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101.
Apart from serving as a parameter in describing the evolution of a system, time appears also as an observable property of
a system in experiments where one measures ‘the time of occurrence’ of an event associated with the system. However, while
the observables normally encountered in quantum theory (and characterized by self-adjoint operators or projection-valued measures)
correspond to instantaneous measurements, a time of occurrence measurement involves continuous observations being performed
on the system to monitor when the event occurs. It is argued that a time of occurrence observable should be represented by
a positive-operator-valued measure on the interval over which the experiment is carried out. It is shown that while the requirement
of time-translation invariance and the spectral condition rule out the possibility of a self-adjoint time operator (Pauli’s
theorem), they do allow for time of occurrence observables to be represented by suitable positive-operator-valued measures.
It is also shown that the uncertainty in the time of occurrence of an event satisfies the time-energy uncertainty relation
as a consequence of the time-translation invariance, only if the time of occurrence experiment is performed on the entire
time axis. 相似文献
102.
103.
M. D. Srinivas M. S. Sriram T. S. Shankara 《International Journal of Theoretical Physics》1972,5(4):263-273
Sudarshan's semi-classical treatment of correlation functions is applied to the study of quantum corrections to the Van Hove function. In its generalised form, it enables one to choose the best correlation function for a given potential. Higher-order correlations are also sketched briefly and the details are similar to those considered by Oppenheim and Bloom. 相似文献
104.
[Chemical reaction: see text] An elimination and stereoselective hydrogenation of alpha-D-glucoheptonic-gamma-lactone derivative has been applied to prepare a differentially protected anti,anti-1,3,5-triol system, the utility of which has been extended for the total synthesis of anti-fungal 1,3-polyol/alpha-pyrone natural products. 相似文献
105.
Several characteristics of fission accompanied by long range alpha particles (LRA) have been studied in the thermal neutron
induced fission of235U. The kinetic energies of fission fragments and the LRA were measured with a back-to-back ionization chamber and semiconductor
detectors respectively. The kinetic energies of the two fragments and the LRA in LRA fission, along with the energies of pair
fragments in the normal binary fissions, were recorded event by event on a magnetic tape by means of a four-parameter data
acquisition system. The data were analysed to study the dependence of different quantities in LRA fission on the fragment
mass ratio, LRA energy and the total kinetic energy of the fission fragments. It is seen that the most probable energy of
LRA increases significantly for near symmetric mass divisions. The total kinetic energy for all mass ratios in LRA fission
is found to be (2.6±0.7) MeV larger than that in binary fission. The difference in the total kinetic energies in LRA and binary
fissions is seen to be dependent on mass ratio. This result may suggest that the scission configuration in LRA fission is
different for different mass ratios. Correlations between the fission fragment and LRA energies have been studied for several
mass ratios. It is seen that the most probable fragment kinetic energyĒ
k varies nearly linearly with the LRA energyE
a for various mass divisions but the variation of the most probable LRA energyĒ
a with fragment kinetic energyE
k is found to deviate from linearity for several mass ratios. From a least square fit to the variation ofĒ
k withE
a it is found that the slope (dĒ
k/dEa) increases with the increase in mass ratio. The present results are discussed to arrive at a better understanding of the
scission configuration in the fission accompanied by LRA emission. 相似文献
106.
107.
Marten Van Dijk Dwaine Clarke Blaise Gassend G. Edward Suh Srinivas Devadas 《Designs, Codes and Cryptography》2006,39(2):253-273
We present protocols for speeding up fixed-base variable-exponent exponentiation and variable-base fixed-exponent exponentiation
using an untrusted computational resource. In the fixed-base protocols, the exponent may be blinded. In the variable-base
protocols, the base may be blinded. The protocols are described for exponentiation in a cyclic group. We describe how to extend
them to exponentiation modulo an integer where the modulus is the product of primes with single multiplicity. The protocols
provide a speedup of
over the square-and-multiply algorithm, where k is the bitlength of the exponent.
One application of the protocols is to speed up exponentiation-based verification in discrete log-based signature and credential
schemes. The protocols also allow signature verifiers to dynamically choose, for each message, the amount of work it would
like to perform to verify the signature. This results in a work-security tradeoff. We introduce a fifth protocol to perform
variable-base variable- exponent exponentiation, which also has this feature.
Our model allows the trusted resource to perform computations in its idle time. The protocols facilitate the offloading of
work to the offline stage, such that the work the trusted resource performs when it has to do an exponentiation is smaller.
Our protocols are unconditionally secure. 相似文献
108.
A microscale cyclical electrical field flow fractionation (CyElFFF) channel is characterized with regard to the effect of various operating parameters and comparison made to recent theoretical developments. Challenges associated with various operating conditions are reported along with some of the optimized operating parameters. The effect of retention wall choice, an offset voltage, relaxation steps, and flow rates, along with the basic operating parameters of voltage, frequency, and electrophoretic mobility are reported. Retention of polystyrene nanoparticle standards is accomplished and the first separations using this technique in a microscale system are also demonstrated. Relaxation steps and offset voltages are found to be effective in eliminating early peaks and in improving plate heights. Plate heights were also found to decrease with increasing flow rates, which is the opposite of the behavior seen in most existing chromatographic systems. The experimental results are compared to the analytical and empirical models of CyElFFF and found to be compatible. Suggestions are made for improving the separation and analysis methods used with CyElFFF. 相似文献
109.
Laursen JB Nielsen J Haack T Pusuluri S David S Balakrishna R Zeng Y Ma Z Doyle TB Mitscher LA 《Combinatorial chemistry & high throughput screening》2006,9(9):663-681
A synthetic reexamination of a series of ketodihydronicotinic acid class antibacterial agents was undertaken in an attempt to improve their therapeutic potential. A convenient new synthesis was developed involving hetero Diels-Alder chemistry producing 74 new analogs in a multiple parallel synthetic manner and these were examined in vitro for their antimicrobial potential. Several compounds demonstrated significant broad-spectrum activity against clinically derived bacterial strains but previously known 1-(2,4-difluorophenyl)-6-(4-dimethylaminophenyl)-4-pyridone-3-carboxylic acid (7) remained the most potent compound in this class. Cross-resistance with ciprofloxacin supported a commonality of mode of action. Permiabilization of Escherichia coli cells by polymyxin B significantly enhanced potency with these agents suggesting that poor cellular uptake was primarily responsible for the disappointing activity against bacteria that some of the analogs exhibited. 相似文献
110.
Algorithms for non-uniform size data placement on parallel disks 总被引:2,自引:0,他引:2
Srinivas Kashyap Samir Khuller 《Journal of Algorithms in Cognition, Informatics and Logic》2006,60(2):144-167
We study an optimization problem that arises in the context of data placement in a multimedia storage system. We are given a collection of M multimedia objects (data items) that need to be assigned to a storage system consisting of N disks d1,d2,…,dN. We are also given sets U1,U2,…,UM such that Ui is the set of clients seeking the ith data item. Data item i has size si. Each disk dj is characterized by two parameters, namely, its storage capacity Cj which indicates the maximum total size of data items that may be assigned to it, and a load capacity Lj which indicates the maximum number of clients that it can serve. The goal is to find a placement of data items to disks and an assignment of clients to disks so as to maximize the total number of clients served, subject to the capacity constraints of the storage system.We study this data placement problem for homogeneous storage systems where all the disks are identical. We assume that all disks have a storage capacity of k and a load capacity of L. Previous work on this problem has assumed that all data items have unit size, in other words si=1 for all i. Even for this case, the problem is NP-hard. For the case where si{1,…,Δ} for some constant Δ, we develop a polynomial time approximation scheme (PTAS). This result is obtained by developing two algorithms, one that works for constant k and one that works for arbitrary k. The algorithm for arbitrary k guarantees that a solution where at least -fraction of all clients are assigned to a disk (under certain assumptions). In addition we develop an algorithm for which we can prove tight bounds when si{1,2}. In fact, we can show that a -fraction of all clients can be assigned (under certain natural assumptions), regardless of the input distribution. 相似文献