249 research outputs found
Honest adaptive confidence bands and self-similar functions
Confidence bands are confidence sets for an unknown function f, containing
all functions within some sup-norm distance of an estimator. In the density
estimation, regression, and white noise models, we consider the problem of
constructing adaptive confidence bands, whose width contracts at an optimal
rate over a range of H\"older classes.
While adaptive estimators exist, in general adaptive confidence bands do not,
and to proceed we must place further conditions on f. We discuss previous
approaches to this issue, and show it is necessary to restrict f to
fundamentally smaller classes of functions.
We then consider the self-similar functions, whose H\"older norm is similar
at large and small scales. We show that such functions may be considered
typical functions of a given H\"older class, and that the assumption of
self-similarity is both necessary and sufficient for the construction of
adaptive bands. Finally, we show that this assumption allows us to resolve the
problem of undersmoothing, creating bands which are honest simultaneously for
functions of any H\"older norm
Spatially-adaptive sensing in nonparametric regression
While adaptive sensing has provided improved rates of convergence in sparse
regression and classification, results in nonparametric regression have so far
been restricted to quite specific classes of functions. In this paper, we
describe an adaptive-sensing algorithm which is applicable to general
nonparametric-regression problems. The algorithm is spatially adaptive, and
achieves improved rates of convergence over spatially inhomogeneous functions.
Over standard function classes, it likewise retains the spatial adaptivity
properties of a uniform design
Near-optimal estimation of jump activity in semimartingales
In quantitative finance, we often model asset prices as semimartingales, with
drift, diffusion and jump components. The jump activity index measures the
strength of the jumps at high frequencies, and is of interest both in model
selection and fitting, and in volatility estimation. In this paper, we give a
novel estimate of the jump activity, together with corresponding confidence
intervals. Our estimate improves upon previous work, achieving near-optimal
rates of convergence, and good finite-sample performance in Monte-Carlo
experiments.The author acknowledges the EPSRC for their support under grant EP/K000993/1.This is the final version of the article. It was first available from Institute of Mathematical Statistics via http://dx.doi.org/10.1214/15-AOS134
A Smirnov-Bickel-Rosenblatt theorem for compactly-supported wavelets
In nonparametric statistical problems, we wish to find an estimator of an
unknown function f. We can split its error into bias and variance terms;
Smirnov, Bickel and Rosenblatt have shown that, for a histogram or kernel
estimate, the supremum norm of the variance term is asymptotically distributed
as a Gumbel random variable. In the following, we prove a version of this
result for estimators using compactly-supported wavelets, a popular tool in
nonparametric statistics. Our result relies on an assumption on the nature of
the wavelet, which must be verified by provably-good numerical approximations.
We verify our assumption for Daubechies wavelets and symlets, with N = 6, ...,
20 vanishing moments; larger values of N, and other wavelet bases, are easily
checked, and we conjecture that our assumption holds also in those cases
Long-wavelength TCF-based fluorescence probes for the detection and intracellular imaging of biological thiols
Two âturn onâ TCF-based fluorescence probes were developed for the detection of biological thiols (TCF-GSH and TCFCl-GSH). TCF-GSH was shown to have a high sensitivity towards glutathione (GSH) with a 0.28 ÎŒM limit of detection. Unfortunately, at higher GSH concentrations the fluorescence intensity of TCF-GSH decreased and toxicity was observed for TCF-GSH in live cells. However, TCFCl-GSH was shown to be able to detect GSH at biologically relevant concentrations with a 0.45 ÎŒM limit of detection. No toxicity was found for TCFCl-GSH and a clear âturn onâ with good photostability was observed for the exogenous addition of GSH, Cys and HCys. Furthermore, TCFCl-GSH was used to evaluate the effects of drug treatment on the levels of GSH in live cells
A simple umbelliferone based fluorescent probe for the detection of nitroreductase
A simple nitrobenzyl-umbelliferone (NCOU1) was synthesised containing a nitroreductase (NTR) trigger moiety. The presence of NTR, resulted in the fragmentation of the parent molecule and release of the highly emissive fluorophore umbelliferone via an NTR-catalyzed reduction of the nitro group. In the presence of the NTR enzyme, NCOU1 gave rise to a 5-fold increase in fluorescence intensity at 455 nm and was selective for NTR over other reductive enzymes. These results indicate that NCOU1 can be used as a simple assay for the detection of NTR
Polymer of Intrinsic Microporosity Induces Host-Guest Substrate Selectivity in Heterogeneous 4-Benzoyloxy-TEMPO-Catalysed Alcohol Oxidations
The free radical 4-benzoyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4B-TEMPO) is active as an electrocatalyst for primary alcohol oxidations when immobilised at an electrode surface and immersed into an aqueous carbonate buffer solution. In order to improve the catalytic process, a composite film electrode is developed based on (i) carbon microparticles of 2â12 ÎŒm diameter to enhance charge transport and (ii) a polymer of intrinsic microporosity (here PIM-EA-TB with a BET surface area of 1027 m2 gâ1). The latter acts as a highly rigid molecular framework for the embedded free radical catalyst with simultaneous access to aqueous phase and substrate. The resulting mechanism for the oxidation of primary alcohols is shown to switch in reaction order from first to zeroth with increasing substrate concentration consistent with a kinetically limited process with competing diffusion of charge at the polymer layer-electrode interface (here the âLEkâ case in Albery-Hillman notation). Reactivity optimisation and screening for a wider range of primary alcohols in conjunction with DFT-based relative reactivity correlation reveals substrate hydrophobicity as an important factor for enhancing catalytic currents. The PIM-EA-TB host matrix is proposed to control substrate partitioning and thereby catalyst reactivity and selectivity
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