565,559 research outputs found
Proof-Relevant Resolution for Elaboration of Programming Languages
Proof-relevant resolution is a new variant of resolution in Horn-clause logic and its extensions. We propose proof-relevant resolution as a systematic approach to elaboration in programming languages that is close to formal specification and hence allows for analysis of semantics of the language. We demonstrate the approach on two case studies; we describe a novel, proof-relevant approach to type inference and term synthesis in dependently types languages and we show how proof-relevant resolution allows for analysis of inductive and coinductive soundness of type class resolution. We conclude by a discussion of overall contributions of our current work
Operational Semantics of Resolution and Productivity in Horn Clause Logic
This paper presents a study of operational and type-theoretic properties of
different resolution strategies in Horn clause logic. We distinguish four
different kinds of resolution: resolution by unification (SLD-resolution),
resolution by term-matching, the recently introduced structural resolution, and
partial (or lazy) resolution. We express them all uniformly as abstract
reduction systems, which allows us to undertake a thorough comparative analysis
of their properties. To match this small-step semantics, we propose to take
Howard's System H as a type-theoretic semantic counterpart. Using System H, we
interpret Horn formulas as types, and a derivation for a given formula as the
proof term inhabiting the type given by the formula. We prove soundness of
these abstract reduction systems relative to System H, and we show completeness
of SLD-resolution and structural resolution relative to System H. We identify
conditions under which structural resolution is operationally equivalent to
SLD-resolution. We show correspondence between term-matching resolution for
Horn clause programs without existential variables and term rewriting.Comment: Journal Formal Aspect of Computing, 201
Richardson Varieties Have Kawamata Log Terminal Singularities
Let be a Richardson variety in the full flag variety associated
to a symmetrizable Kac-Moody group . Recall that is the intersection
of the finite dimensional Schubert variety with the finite codimensional
opposite Schubert variety . We give an explicit \bQ-divisor on
and prove that the pair has Kawamata log terminal
singularities. In fact, is ample, which additionally
proves that is log Fano.
We first give a proof of our result in the finite case (i.e., in the case
when is a finite dimensional semisimple group) by a careful analysis of an
explicit resolution of singularities of (similar to the BSDH resolution
of the Schubert varieties). In the general Kac-Moody case, in the absence of an
explicit resolution of as above, we give a proof that relies on the
Frobenius splitting methods. In particular, we use Mathieu's result asserting
that the Richardson varieties are Frobenius split, and combine it with a result
of N. Hara and K.-I. Watanabe relating Frobenius splittings with log canonical
singularities.Comment: 15 pages, improved exposition and explanation. To appear in the
International Mathematics Research Notice
The Gravitational Lensing in the QSO 1208+10 from the Proximity Effect in its Lyman alpha Forest
The quasar Q1208+1011 (z_{em}=3.8) is the second highest redshift double
quasar ever detected. Several indications point toward it being a gravitational
lensed system, although a definitive proof is still lacking. We present new
evidence of its lensed nature based on the weakness of the ``proximity effect''
measured in the high resolution Lyman absorption spectrum of the QSO. A
luminosity amplification as large as 22 has been derived from this analysis.
Indications on the redshift of the lensing galaxy can be obtained from the
analysis of the intervening heavy element absorption systems discovered in the
QSO high resolution spectrum. On statistical and dynamical grounds a MgII
system present at z=1.13 appears as the most likely candidate for the lensing
galaxy. We compare the observed parameters with a simple isothermal model for
the lens to derive the properties of the lensing galaxy. The resulting
magnification factor is smaller, although marginally consistent with that
derived by the analysis of the proximity effect.Comment: 11 pages, 2 Postscript figures, ApJ in pres
Elemental surface analysis at ambient pressure by electron-induced x-ray fluorescence
The development of a portable surface elemental analysis tool, based on the excitation of characteristic x rays from samples at ambient pressure with a focused electron beam is described. This instrument relies on the use of a thin electron transmissive membrane to isolate the vacuum of the electron source from the ambient atmosphere. The major attributes of this instrument include rapid (several minutes) spectrum acquisition, nondestructive evaluation of elemental composition, no sample preparation, and high-to-medium (several hundreds µm) spatial resolution. The instrument proof-of-principle has been demonstrated in a laboratory setup by obtaining energy dispersive x-ray spectra from metal and mineral samples
Well-posedness of the plasma-vacuum interface problem
We consider the free boundary problem for the plasma-vacuum interface in
ideal compressible magnetohydrodynamics (MHD). In the plasma region the flow is
governed by the usual compressible MHD equations, while in the vacuum region we
consider the pre-Maxwell dynamics for the magnetic field. At the
free-interface, driven by the plasma velocity, the total pressure is continuous
and the magnetic field on both sides is tangent to the boundary. The
plasma-vacuum system is not isolated from the outside world, because of a given
surface current on the fixed boundary that forces oscillations.
Under a suitable stability condition satisfied at each point of the initial
interface, stating that the magnetic fields on either side of the interface are
not collinear, we show the existence and uniqueness of the solution to the
nonlinear plasma-vacuum interface problem in suitable anisotropic Sobolev
spaces. The proof is based on the results proved in the companion paper
arXiv:1112.3101, about the well-posedness of the homogeneous linearized problem
and the proof of a basic a priori energy estimate. The proof of the resolution
of the nonlinear problem given in the present paper follows from the analysis
of the elliptic system for the vacuum magnetic field, a suitable tame estimate
in Sobolev spaces for the full linearized equations, and a Nash-Moser
iteration.Comment: 58 page
The Singular Supports of IC sheaves on Quasimaps' Spaces are Irreducible
Let be a smooth projective curve of genus 0. Let be the variety of
complete flags in an -dimensional vector space . Given an -tuple
of positive integers one can consider the space of
algebraic maps of degree from to . This space admits some
remarkable compactifications (Quasimaps),
(Quasiflags) constructed by Drinfeld and Laumon respectively. In [Kuznetsov] it
was proved that the natural map is a small
resolution of singularities. The aim of the present note is to study the
singular support of the Goresky-MacPherson sheaf on the Quasimaps'
space . Namely, we prove that this singular support
is irreducible. The proof is based on the factorization property of Quasimaps'
space and on the detailed analysis of Laumon's resolution .Comment: 8 pages, AmsLatex 1.
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