3,657 research outputs found
Rung-singlet phase of the S=1/2 two-leg spin-ladder with four-spin cyclic exchange
Using continuous unitary transformations (CUT) we calculate the one-triplet
gap for the antiferromagnetic S=1/2 two-leg spin ladder with additional
four-spin exchange interactions in a high order series expansion about the
limit of isolated rungs. By applying a novel extrapolation technique we
calculate the transition line between the rung-singlet phase and a
spontaneously dimerized phase with dimers on the legs. Using this efficient
extrapolation technique we are able to analyze the crossover from strong rung
coupling to weakly coupled chains.Comment: 4 pages, 4 figures included, submitted to Phys Rev
Characterizing steady states of genome-scale metabolic networks in continuous cell cultures
We present a model for continuous cell culture coupling intra-cellular
metabolism to extracellular variables describing the state of the bioreactor,
taking into account the growth capacity of the cell and the impact of toxic
byproduct accumulation. We provide a method to determine the steady states of
this system that is tractable for metabolic networks of arbitrary complexity.
We demonstrate our approach in a toy model first, and then in a genome-scale
metabolic network of the Chinese hamster ovary cell line, obtaining results
that are in qualitative agreement with experimental observations. More
importantly, we derive a number of consequences from the model that are
independent of parameter values. First, that the ratio between cell density and
dilution rate is an ideal control parameter to fix a steady state with desired
metabolic properties invariant across perfusion systems. This conclusion is
robust even in the presence of multi-stability, which is explained in our model
by the negative feedback loop on cell growth due to toxic byproduct
accumulation. Moreover, a complex landscape of steady states in continuous cell
culture emerges from our simulations, including multiple metabolic switches,
which also explain why cell-line and media benchmarks carried out in batch
culture cannot be extrapolated to perfusion. On the other hand, we predict
invariance laws between continuous cell cultures with different parameters. A
practical consequence is that the chemostat is an ideal experimental model for
large-scale high-density perfusion cultures, where the complex landscape of
metabolic transitions is faithfully reproduced. Thus, in order to actually
reflect the expected behavior in perfusion, performance benchmarks of
cell-lines and culture media should be carried out in a chemostat
ALTERNATIVE METHODS OF FORECASTING AGRICULTURAL WATER DEMAND: A CASE STUDY ON THE FLINT RIVER BASIN IN GEORGIA
Future agricultural water demands are determined by employing forecasts from irrigated crop acreage models. Forecasts of prices and yields, and variances and covariances of crop returns are employed for forecasting crop acreage. Results provide insights into the value of rational expectations in forecasting agricultural water demand.Resource /Energy Economics and Policy,
Sensitivity of optimum solutions to problem parameters
Derivation of the sensitivity equations that yield the sensitivity derivatives directly, which avoids the costly and inaccurate perturb-and-reoptimize approach, is discussed and solvability of the equations is examined. The equations apply to optimum solutions obtained by direct search methods as well as those generated by procedures of the sequential unconstrained minimization technique class. Applications are discussed for the use of the sensitivity derivatives in extrapolation of the optimal objective function and design variable values for incremented parameters, optimization with multiple objectives, and decomposition of large optimization problems
The Gluon Propagator without lattice Gribov copies
We study the gluon propagator in quenched lattice QCD using the Laplacian
gauge which is free of lattice Gribov copies. We compare our results with those
obtained in the Landau gauge on the lattice, as well as with various
approximate solutions of the Dyson Schwinger equations. We find a finite value
for the renormalized zero-momentum propagator
(taking our renormalization point at 1.943 GeV), and a pole mass MeV.Comment: Discussion of the renormalized gluon propagator and of the Laplacian
gauge fixing procedure extended. Version to appear in Phys. Rev. D. 15 pages,
8 figure
Phase Transitions of the Typical Algorithmic Complexity of the Random Satisfiability Problem Studied with Linear Programming
Here we study the NP-complete -SAT problem. Although the worst-case
complexity of NP-complete problems is conjectured to be exponential, there
exist parametrized random ensembles of problems where solutions can typically
be found in polynomial time for suitable ranges of the parameter. In fact,
random -SAT, with as control parameter, can be solved quickly
for small enough values of . It shows a phase transition between a
satisfiable phase and an unsatisfiable phase. For branch and bound algorithms,
which operate in the space of feasible Boolean configurations, the empirically
hardest problems are located only close to this phase transition. Here we study
-SAT () and the related optimization problem MAX-SAT by a linear
programming approach, which is widely used for practical problems and allows
for polynomial run time. In contrast to branch and bound it operates outside
the space of feasible configurations. On the other hand, finding a solution
within polynomial time is not guaranteed. We investigated several variants like
including artificial objective functions, so called cutting-plane approaches,
and a mapping to the NP-complete vertex-cover problem. We observed several
easy-hard transitions, from where the problems are typically solvable (in
polynomial time) using the given algorithms, respectively, to where they are
not solvable in polynomial time. For the related vertex-cover problem on random
graphs these easy-hard transitions can be identified with structural properties
of the graphs, like percolation transitions. For the present random -SAT
problem we have investigated numerous structural properties also exhibiting
clear transitions, but they appear not be correlated to the here observed
easy-hard transitions. This renders the behaviour of random -SAT more
complex than, e.g., the vertex-cover problem.Comment: 11 pages, 5 figure
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