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Irregular sampling, Toeplitz matrices, and the approximation of entire functions of exponential type
In many applications one seeks to recover an entire function of exponential type from its non-uniformly spaced samples. Whereas the mathematical theory usually addresses the question of when such a function in can be recovered, numerical methods operate with a finite-dimensional model. The numerical reconstruction or approximation of the original function amounts to the solution of a large linear system. We show that the solutions of a particularly efficient discrete model in which the data are fit by trigonometric polynomials converge to the solution of the original infinite-dimensional reconstruction problem. This legitimatizes the numerical computations and explains why the algorithms employed produce reasonable results. The main mathematical result is a new type of approximation theorem for entire functions of exponential type from a finite number of values. From another point of view our approach provides a new method for proving sampling theorems.
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Karlheinz Seeger 《Zeitschrift für Physik A Hadrons and Nuclei》1965,182(5):510-520
The microwave induced breakdown characteristic inn-type germanium at 4.2 °K has been observed and compared with the d.c. induced breakdown characteristic obtained from the same sample. The effective microwave breakdown field intensity is nearly equal to the field intensity observed in d.c. induced breakdown. However, in the breakdown region with conductivities greater than 0.1 ohm?1 cm?1 a relaxation effect was found and interpreted qualitatively as momentun relaxation. In the initial breakdown the relaxation time τ m is small,ω 2τ m 2 being ?1 whereω/2π=9·109s?1 is the microwave frequency. The relaxation time is determined by predominant neutral impurity scattering with at last 5·1014 impurities per cm3. This scattering mechanism becomes ineffective when the impurities are ionized by hot carriers. Ionized impurity scattering or acoustic phonon scattering will then be predominant with increased and energy dependent values of τ m . The increased phase shift between carriers and field causes a decreased energy transfer from the field to the carriers, an accordingly smaller ionization rate, and finally results in a nearly constant a.c. conductivity. The observed anisotropy of the breakdown field intensity is in qualitative agreement with the assumption that only carriers in “hot” valleys of the conduction band initiate the breakdown by impact ionization. The unsufficient quantitative agreement may be due to an inhomogeneity of doping which is suggested by comparing the values of the ohmic low temperature conductivity of the samples. 相似文献
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We introduce a new concept to describe the localization of frames. In our main
result we show that the frame operator preserves this localization and that the dual frame possesses
the same localization property. As an application we show that certain frames for Hilbert spaces
extend automatically to Banach frames. Using this abstract theory, we derive new results on the
construction of nonuniform Gabor frames and solve a problem about non-uniform sampling in
shift-invariant spaces. 相似文献
25.
Cyclometalated Iridium(III) and Rhodium(III) Complexes Containing Naphthyridine Ligands: Synthesis,Characterization and Biological Studies
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The synthesis, crystal structure, and biological activity of new bis‐cyclometalated compounds [M(ptpy)2(4‐chloro‐2‐methyl‐1,8‐naphthyridine)]PF6 [M = Rh ( 1 ); M = Ir ( 2 ); ptpy = 2‐(p‐tolyl)pyridinato] and [M(ptpy)2(2‐methyl‐1,8‐naphthyridine)]PF6 [M = Rh ( 3 ); M = Ir ( 4 )] are described. The new compounds were prepared by the reaction of [{M(μ‐Cl)(ptpy)2}2] (M = Rh, Ir) with the corresponding naphthyridine ligands. The molecular structures of compounds 1 , 3 , and 4 were confirmed by single‐crystal X‐ray diffraction studies. 相似文献
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We study the composition of time-frequency localization operators (wavepacket operators) and develop a symbolic calculus of
such operators on modulation spaces. The use of time-frequency methods (phase space methods) allows the use of rough symbols
of ultra-rapid growth in place of smooth symbols in the standard classes. As the main application it is shown that, in general,
a localization operator possesses the Fredholm property, and thus its range is closed in the target space. 相似文献
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