171 research outputs found
Matrix probing and its conditioning
When a matrix A with n columns is known to be well approximated by a linear
combination of basis matrices B_1,..., B_p, we can apply A to a random vector
and solve a linear system to recover this linear combination. The same
technique can be used to recover an approximation to A^-1. A basic question is
whether this linear system is invertible and well-conditioned. In this paper,
we show that if the Gram matrix of the B_j's is sufficiently well-conditioned
and each B_j has a high numerical rank, then n {proportional} p log^2 n will
ensure that the linear system is well-conditioned with high probability. Our
main application is probing linear operators with smooth pseudodifferential
symbols such as the wave equation Hessian in seismic imaging. We demonstrate
numerically that matrix probing can also produce good preconditioners for
inverting elliptic operators in variable media
Riesz transforms on generalized Heisenberg groups and Riesz transforms
Let 1 < q < \infty. We prove that the Riesz transforms on a generalized Heisenberg group satisfy where , are respectively the dimensions of the first and second layer of the Lie algebra of . We prove similar inequalities on Schatten spaces , with dimension free constants, for Riesz transforms associated to commuting inner -derivations and a suitable substitute of the square function. An example is given by the derivations associated to commuting pairs of operators on a Hilbert space satisfying the canonical commutation relations [P
Structured Random Matrices
Random matrix theory is a well-developed area of probability theory that has
numerous connections with other areas of mathematics and its applications. Much
of the literature in this area is concerned with matrices that possess many
exact or approximate symmetries, such as matrices with i.i.d. entries, for
which precise analytic results and limit theorems are available. Much less well
understood are matrices that are endowed with an arbitrary structure, such as
sparse Wigner matrices or matrices whose entries possess a given variance
pattern. The challenge in investigating such structured random matrices is to
understand how the given structure of the matrix is reflected in its spectral
properties. This chapter reviews a number of recent results, methods, and open
problems in this direction, with a particular emphasis on sharp spectral norm
inequalities for Gaussian random matrices.Comment: 46 pages; to appear in IMA Volume "Discrete Structures: Analysis and
Applications" (Springer
Quasiparticle Andreev scattering in the fractional quantum Hall regime
The scattering of exotic quasiparticles may follow different rules than
electrons. In the fractional quantum Hall regime, a quantum point contact (QPC)
provides a source of quasiparticles with field effect selectable charges and
statistics, which can be scattered on an 'analyzer' QPC to investigate these
rules. Remarkably, for incident quasiparticles dissimilar to those naturally
transmitted across the analyzer, electrical conduction conserves neither the
nature nor the number of the quasiparticles. In contrast with standard elastic
scattering, theory predicts the emergence of a mechanism akin to the Andreev
reflection at a normal-superconductor interface. Here, we observe the predicted
Andreev-like reflection of an quasiparticle into a hole
accompanied by the transmission of an quasielectron. Combining shot noise
and cross-correlation measurements, we independently determine the charge of
the different particles and ascertain the coincidence of quasielectron and
fractional hole. The present work advances our understanding on the
unconventional behavior of fractional quasiparticles, with implications toward
the generation of novel quasi-particles/holes and non-local entanglements
Observing the universal screening of a Kondo impurity
The Kondo effect, deriving from a local magnetic impurity mediating
electron-electron interactions, constitutes a flourishing basis for
understanding a large variety of intricate many-body problems. Its experimental
implementation in tunable circuits has made possible important advances through
well-controlled investigations. However, these have mostly concerned transport
properties, whereas thermodynamic observations - notably the fundamental
measurement of the spin of the Kondo impurity - remain elusive in test-bed
circuits. Here, with a novel combination of a "charge" Kondo circuit with a
charge sensor, we directly observe the state of the impurity and its
progressive screening. We establish the universal renormalization flow from a
single free spin to a screened singlet, the associated reduction in the
magnetization, and the relationship between scaling Kondo temperature and
microscopic parameters. In our device, a Kondo pseudospin is realized by two
degenerate charge states of a metallic island, which we measure with a
non-invasive, capacitively coupled charge sensor. Such pseudospin probe of an
engineered Kondo system opens the way to the thermodynamic investigation of
many exotic quantum states, including the clear observation of Majorana zero
modes through their fractional entropy
User-friendly tail bounds for sums of random matrices
This paper presents new probability inequalities for sums of independent,
random, self-adjoint matrices. These results place simple and easily verifiable
hypotheses on the summands, and they deliver strong conclusions about the
large-deviation behavior of the maximum eigenvalue of the sum. Tail bounds for
the norm of a sum of random rectangular matrices follow as an immediate
corollary. The proof techniques also yield some information about matrix-valued
martingales.
In other words, this paper provides noncommutative generalizations of the
classical bounds associated with the names Azuma, Bennett, Bernstein, Chernoff,
Hoeffding, and McDiarmid. The matrix inequalities promise the same diversity of
application, ease of use, and strength of conclusion that have made the scalar
inequalities so valuable.Comment: Current paper is the version of record. The material on Freedman's
inequality has been moved to a separate note; other martingale bounds are
described in Caltech ACM Report 2011-0
Systematic stratospheric observations on the Antarctic continent at Dumont d'Urville
Results of different routine measurements performed in Dumont d'Urville (66 deg S, 140 deg E) since 1988 are presented. They include the seasonal variation of total ozone and NO2 as measured by a SAOZ UV-Visible spectrometer, Polar Stratospheric Cloud observations by a backscatter lidar and more recently, vertical ozone profiles by ECC sondes and ozone and aerosols stratospheric profiles by a DIAL lidar. The particular results of 1991 in relation with the volcanic events of Mount Pinatubo and Mount Hudson, and the position of the polar vortex over Dumont d'Urville are discussed
p53-dependent control of transactivation of the Pen2 promoter by presenilins
The senile plaques found in the brains of patients with Alzheimer's disease are mainly due to the accumulation of amyloid β-peptides (Aβ) that are liberated by γ-secretase, a high molecular weight complex including presenilins, PEN-2, APH-1 and nicastrin. The depletion of each of these proteins disrupts the complex assembly into a functional protease. Here, we describe another level of regulation of this multimeric protease. The depletion of both presenilins drastically reduces Pen2 mRNA levels and its promoter transactivation. Furthermore, overexpression of presenilin-1 lowers Pen2 promoter transactivation, a phenotype abolished by a double mutation known to prevent presenilin-dependent γ-secretase activity. PEN-2 expression is decreased by depletion of β-amyloid precursor protein (APP) and increased by the APP intracellular domain (AICD). We show that AICD and APP complement for Pen2 mRNA levels in APP/APLP1-2 knockout fibroblasts. Interestingly, overexpression of presenilin-2 greatly increases Pen2 promoter transactivation. The opposite effect triggered by both presenilins was reminiscent of our previous study, which showed that these two proteins elicit antagonistic effects on p53. Therefore, we examined the contribution of p53 on Pen2 transcription. Pen2 promoter transactivation, and Pen2 mRNA and protein levels were drastically reduced in p53–/– fibroblasts. Furthermore, PEN-2 expression could be rescued by p53 complementation in p53- and APP-deficient cells. Interestingly, PEN-2 expression was also reduced in p53-deficient mouse brain. Overall, our study describes a p53-dependent regulation of PEN-2 expression by other members of the γ-secretase complex, namely presenilins
Cellular Prion Protein Expression Is Not Regulated by the Alzheimer's Amyloid Precursor Protein Intracellular Domain
There is increasing evidence of molecular and cellular links between Alzheimer's disease (AD) and prion diseases. The cellular prion protein, PrPC, modulates the post-translational processing of the AD amyloid precursor protein (APP), through its inhibition of the β-secretase BACE1, and oligomers of amyloid-β bind to PrPC which may mediate amyloid-β neurotoxicity. In addition, the APP intracellular domain (AICD), which acts as a transcriptional regulator, has been reported to control the expression of PrPC. Through the use of transgenic mice, cell culture models and manipulation of APP expression and processing, this study aimed to clarify the role of AICD in regulating PrPC. Over-expression of the three major isoforms of human APP (APP695, APP751 and APP770) in cultured neuronal and non-neuronal cells had no effect on the level of endogenous PrPC. Furthermore, analysis of brain tissue from transgenic mice over-expressing either wild type or familial AD associated mutant human APP revealed unaltered PrPC levels. Knockdown of endogenous APP expression in cells by siRNA or inhibition of γ-secretase activity also had no effect on PrPC levels. Overall, we did not detect any significant difference in the expression of PrPC in any of the cell or animal-based paradigms considered, indicating that the control of cellular PrPC levels by AICD is not as straightforward as previously suggested
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