23 research outputs found
Greedy optimal control for elliptic problems and its application to turnpike problems
This is a post-peer-review, pre-copyedit version of an article published in Numerische Mathematik. The final authenticated version is available online at: https://doi.org/10.1007/s00211-018-1005-zWe adapt and apply greedy methods to approximate in an efficient way the optimal controls for parameterized elliptic control problems. Our results yield an optimal approximation procedure that, in particular, performs better than simply sampling the parameter-space to compute controls for each parameter value. The same method can be adapted for parabolic control problems, but this leads to greedy selections of the realizations of the parameters that depend on the initial datum under consideration. The turnpike property (which ensures that parabolic optimal control problems behave nearly in a static manner when the control horizon is long enough) allows using the elliptic greedy choice of the parameters in the parabolic setting too. We present various numerical experiments and an extensive discussion of the efficiency of our methodology for parabolic control and indicate a number of open problems arising when analyzing the convergence of the proposed algorithmsThis project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 694126-DyCon). Part of this research was done while the second author visited DeustoTech and Univesity of Deusto with the support of the DyCon project. The second author was also partially supported by Croatian Science Foundation under ConDyS Project, IP-2016-06-2468. The work of the third author was partially supported by the Grants MTM2014-52347, MTM2017-92996 of MINECO (Spain) and ICON of the French AN
The Qitai Radio Telescope
This study presents a general outline of the Qitai radio telescope (QTT)
project. Qitai, the site of the telescope, is a county of Xinjiang Uygur
Autonomous Region of China, located in the east Tianshan Mountains at an
elevation of about 1800 m. The QTT is a fully steerable, Gregorian type
telescope with a standard parabolic main reflector of 110 m diameter. The QTT
has adopted an um-brella support, homology-symmetric lightweight design. The
main reflector is active so that the deformation caused by gravity can be
corrected. The structural design aims to ultimately allow high-sensitivity
observations from 150 MHz up to 115 GHz. To satisfy the requirements for early
scientific goals, the QTT will be equipped with ultra-wideband receivers and
large field-of-view mul-ti-beam receivers. A multi-function signal-processing
system based on RFSoC and GPU processor chips will be developed. These will
enable the QTT to operate in pulsar, spectral line, continuum and Very Long
Baseline Interferometer (VLBI) observing modes. Electromagnetic compatibility
(EMC) and radio frequency interference (RFI) control techniques are adopted
throughout the system design. The QTT will form a world-class observational
platform for the detection of low-frequency (nanoHertz) gravitational waves
through pulsar timing array (PTA) techniques, pulsar surveys, the discovery of
binary black-hole systems, and exploring dark matter and the origin of life in
the universe.Comment: 12 pages, 11 figures, accepted for publication in Science China
Physics, Mechanics & Astronom
Aviation effects on already-existing cirrus clouds.
Determining the effects of the formation of contrails within natural cirrus clouds has proven to be challenging. Quantifying any such effects is necessary if we are to properly account for the influence of aviation on climate. Here we quantify the effect of aircraft on the optical thickness of already-existing cirrus clouds by matching actual aircraft flight tracks to satellite lidar measurements. We show that there is a systematic, statistically significant increase in normalized cirrus cloud optical thickness inside mid-latitude flight tracks compared with adjacent areas immediately outside the tracks
Accelerating GW calculations with optimal polarizability basis
We present a method for accelerating GW quasi-particle (QP) calculations. This is achieved through the introduction of optimal basis sets for representing polarizability matrices. First the real-space products of Wannier like orbitals are constructed and then optimal basis sets are obtained through singular value decomposition. Our method is validated by calculating the vertical ionization energies of the benzene molecule and the band structure of crystalline silicon. Its potentialities are illustrated by calculating the QP spectrum of a model structure of vitreous silica. Finally, we apply our method for studying the electronic structure properties of a model of quasi-stoichiometric amorphous silicon nitride and of its point defects