37,936 research outputs found
The formation of planetary disks and winds: an ultraviolet view
Planetary systems are angular momentum reservoirs generated during star
formation. This accretion process produces very powerful engines able to drive
the optical jets and the molecular outflows. A fraction of the engine energy is
released into heating thus the temperature of the engine ranges from the 3000K
of the inner disk material to the 10MK in the areas where magnetic reconnection
occurs. There are important unsolved problems concerning the nature of the
engine, its evolution and the impact of the engine in the chemical evolution of
the inner disk. Of special relevance is the understanding of the shear layer
between the stellar photosphere and the disk; this layer controls a significant
fraction of the magnetic field building up and the subsequent dissipative
processes ougth to be studied in the UV.
This contribution focus on describing the connections between 1 Myr old suns
and the Sun and the requirements for new UV instrumentation to address their
evolution during this period. Two types of observations are shown to be needed:
monitoring programmes and high resolution imaging down to, at least,
milliarsecond scales.Comment: Accepted for publication in Astrophysics and Space Science 9 figure
Multiple Testing and Variable Selection along Least Angle Regression's path
In this article, we investigate multiple testing and variable selection using
Least Angle Regression (LARS) algorithm in high dimensions under the Gaussian
noise assumption. LARS is known to produce a piecewise affine solutions path
with change points referred to as knots of the LARS path. The cornerstone of
the present work is the expression in closed form of the exact joint law of
K-uplets of knots conditional on the variables selected by LARS, namely the
so-called post-selection joint law of the LARS knots. Numerical experiments
demonstrate the perfect fit of our finding.
Our main contributions are three fold. First, we build testing procedures on
variables entering the model along the LARS path in the general design case
when the noise level can be unknown. This testing procedures are referred to as
the Generalized t-Spacing tests (GtSt) and we prove that they have exact
non-asymptotic level (i.e., Type I error is exactly controlled). In that way,
we extend a work from (Taylor et al., 2014) where the Spacing test works for
consecutive knots and known variance. Second, we introduce a new exact multiple
false negatives test after model selection in the general design case when the
noise level can be unknown. We prove that this testing procedure has exact
non-asymptotic level for general design and unknown noise level. Last, we give
an exact control of the false discovery rate (FDR) under orthogonal design
assumption. Monte-Carlo simulations and a real data experiment are provided to
illustrate our results in this case. Of independent interest, we introduce an
equivalent formulation of LARS algorithm based on a recursive function.Comment: 62 pages; new: FDR control and power comparison between Knockoff,
FCD, Slope and our proposed method; new: the introduction has been revised
and now present a synthetic presentation of the main results. We believe that
this introduction brings new insists compared to previous version
Some exact solutions of the Dirac equation
Exact analytic solutions are found to the Dirac equation for a combination of
Lorentz scalar and vector Coulombic potentials with additional non-Coulombic
parts. An appropriate linear combination of Lorentz scalar and vector
non-Coulombic potentials, with the scalar part dominating, can be chosen to
give exact analytic Dirac wave functions.Comment: 4 pages. No figures. Presented in Hadron 2000: International Workshop
on Hadron Physics, Caraguatatuba, SP, Brasil, April 200
A Rice method proof of the Null-Space Property over the Grassmannian
The Null-Space Property (NSP) is a necessary and sufficient condition for the
recovery of the largest coefficients of solutions to an under-determined system
of linear equations. Interestingly, this property governs also the success and
the failure of recent developments in high-dimensional statistics, signal
processing, error-correcting codes and the theory of polytopes. Although this
property is the keystone of -minimization techniques, it is an open
problem to derive a closed form for the phase transition on NSP. In this
article, we provide the first proof of NSP using random processes theory and
the Rice method. As a matter of fact, our analysis gives non-asymptotic bounds
for NSP with respect to unitarily invariant distributions. Furthermore, we
derive a simple sufficient condition for NSP.Comment: 18 Pages, some Figure
Science with the World Space Observatory - Ultraviolet
The World Space Observatory-Ultraviolet (WSO-UV) will provide access to the
UV range during the next decade. The instrumentation on board will allow to
carry out high resolution imaging, high sensitivity imaging, high resolution
(R~55000) spectroscopy and low resolution (R~2500) long slit spectroscopy. In
this contribution, we briefly outline some of the key science issues that
WSO-UV will address during its lifetime. Among them, of special interest are:
the study of galaxy formation and the intergalactic medium; the astronomical
engines; the Milky Way formation and evol ution, and the formation of the Solar
System and the atmospheres of extrasolar p lanets.Comment: Just one text file (aigomezdecastro.tex). To be published in the
proceeding of the conference: "New Quest in Stellar Astrophysics II: UV
properties of evolved stellar populations" held in Puerto Vallarta - Mexico,
in april 200
Of the significance of business relationships
The Industrial Network Theory aims to describe and explain the business relationships and networks in which the focal firm is deeply embedded. One of its major propositions is that business relationships somehow influence, to different extents and over time, the focal firm’s survival. This pertains to the diverse and time-varying significance of business relationships for the focal firm. It has often been implicitly sustained that such significance is strongly related to the role played by business relationships and consequently the relationship outcomes accruing to the focal firm. The logic underlying the relationship significance proposition is outwardly oriented, somewhat overlooking the focal firm’s inside and in particular the conspicuous influence of business relationships on what the focal firm does competently both within and across its vertical boundaries. Arguably, the (predominantly ‘functional’) network-based arguments currently advanced represent a necessary but not sufficient condition for relationship significance. This conceptual paper tentatively suggests that there may be missing a supplementary (essentially internal) explanation supported by Competence-based Theories of the Firm.Industrial Network Theory; relationship significance proposition
The strategic relevance of business relationships: a preliminary assessment
The ubiquitous contention within the Industrial Networks literature - that business relationships are one of the firm´s most important resources - has not been, in our viewpoint, thoroughly explored. Hence we argue that the ‘Resource-based View of the Firm’ (‘RBV’) may complement the network-based reasoning on the strategic relevance of business relationships. A theoretical framework is proposed – a competence-based view of the firm – which solves RBV´s terminological and inconsistency problems and, more importantly, assures compatibility with the network perspective´s assumptions. The possibility of cross-fertilizing the Industrial Networks and RBV theories seems not only real, but also conceptually profitable for both theoretical fields.Business Relationships, Industrial Networks, Resource-Based View of the Firm, Competence-Based View of the Firm
How is the relationship significance brought about? A critical realist approach
The markets-as-networks theorists contend, at least tacitly, the significance of business relationships for the focal firm – that is, business relationships contribute somewhat to the focal firm’s survival and growth. We do not deny the existence of significant business relationships but sustain, in contrast to the consensus within the Markets-as-Networks Theory, that relationship significance should not be a self-evident assumption. Significance cannot be a taken-for-granted property of each and every one of the focal firm’s business relationships. We adopt explicitly a critical realist position in this conceptual paper and claim that the relationship significance is an event of the business world, whose causes remain yet largely unidentified. Where the powers and liabilities of business relationships (i.e., their functions and dysfunctions) are put to work, inevitably under certain contingencies (namely the surrounding networks and markets), effects result for the focal firm (often benefits in excess of sacrifices, i.e., relationship value) and as a result the relationship significance is likely to be brought about. In addition, the relationship significance can result from the dual influence that business relationships have on a great part of the structure and powers and liabilities of the focal firm, i.e., its nature and scope respectively.Markets-as-Networks Theory, relationship significance, business relationships, focal firm, resources, competences, activities
Relationship significance: is it sufficiently explained?
The Industrial Networks Theory (cf. Axelsson and Easton, 1992, Hakansson and Snehota, 1995) sets out to describe and explain the business relationships and markets in which the focal firm is deeply embedded. One of its major propositions pertains to the (time-varying) significance of business relationships for the focal firm (Gadde et al., 2003), i.e., business relationships influence to some extent the focal firm’s survival. Such significance seems strongly related to the role played by business relationships and consequently the relationship outcomes accruing to the focal firm. The theoretical justification underlying this proposition is outwardly oriented, somewhat overlooking the inside of the focal firm - in particular the influence of business relationships on what the focal firm does competently within and across its boundaries. Arguably, the creation and appropriation of relationship value by the focal firm is a necessary but not sufficient condition for relationship significance. A supplementary (internal) explanation supported by Knowledge-based Theories of the Firm (e.g., see Kogut and Zander, 1992), we suggest, may be missing. Our aim here has been to intuitively pinpoint a theoretical flaw, further suggesting a feasible path for its solution.Industrial Networks Theory; relationship significance proposition; relationship functions, dysfunctions, benefits, sacrifices, and value
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