27,847 research outputs found
Exact Post Model Selection Inference for Marginal Screening
We develop a framework for post model selection inference, via marginal
screening, in linear regression. At the core of this framework is a result that
characterizes the exact distribution of linear functions of the response ,
conditional on the model being selected (``condition on selection" framework).
This allows us to construct valid confidence intervals and hypothesis tests for
regression coefficients that account for the selection procedure. In contrast
to recent work in high-dimensional statistics, our results are exact
(non-asymptotic) and require no eigenvalue-like assumptions on the design
matrix . Furthermore, the computational cost of marginal regression,
constructing confidence intervals and hypothesis testing is negligible compared
to the cost of linear regression, thus making our methods particularly suitable
for extremely large datasets. Although we focus on marginal screening to
illustrate the applicability of the condition on selection framework, this
framework is much more broadly applicable. We show how to apply the proposed
framework to several other selection procedures including orthogonal matching
pursuit, non-negative least squares, and marginal screening+Lasso
Knowledge-based vision and simple visual machines
The vast majority of work in machine vision emphasizes the representation of perceived objects and events: it is these internal representations that incorporate the 'knowledge' in knowledge-based vision or form the 'models' in model-based vision. In this paper, we discuss simple machine vision systems developed by artificial evolution rather than traditional engineering design techniques, and note that the task of identifying internal representations within such systems is made difficult by the lack of an operational definition of representation at the causal mechanistic level. Consequently, we question the nature and indeed the existence of representations posited to be used within natural vision systems (i.e. animals). We conclude that representations argued for on a priori grounds by external observers of a particular vision system may well be illusory, and are at best place-holders for yet-to-be-identified causal mechanistic interactions. That is, applying the knowledge-based vision approach in the understanding of evolved systems (machines or animals) may well lead to theories and models that are internally consistent, computationally plausible, and entirely wrong
Comparing Probabilistic Models for Melodic Sequences
Modelling the real world complexity of music is a challenge for machine
learning. We address the task of modeling melodic sequences from the same music
genre. We perform a comparative analysis of two probabilistic models; a
Dirichlet Variable Length Markov Model (Dirichlet-VMM) and a Time Convolutional
Restricted Boltzmann Machine (TC-RBM). We show that the TC-RBM learns
descriptive music features, such as underlying chords and typical melody
transitions and dynamics. We assess the models for future prediction and
compare their performance to a VMM, which is the current state of the art in
melody generation. We show that both models perform significantly better than
the VMM, with the Dirichlet-VMM marginally outperforming the TC-RBM. Finally,
we evaluate the short order statistics of the models, using the
Kullback-Leibler divergence between test sequences and model samples, and show
that our proposed methods match the statistics of the music genre significantly
better than the VMM.Comment: in Proceedings of the ECML-PKDD 2011. Lecture Notes in Computer
Science, vol. 6913, pp. 289-304. Springer (2011
Asymmetry of localised states in a single quantum ring: polarization dependence of excitons and biexcitons
We performed spectroscopic studies of a single GaAs quantum ring with an
anisotropy in the rim height. The presence of an asymmetric localised state was
suggested by the adiabatic potential. The asymmetry was investigated in terms
of the polarization dependence of excitons and biexcitons, where a large energy
di erence (0.8 meV) in the exciton emission energy for perpendicular
polarizations was observed and the oscillator strengths were also compared
using the photoluminescence decay rate. For perpendicular polarizations the
biexciton exhibits twice the energy di erence seen for the exciton, a fact that
may be attributed to a possible change in the selection rules for the lowered
symmetry.Comment: accepted in Applied physics Letter
Mycorhizal diversity of epiphytic orchids in a hyperdiverse tropical forest: insights on temporal and life stage changes
Few studies have examined the diversity of fungal communities of epiphytic orchids from hyperdiverse tropical forests and its temporal dynamics. This study aims to describe the fungal diversity associated with three tropical, epiphytic orchid species from a Costa Rican hyperdiverse forest across a three year-period at the juvenile and adult stages, by applying standard Sanger sequencing methods.Universidad de Costa Rica/[814-B6-139]/UCR/Costa RicaUCR::VicerrectorÃa de Investigación::Unidades de Investigación::Ciencias Agroalimentarias::JardÃn Botánico Lankester (JBL
Excited exciton and biexciton localised states in a single quantum ring
We observe excited exciton and biexciton states of localised excitons in an
anisotropic quantum ring, where large polarisation asymmetry supports the
presence of a crescent-like localised structure. We also find that saturation
of the localised ground state exciton with increasing excitation can be
attributed to relatively fast dissociation of biexcitons (? 430 ps) compared to
slow relaxation from the excited state to the ground state (? 1000 ps). As no
significant excitonic Aharonov-Bohm oscillations occur up to 14 T, we conclude
that phase coherence around the rim is inhibited as a consequence of height
anisotropy in the quantum ring.Comment: 4 pages, 4 figure
Observation of Competing Order in a High- Superconductor with Femtosecond Optical Pulses
We present studies of the photoexcited quasiparticle dynamics in
TlBaCaCuO (Tl-2223) using femtosecond optical
techniques. Deep into the superconducting state (below 40 K), a dramatic change
occurs in the temporal dynamics associated with photoexcited quasiparticles
rejoining the condensate. This is suggestive of entry into a coexistence phase
which, as our analysis reveals, opens a gap in the density of states (in
addition to the superconducting gap), and furthermore, competes with
superconductivity resulting in a depression of the superconducting gap.Comment: 5 pages, 3 figure
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