从卫星STARLETTE的激光测距推得1980年8—10月的地球自转参数值

本文分析了在1980年8—10月,即地球自转短期联测(MERIT Short Campaign)期间,全世界13个测站对卫星STARLETTE进行激光测距所获得的631圈共54189个数据。由此推得了在这期间的极移两分量(x_p和y_p)以及日长的变化量(D=日长—86400秒)。经多项式拟合以后,所得解中,x_p的残差的均方值是0″.014;y_p的残差的均方值是0″.013;D的残差的均方值是0⁵.0005。     

A novel approach to rigid spheroid models in viscous flows using operator splitting methods

Numerical Algorithms - Calculating cost-effective solutions to particle dynamics in viscous flows is an important problem in many areas of industry and nature. We implement a second-order symmetric...     

Absolute reactivity of arylallyl carbocations

A series of α‐vinyl arylmethyl cations were generated and studied using nanosecond laser flash photolysis. Rate constants for the decay of the substituted α‐vinyl arylmethyl cations were determined under solvolytic conditions in pure solvents and solvent mixtures of 1,1,1,3,3,3‐hexafluoro‐2‐propanol (HFIP) and 2,2,2‐trifluoroethanol (TFE). In addition the absolute reactivity of the carbocations with added nucleophiles were obtained. The reactivities of the α‐vinyl arylmethyl cations were then compared to the reactivities of the corresponding α‐methyl, α‐phenyl, and α‐cyclopropyl arylmethyl cations. Hammett σ⁺ plots of each of the series of carbocations were obtained and the substituent effects on carbocation reactivity analyzed. These data show that the influence of substituent on the reactivity of the α‐vinyl carbocations was different from the substituents effects on the reactivity of the α‐methyl, α‐phenyl, and α‐cyclopropyl series. Copyright © 2008 John Wiley & Sons, Ltd.     

Comparison of several numerical optimization methods

Several numerical methods for solving the nonlinear two-point boundary value problem associated with an optimum spacecraft trajectory are considered. A comparative evaluation of the methods is made to determine the relative merits of each method. Particular attention is given to such characteristics as the simplicity of formulation and implementation, the convergence sensitivity, the computing time required, and the computer storage requirements. The methods considered are the perturbation method, the quasilinearization method, and the gradient method. The numerical comparison is made by considering a two-dimensional, low-thrust, minimum-time, Earth-Mars trajectory.The authors are greatly indebted to Mr. Robert D. Witty, Lockheed Electronics Company, for providing the excellent programming support.     

PHOTOCHEMISTRY AND PHOTOBIOLOGY OF PSORALENS

Abstract— The photochemistry and photobiology of psoralens have been rcvicwed on the basis of selected literature. In addition to the well establishcd photoaddition of psoralens to DNA. photoreac-tions involving RNA. tRNA and proteins warrant further studies. Although thymine seems to be one of the predominant bases for the psoralen photocycloaddition in DNA. photoreactions of other bases such as adenine and the mechanisms other than cycloaddition should also be investigated in order to fully assess the specificity of psoralen photoreactions and their application BS a probe for biopolymer structure and as a tool for photobiological studies of microorganisms. The relativc photobiological importance of monoaddition vs crosslinking of psoralens to nucleic acids also remains to be fully elucidated.     

Square-root variable-metric methods for minimization

Variable-metric methods are presented which do not need an accurate one-dimensional search and eliminate roundoff error problems which can occur in updating the metric for large-dimension systems. The methods are based on updating the square root of the metric, so that a positive-definite metric always results. The disadvantage of intentionally relaxing the accuracy of the one-dimensional search is that the number of iterations (and hence, gradient evaluations) increases. For problems involving a large number of variables, the square-root method is presented in a triangular form to reduce the amount of computation. Also, for usual optimization problems, the square-root procedure can be carried out entirely in terms of the metric, eliminating storage and computer time associated with computations of the square root of the metric.