77,710 research outputs found
On the Complexity and Performance of Parsing with Derivatives
Current algorithms for context-free parsing inflict a trade-off between ease
of understanding, ease of implementation, theoretical complexity, and practical
performance. No algorithm achieves all of these properties simultaneously.
Might et al. (2011) introduced parsing with derivatives, which handles
arbitrary context-free grammars while being both easy to understand and simple
to implement. Despite much initial enthusiasm and a multitude of independent
implementations, its worst-case complexity has never been proven to be better
than exponential. In fact, high-level arguments claiming it is fundamentally
exponential have been advanced and even accepted as part of the folklore.
Performance ended up being sluggish in practice, and this sluggishness was
taken as informal evidence of exponentiality.
In this paper, we reexamine the performance of parsing with derivatives. We
have discovered that it is not exponential but, in fact, cubic. Moreover,
simple (though perhaps not obvious) modifications to the implementation by
Might et al. (2011) lead to an implementation that is not only easy to
understand but also highly performant in practice.Comment: 13 pages; 12 figures; implementation at
http://bitbucket.org/ucombinator/parsing-with-derivatives/ ; published in
PLDI '16, Proceedings of the 37th ACM SIGPLAN Conference on Programming
Language Design and Implementation, June 13 - 17, 2016, Santa Barbara, CA,
US
A General Framework for the Derivation of Regular Expressions
The aim of this paper is to design a theoretical framework that allows us to
perform the computation of regular expression derivatives through a space of
generic structures. Thanks to this formalism, the main properties of regular
expression derivation, such as the finiteness of the set of derivatives, need
only be stated and proved one time, at the top level. Moreover, it is shown how
to construct an alternating automaton associated with the derivation of a
regular expression in this general framework. Finally, Brzozowski's derivation
and Antimirov's derivation turn out to be a particular case of this general
scheme and it is shown how to construct a DFA, a NFA and an AFA for both of
these derivations.Comment: 22 page
Efficient Dynamic Access Analysis Using JavaScript Proxies
JSConTest introduced the notions of effect monitoring and dynamic effect
inference for JavaScript. It enables the description of effects with path
specifications resembling regular expressions. It is implemented by an offline
source code transformation.
To overcome the limitations of the JSConTest implementation, we redesigned
and reimplemented effect monitoring by taking advantange of JavaScript proxies.
Our new design avoids all drawbacks of the prior implementation. It guarantees
full interposition; it is not restricted to a subset of JavaScript; it is
self-maintaining; and its scalability to large programs is significantly better
than with JSConTest.
The improved scalability has two sources. First, the reimplementation is
significantly faster than the original, transformation-based implementation.
Second, the reimplementation relies on the fly-weight pattern and on trace
reduction to conserve memory. Only the combination of these techniques enables
monitoring and inference for large programs.Comment: Technical Repor
Distributions generated by perturbation of symmetry with emphasis on a multivariate skew distribution
A fairly general procedure is studied to perturbate a multivariate density
satisfying a weak form of multivariate symmetry, and to generate a whole set of
non-symmetric densities. The approach is general enough to encompass a number
of recent proposals in the literature, variously related to the skew normal
distribution. The special case of skew elliptical densities is examined in
detail, establishing connections with existing similar work. The final part of
the paper specializes further to a form of multivariate skew density.
Likelihood inference for this distribution is examined, and it is illustrated
with numerical examples.Comment: full-length version of the published paper, 31 pages with 9 figure
Investigating the functionality of an OCT4-short response element in human induced pluripotent stem cells.
Pluripotent stem cells offer great therapeutic promise for personalized treatment platforms for numerous injuries, disorders, and diseases. Octamer-binding transcription factor 4 (OCT4) is a key regulatory gene maintaining pluripotency and self-renewal of mammalian cells. With site-specific integration for gene correction in cellular therapeutics, use of the OCT4 promoter may have advantages when expressing a suicide gene if pluripotency remains. However, the human OCT4 promoter region is 4 kb in size, limiting the capacity of therapeutic genes and other regulatory components for viral vectors, and decreasing the efficiency of homologous recombination. The purpose of this investigation was to characterize the functionality of a novel 967bp OCT4-short response element during pluripotency and to examine the OCT4 titer-dependent response during differentiation to human derivatives not expressing OCT4. Our findings demonstrate that the OCT4-short response element is active in pluripotency and this activity is in high correlation with transgene expression in vitro, and the OCT4-short response element is inactivated when pluripotent cells differentiate. These studies demonstrate that this shortened OCT4 regulatory element is functional and may be useful as part of an optimized safety component in a site-specific gene transferring system that could be used as an efficient and clinically applicable safety platform for gene transfer in cellular therapeutics
Singular potentials and annihilation
We discuss the regularization of attractive singular potentials , by infinitesimal imaginary addition to interaction
constant . Such a procedure enables unique
definition of scattering observables and is equal to an absorption (creation)
of particles in the origin. It is shown, that suggested regularization is an
analytical continuation of the scattering amplitudes of repulsive singular
potential in interaction constant . The nearthreshold properties of
regularized in a mentioned way singular potential are examined. We obtain
expressions for the scattering lengths, which turn to be complex even for
infinitesimal imaginary part of interaction constant. The problem of
perturbation of nearthreshold states of regular potential by a singular one is
treated, the expressions for level shifts and widths are obtained. We show,
that the physical sense of suggested regularization is that the scattering
observables are insensitive to any details of the short range modification of
singular potential, if there exists sufficiently strong inelastic short range
interaction. In this case the scattering observables are determined by
solutions of Schrodinger equation with regularized potential . We point out that the developed formalism can be applied for the
description of systems with short range annihilation, in particular low energy
nucleon-antinucleon scattering.Comment: 10 page
Euclidean spinor Green's functions in the spacetime of a straight cosmic string
We determine generally the spinor Green's function and the twisted spinor
Green's function in an Euclidean space with a conical-type line singularity. In
particular, in the neighbourhood of the point source, we expree them as a sum
of the usual Euclidean spinor Green's functin and a regular term. In four
dimensions, we use these determinations to calculate the vacuum energy density
and the twisted one for a massless spinor field in the spacetime of a straight
cosmic string. In the Minkowski spacetime, we determine explicitly the vacuum
energy density for a massive twisted spinor field.Comment: 20 pages, latex, no figure
Local Casimir Effect for a Scalar Field in Presence of a Point Impurity
The Casimir effect for a scalar field in presence of delta-type potentials
has been investigated for a long time in the case of surface delta functions,
modelling semi-transparent boundaries. More recently Albeverio, Cacciapuoti,
Cognola, Spreafico and Zerbini [9,10,51] have considered some configurations
involving delta-type potentials concentrated at points of ; in
particular, the case with an isolated point singularity at the origin can be
formulated as a field theory on , with
self-adjoint boundary conditions at the origin for the Laplacian. However, the
above authors have discussed only global aspects of the Casimir effect,
focusing their attention on the vacuum expectation value (VEV) of the total
energy. In the present paper we analyze the local Casimir effect with a point
delta-type potential, computing the renormalized VEV of the stress-energy
tensor at any point of ; to this purpose
we follow the zeta regularization approach, in the formulation already employed
for different configurations in previous works of ours (see [29-31] and
references therein).Comment: 20 pages, 6 figures; the final version accepted for publication. In
the initial part of the paper, possible text overlaps with our previous works
arXiv:1104.4330, arXiv:1505.00711, arXiv:1505.01044, arXiv:1505.01651,
arXiv:1505.03276. These overlaps aim to make the present paper
self-contained, and do not involve the main result
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