: In Proceedings EXPRESS/SOS 2012, arXiv:1208.244
Classroom Demonstrations: Learning Tools Or Entertainment?
We compared student learning from different modes of presenting classroom demonstrations to determine how much students learn from traditionally presented demonstrations, and whether learning can be enhanced by simply changing the mode of presentation to increase student engagement. We find that students who passively observe demonstrations understand the underlying concepts no better than students who do not see the demonstration at all, in agreement with previous studies. Learning is enhanced, however, by increasing student engagement; students who predict the demonstration outcome before seeing it, however, display significantly greater understanding
Wigner functions, contact interactions, and matching
Quantum mechanics in phase space (or deformation quantization) appears to
fail as an autonomous quantum method when infinite potential walls are present.
The stationary physical Wigner functions do not satisfy the normal eigen
equations, the *-eigen equations, unless an ad hoc boundary potential is added
[Dias-Prata]. Alternatively, they satisfy a different, higher-order,
``*-eigen-* equation'', locally, i.e. away from the walls [Kryukov-Walton].
Here we show that this substitute equation can be written in a very simple
form, even in the presence of an additional, arbitrary, but regular potential.
The more general applicability of the -eigen- equation is then demonstrated.
First, using an idea from [Fairlie-Manogue], we extend it to a dynamical
equation describing time evolution. We then show that also for general contact
interactions, the -eigen- equation is satisfied locally. Specifically, we treat
the most general possible (Robin) boundary conditions at an infinite wall,
general one-dimensional point interactions, and a finite potential jump.
Finally, we examine a smooth potential, that has simple but different
expressions for x positive and negative. We find that the -eigen- equation is
again satisfied locally. It seems, therefore, that the -eigen- equation is
generally relevant to the matching of Wigner functions; it can be solved
piece-wise and its solutions then matched.Comment: 20 pages, no figure
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