5,607 research outputs found
A series solution and a fast algorithm for the inversion of the spherical mean Radon transform
An explicit series solution is proposed for the inversion of the spherical
mean Radon transform. Such an inversion is required in problems of thermo- and
photo- acoustic tomography. Closed-form inversion formulae are currently known
only for the case when the centers of the integration spheres lie on a sphere
surrounding the support of the unknown function, or on certain unbounded
surfaces. Our approach results in an explicit series solution for any closed
measuring surface surrounding a region for which the eigenfunctions of the
Dirichlet Laplacian are explicitly known - such as, for example, cube, finite
cylinder, half-sphere etc. In addition, we present a fast reconstruction
algorithm applicable in the case when the detectors (the centers of the
integration spheres) lie on a surface of a cube. This algorithm reconsrtucts
3-D images thousands times faster than backprojection-type methods
Coupling of Two Motor Proteins: a New Motor Can Move Faster
We study the effect of a coupling between two motor domains in
highly-processive motor protein complexes. A simple stochastic discrete model,
in which the two parts of the protein molecule interact through some energy
potential, is presented. The exact analytical solutions for the dynamic
properties of the combined motor species, such as the velocity and dispersion,
are derived in terms of the properties of free individual motor domains and the
interaction potential. It is shown that the coupling between the motor domains
can create a more efficient motor protein that can move faster than individual
particles. The results are applied to analyze the motion of helicase RecBCD
molecules
Effects of solar wind magnetosphere coupling recorded at different geomagnetic latitudes: Separation of directly-driven and storage/release systems
The effect on geomagnetic activity of solar wind speed, compared with that of the strength of the interplanetary magnetic field, differs with geomagnetic latitude. In this study we construct a new index based on monthly standard deviations in the H-component of the geomagnetic field for all geomagnetic latitudes. We demonstrate that for this index the response at auroral regions correlates best with interplanetary coupling functions which include the solar wind speed while mid- and low-latitude regions respond to variations in the interplanetary magnetic field strength. These results are used to isolate the responsible geomagnetic current systems
Universal Baxterization for -graded Hopf algebras
We present a method for Baxterizing solutions of the constant Yang-Baxter
equation associated with -graded Hopf algebras. To demonstrate the
approach, we provide examples for the Taft algebras and the quantum group
.Comment: 8 page
Principal forms X^2 + nY^2 representing many integers
In 1966, Shanks and Schmid investigated the asymptotic behavior of the number
of positive integers less than or equal to x which are represented by the
quadratic form X^2+nY^2. Based on some numerical computations, they observed
that the constant occurring in the main term appears to be the largest for n=2.
In this paper, we prove that in fact this constant is unbounded as n runs
through positive integers with a fixed number of prime divisors.Comment: 10 pages, title has been changed, Sections 2 and 3 are new, to appear
in Abh. Math. Sem. Univ. Hambur
The elements of a computational infrastructure for social simulation
Applications of simulation modelling in social science domains are varied and increasingly widespread. The effective deployment of simulation models depends on access to diverse datasets, the use of analysis capabilities, the ability to visualize model outcomes and to capture, share and re-use simulations as evidence in research and policy-making. We describe three applications of e-social science that promote social simulation modelling, data management and visualization. An example is outlined in which the three components are brought together in a transport planning context. We discuss opportunities and benefits for the combination of these and other components into an e-infrastructure for social simulation and review recent progress towards the establishment of such an infrastructure
- …