1,315 research outputs found
Orthogonal polynomial kernels and canonical correlations for Dirichlet measures
We consider a multivariate version of the so-called Lancaster problem of
characterizing canonical correlation coefficients of symmetric bivariate
distributions with identical marginals and orthogonal polynomial expansions.
The marginal distributions examined in this paper are the Dirichlet and the
Dirichlet multinomial distribution, respectively, on the continuous and the
N-discrete d-dimensional simplex. Their infinite-dimensional limit
distributions, respectively, the Poisson-Dirichlet distribution and Ewens's
sampling formula, are considered as well. We study, in particular, the
possibility of mapping canonical correlations on the d-dimensional continuous
simplex (i) to canonical correlation sequences on the d+1-dimensional simplex
and/or (ii) to canonical correlations on the discrete simplex, and vice versa.
Driven by this motivation, the first half of the paper is devoted to providing
a full characterization and probabilistic interpretation of n-orthogonal
polynomial kernels (i.e., sums of products of orthogonal polynomials of the
same degree n) with respect to the mentioned marginal distributions. We
establish several identities and some integral representations which are
multivariate extensions of important results known for the case d=2 since the
1970s. These results, along with a common interpretation of the mentioned
kernels in terms of dependent Polya urns, are shown to be key features leading
to several non-trivial solutions to Lancaster's problem, many of which can be
extended naturally to the limit as .Comment: Published in at http://dx.doi.org/10.3150/11-BEJ403 the Bernoulli
(http://isi.cbs.nl/bernoulli/) by the International Statistical
Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm
Two qubit copying machine for economical quantum eavesdropping
We study the mapping which occurs when a single qubit in an arbitrary state
interacts with another qubit in a given, fixed state resulting in some unitary
transformation on the two qubit system which, in effect, makes two copies of
the first qubit. The general problem of the quality of the resulting copies is
discussed using a special representation, a generalization of the usual Schmidt
decomposition, of an arbitrary two-dimensional subspace of a tensor product of
two 2-dimensional Hilbert spaces. We exhibit quantum circuits which can
reproduce the results of any two qubit copying machine of this type. A simple
stochastic generalization (using a ``classical'' random signal) of the copying
machine is also considered. These copying machines provide simple embodiments
of previously proposed optimal eavesdropping schemes for the BB84 and B92
quantum cryptography protocols.Comment: Minor changes. 26 pages RevTex including 7 PS figure
Optimal Eavesdropping in Quantum Cryptography. II. Quantum Circuit
It is shown that the optimum strategy of the eavesdropper, as described in
the preceding paper, can be expressed in terms of a quantum circuit in a way
which makes it obvious why certain parameters take on particular values, and
why obtaining information in one basis gives rise to noise in the conjugate
basis.Comment: 7 pages, 1 figure, Latex, the second part of quant-ph/970103
Collaborative working within UK NHS secondary care and across sectors for COPD and the impact of peer review : qualitative findings from the UK National COPD Resources and Outcomes Project
Introduction: We investigated the effects on collaborative work within the UK National Health Service (NHS) of an intervention for service quality improvement: informal, structured, reciprocated, multidisciplinary peer review with feedback and action plans. The setting was care for chronic obstructive pulmonary disease (COPD).
Theory and methods: We analysed semi-structured interviews with 43 hospital respiratory consultants, nurses and general managers at 24 intervention and 11 control sites, as part of a UK randomised controlled study, the National COPD Resources and Outcomes Project (NCROP), using Scottâs conceptual framework for action (inter-organisational, intra-organisational, inter-professional and inter-individual). Three areas of care targeted by NCROP involved collaboration across primary and secondary care.
Results: Hospital respiratory department collaborations with commissioners and hospital managers varied. Analysis suggested that this is related to team responses to barriers. Clinicians in unsuccessful collaborations told âatrocity storiesâ of organisational, structural and professional barriers to service improvement. The others removed barriers by working with government and commissioner agendas to ensure continued involvement in patientsâ care. Multidisciplinary peer review facilitated collaboration between participants, enabling them to meet, reconcile differences and exchange ideas across boundaries.
Conclusions: The data come from the first randomised controlled trial of organisational peer review, adding to research into UK health service collaborative work, which has had a more restricted focus on inter-professional relations. NCROP peer review may only modestly improve collaboration but these data suggest it might be more effective than top-down exhortations to change when collaboration both across and within organisations is required
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