160 research outputs found
Marginal empirical likelihood and sure independence feature screening
We study a marginal empirical likelihood approach in scenarios when the
number of variables grows exponentially with the sample size. The marginal
empirical likelihood ratios as functions of the parameters of interest are
systematically examined, and we find that the marginal empirical likelihood
ratio evaluated at zero can be used to differentiate whether an explanatory
variable is contributing to a response variable or not. Based on this finding,
we propose a unified feature screening procedure for linear models and the
generalized linear models. Different from most existing feature screening
approaches that rely on the magnitudes of some marginal estimators to identify
true signals, the proposed screening approach is capable of further
incorporating the level of uncertainties of such estimators. Such a merit
inherits the self-studentization property of the empirical likelihood approach,
and extends the insights of existing feature screening methods. Moreover, we
show that our screening approach is less restrictive to distributional
assumptions, and can be conveniently adapted to be applied in a broad range of
scenarios such as models specified using general moment conditions. Our
theoretical results and extensive numerical examples by simulations and data
analysis demonstrate the merits of the marginal empirical likelihood approach.Comment: Published in at http://dx.doi.org/10.1214/13-AOS1139 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Cutting the traintracks: Cauchy, Schubert and Calabi-Yau
In this note we revisit the maximal-codimension residues, or leading
singularities, of four-dimensional -loop traintrack integrals with massive
legs, both in Feynman parameter space and in momentum (twistor) space. We
identify a class of "half traintracks" as the most general degenerations of
traintracks with conventional (0-form) leading singularities, although the
integrals themselves still have rigidity due to
lower-loop "full traintrack'' subtopologies. As a warm-up exercise, we derive
closed-form expressions for their leading singularities both via (Cauchy's)
residues in Feynman parameters, and more geometrically using the so-called
Schubert problems in momentum twistor space. For -loop full traintracks, we
compute their leading singularities as integrals of -forms, which
proves that the rigidity is as expected; the form is given by an
inverse square root of an irreducible polynomial quartic with respect to each
variable, which characterizes an -dim Calabi-Yau manifold (elliptic
curve, K3 surface, etc.) for any . We also briefly comment on the
implications for the "symbology" of these traintrack integrals.Comment: refs updated; 36 pages, 12 figure
Miniaturized Power Electronic Interfaces for Ultra-compact Electromechanical Systems
Advanced and ultra-compact electromechanical (EM) systems, such as kinetic energy harvesting and microrobotic systems are deemed as enabling solutions to provide efficient energy conversion. One of the most critical challenges in such systems is to develop tiny power electronic interfaces (PEIs) capable of addressing power conditioning between EM devices and energy storage units. This dissertation presents technologies and topological solutions toward fabricating miniaturized PEIs to efficiently regulate erratic power/voltage for kinetic energy harvesting and drive high-voltage actuators for microrobotic systems. High-frequency resonant-switching topologies are introduced as power stages of PEIs that allow small footprint of the circuit without suffering from switching losses. Two types of bridgeless resonant ac-dc converters are first introduced and developed to efficiently convert arbitrary input voltages into a regulated dc output voltage. The proposed topologies provide direct ac-dc power conversion with less number of components, in comparison to other resonant topologies. A 5-mm×6-mm, 100-mg, 2-MHz and 650-mW prototype is fabricated for validation of capability of converting very-low ac voltages into a relatively higher voltage. A resonant gate drive circuit is designed and utilized to further reduce gating losses under high-frequency switching and light-load condition. The closed-loop efficiency reaches higher than 70% across wide range of input voltages and output powers. In a multi-channel energy harvesting system, a multi-input bridgeless resonant ac-dc converter is developed to achieve ac-dc conversion, step up voltage and match optimal impedance. Alternating voltage of each energy harvesting channel is stepped up through the switching LC network and then rectified by a freewheeling diode. The optimal electrical impedance can be adjusted through resonance impedance matching and pulse-frequency-modulation (PFM) control. In addition, a six-input standalone prototype is fabricated to address power conditioning for a six-channel wind panel. Furthermore, the concepts of miniaturization are incorporated in the context of microrobots. In a mobile microrobotic system, conventional bulky power supplies and electronics used to drive electroactive polymer (EAP) actuators are not practical as on-board energy sources for microrobots. A bidirectional single-stage resonant dc-dc step-up converter is introduced and developed to efficiently drive high-voltage EAP actuators. The converter utilizes resonant capacitors and a coupled-inductor as a soft-switched LC network to step up low input voltages. The circuit is capable of generating explicit high-voltage actuation signals, with capability of recovering unused energy from EAP actuators. A 4-mm × 8-mm, 100-mg and 600-mW prototype has been designed and fabricated to drive an in-plane gap-closing electrostatic inchworm motor. Experimental validations have been carried out to verify the circuit’s ability to step up voltage from 2 V to 100 V and generate two 1-kHz, 100-V driving voltages at 2-nF capacitive loads
On constructibility of AdS supergluon amplitudes
We prove that all tree-level -point supergluon (scalar) amplitudes in
AdS can be recursively constructed, using factorization and flat-space
limit. Our method is greatly facilitated by a natural R-symmetry basis for
planar color-ordered amplitudes, which reduces the latter to "partial
amplitudes" with simpler pole structures and factorization properties. Given
the -point scalar amplitude, we first extract spinning amplitudes with
scalars and one gluon by imposing "gauge invariance", and then use a
special "no-gluon kinematics" to determine the -point scalar amplitude
completely (which in turn contains the -point single-gluon amplitude).
Explicit results of up to 8-point scalar amplitudes and up to 6-point
single-gluon amplitudes are included as supplemental materials.Comment: 5 pages, 4 figures, major revision from v2 including new ancillary
fil
Bootstrapping octagons in reduced kinematics from cluster algebras
Multi-loop scattering amplitudes/null polygonal Wilson loops in super-Yang-Mills are known to simplify significantly in reduced
kinematics, where external legs/edges lie in an dimensional subspace of
Minkowski spacetime (or boundary of the subspace). Since the edges
of a -gon with even and odd labels go along two different null directions,
the kinematics is reduced to two copies of . In the
simplest octagon case, we conjecture that all loop amplitudes and Feynman
integrals are given in terms of two overlapping functions (a special case
of two-dimensional harmonic polylogarithms): in addition to the letters of , there are two letters mixing
the two sectors but they never appear together in the same term; these are the
reduced version of four-mass-box algebraic letters. Evidence supporting our
conjecture includes all known octagon amplitudes as well as new computations of
multi-loop integrals in reduced kinematics. By leveraging this alphabet and
conditions on first and last entries, we initiate a bootstrap program in
reduced kinematics: within the remarkably simple space of overlapping
functions, we easily obtain octagon amplitudes up to two-loop NMHV and
three-loop MHV. We also briefly comment on the generalization to -gons in
terms of functions and beyond.Comment: 26 pages, several figures and tables, an ancilary fil
Development and application of a packer-type drilling-free liner hanger
AbstractIn liner cementing, the upper cement plug and inner components of a common hanger needs to be drilled out after cementing, which will result in a poor cementing quality or even gas leakage at the flare opening. Therefore, a new packer-type drilling-free liner hanger has been developed, and a hydraulic setting-control packer, a flexible drilling-free seal box, and an auxiliary bearing back-off mechanism that go with the line hanger have been designed at the same time. Specific operation procedures include: (1) run in the liner string to the designed depth, then fully circulate the drilling fluid, finally drop the ball. When the tripping ball gets into the seat, the pressure will go up to cut off the hanging control pin and set the hanger; (2) continue to hold the pressure and cut off the ball seat pin to form circulation; (3) trip in the drill pipe to exert pressure on the hanger, back off to release the hanger from the running tool; (4) lower the drill pipe plug upon the completion of cement injection, cut off the releasing control pin of hollow casing plug, and run down further to bump with the bumping assembly; (5) remove the cementing head and connect the kelly driver, hold pressure again, then slowly pull up the drill tools, exert hydraulic pressure on the setting hydraulic cylinder of the packer assembly to cut off the setting control pin and set the packer; and (6) pull up the tools to the flare opening and wash out excessive cement slurry by circulating to realize free drilling of the whole hole. The successful application of the liner hanger in 127Â mm diameter liner in Well BQ203-H1 indicates that the packer-type liner hanger has such advantages as easy hanging and back-off, accurate bumping, simple setting, and sound sealing performance
- …