219,842 research outputs found
Resolution of seven-axis manipulator redundancy: A heuristic issue
An approach is presented for the resolution of the redundancy of a seven-axis manipulator arm from the AI and expert systems point of view. This approach is heuristic, analytical, and globally resolves the redundancy at the position level. When compared with other approaches, this approach has several improved performance capabilities, including singularity avoidance, repeatability, stability, and simplicity
Coherent spin rotation in the presence of a phonon-bottleneck effect
A characteristic of spin reversal in the presence of phonon-bottleneck is the
deviation of the magnetization cycle from a reversible function into an opened
hysterezis cycle. In recent experiments on molecular magnets (e.g. V and
Ru), the zero-field level repulsion was sufficiently large to ensure an
otherwise adiabatic passage through zero-field and the magnetization curves can
be described by using only a phonon-bottleneck model. Here, we generalize the
phonon-bottleneck model into a model able to blend the non-adiabatic dynamics
of spins with the presence of a non-equilibrium phonon bath. In this simple
phenomenological model, Bloch equations are written in the eigenbasis of the
effective spin Hamiltonian, considered to be a two-level system at low
temperatures. The relaxation term is given by the phonon-bottleneck mechanism.
To the expense of calculus time, the method can be generalized to multi-level
systems, where the notion of Bloch sphere does not apply but the density matrix
formalism is still applicable.Comment: as published in EuroPhys Letter
Propagation of boundary-induced discontinuity in stationary radiative transfer
We consider the boundary value problem of the stationary transport equation
in the slab domain of general dimensions. In this paper, we discuss the
relation between discontinuity of the incoming boundary data and that of the
solution to the stationary transport equation. We introduce two conditions
posed on the boundary data so that discontinuity of the boundary data
propagates along positive characteristic lines as that of the solution to the
stationary transport equation. Our analysis does not depend on the celebrated
velocity averaging lemma, which is different from previous works. We also
introduce an example in two dimensional case which shows that piecewise
continuity of the boundary data is not a sufficient condition for the main
result.Comment: 15 pages, no figure
Lethal Mutagenesis in Viruses and Bacteria
Here we study how mutations which change physical properties of cell proteins
(stability) impact population survival and growth. In our model the genotype is
presented as a set of N numbers, folding free energies of cells N proteins.
Mutations occur upon replications so that stabilities of some proteins in
daughter cells differ from those in parent cell by random amounts drawn from
experimental distribution of mutational effects on protein stability. The
genotype-phenotype relationship posits that unstable proteins confer lethal
phenotype to a cell and in addition the cells fitness (duplication rate) is
proportional to the concentration of its folded proteins. Simulations reveal
that lethal mutagenesis occurs at mutation rates close to 7 mutations per
genome per replications for RNA viruses and about half of that for DNA based
organisms, in accord with earlier predictions from analytical theory and
experiment. This number appears somewhat dependent on the number of genes in
the organisms and natural death rate. Further, our model reproduces the
distribution of stabilities of natural proteins in excellent agreement with
experiment. Our model predicts that species with high mutation rates, tend to
have less stable proteins compared to species with low mutation rate
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