8,598 research outputs found
Exploring student difficulties with observation location
Throughout introductory physics, students create and interpret free body
diagrams in which multiple forces act on an object, typically at a single
location (the object's center of mass). The situation increases in difficulty
when multiple objects are involved, and further when electric and magnetic
fields are present. In the latter, sources of the fields are often identified
as a set of electric charges or current-carrying wires, and students are asked
to determine the electric or magnetic field at a separate location defined as
the observation location. Previous research suggests students struggle with
accounting for how a measurement or calculation depends on the observation
location. We present preliminary results from a studio-style, algebra-based,
introductory electricity and magnetism course showing the prevalence of correct
and incorrect responses to questions about observation location by analyzing
student written work involving vector addition of fields
Post-Swift Gamma-ray Burst Science and Capabilities Needed to EXIST
The exhilerating results from Swift in its first year of operations have
opened a new era of exploration of the high energy universe. The surge to
higher redshifts of the Gamma-ray bursts now imaged with increased sensitivity
establishes them as viable cosmic probes of the early universe. Wide-field
coded aperture imaging with solid-state pixel detectors (Cd-Zn-Te) has been
also established as the optimum approach for GRB discovery and location as well
as to conduct sensitive full-sky hard X-ray sky surveys. I outline the current
and future major science questions likely to dominate the post-Swift era for
GRBs and several related disciplines and the mission requirements to tackle
these. The EXIST mission, under study for NASA's Black Hole Finder Probe (BHFP)
in the Beyond Einstein Program, could achieve these objectives as the Next
Generation GRB Mission with `ultimate' sensitivity and wide-field survey
capability. Analysis tools for processing Swift/BAT slew data are under
development at CfA and will both test EXIST scanning imaging and provide new
data on GRBs and transients.Comment: Invited review to appear in the Proceedings of the 16th Annual
October Astrophysics Conference in Maryland, "Gamma Ray Bursts in the Swift
Era", eds. S. Holt, N. Gehrels and J. Nousek; 11 pages, 5 figure
Principal Component Analysis for Functional Data on Riemannian Manifolds and Spheres
Functional data analysis on nonlinear manifolds has drawn recent interest.
Sphere-valued functional data, which are encountered for example as movement
trajectories on the surface of the earth, are an important special case. We
consider an intrinsic principal component analysis for smooth Riemannian
manifold-valued functional data and study its asymptotic properties. Riemannian
functional principal component analysis (RFPCA) is carried out by first mapping
the manifold-valued data through Riemannian logarithm maps to tangent spaces
around the time-varying Fr\'echet mean function, and then performing a
classical multivariate functional principal component analysis on the linear
tangent spaces. Representations of the Riemannian manifold-valued functions and
the eigenfunctions on the original manifold are then obtained with exponential
maps. The tangent-space approximation through functional principal component
analysis is shown to be well-behaved in terms of controlling the residual
variation if the Riemannian manifold has nonnegative curvature. Specifically,
we derive a central limit theorem for the mean function, as well as root-
uniform convergence rates for other model components, including the covariance
function, eigenfunctions, and functional principal component scores. Our
applications include a novel framework for the analysis of longitudinal
compositional data, achieved by mapping longitudinal compositional data to
trajectories on the sphere, illustrated with longitudinal fruit fly behavior
patterns. RFPCA is shown to be superior in terms of trajectory recovery in
comparison to an unrestricted functional principal component analysis in
applications and simulations and is also found to produce principal component
scores that are better predictors for classification compared to traditional
functional functional principal component scores
Single-Trial Phase Precession in the Hippocampus
During the crossing of the place field of a pyramidal cell in the rat hippocampus, the firing phase of the cell decreases with respect to the local theta rhythm. This phase precession is usually studied on the basis of data in which many place field traversals are pooled together. Here we study properties of phase precession in single trials. We found that single-trial and pooled-trial phase precession were different with respect to phase-position correlation, phase-time correlation, and phase range. Whereas pooled-trial phase precession may span 360°, the most frequent single-trial phase range was only ∼180°. In pooled trials, the correlation between phase and position (r = −0.58) was stronger than the correlation between phase and time (r = −0.27), whereas in single trials these correlations (r = −0.61 for both) were not significantly different. Next, we demonstrated that phase precession exhibited a large trial-to-trial variability. Overall, only a small fraction of the trial-to-trial variability in measures of phase precession (e.g., slope or offset) could be explained by other single-trial properties (such as running speed or firing rate), whereas the larger part of the variability remains to be explained. Finally, we found that surrogate single trials, created by randomly drawing spikes from the pooled data, are not equivalent to experimental single trials: pooling over trials therefore changes basic measures of phase precession. These findings indicate that single trials may be better suited for encoding temporally structured events than is suggested by the pooled data
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