3,808 research outputs found
Non-linear eigenvalue problems arising from growth maximization of positive linear dynamical systems
We study a growth maximization problem for a continuous time positive linear
system with switches. This is motivated by a problem of mathematical biology
(modeling growth-fragmentation processes and the PMCA protocol). We show that
the growth rate is determined by the non-linear eigenvalue of a max-plus
analogue of the Ruelle-Perron-Frobenius operator, or equivalently, by the
ergodic constant of a Hamilton-Jacobi (HJ) partial differential equation, the
solutions or subsolutions of which yield Barabanov and extremal norms,
respectively. We exploit contraction properties of order preserving flows, with
respect to Hilbert's projective metric, to show that the non-linear eigenvector
of the operator, or the "weak KAM" solution of the HJ equation, does exist. Low
dimensional examples are presented, showing that the optimal control can lead
to a limit cycle.Comment: 8 page
Spatially-constrained clustering of ecological networks
Spatial ecological networks are widely used to model interactions between
georeferenced biological entities (e.g., populations or communities). The
analysis of such data often leads to a two-step approach where groups
containing similar biological entities are firstly identified and the spatial
information is used afterwards to improve the ecological interpretation. We
develop an integrative approach to retrieve groups of nodes that are
geographically close and ecologically similar. Our model-based
spatially-constrained method embeds the geographical information within a
regularization framework by adding some constraints to the maximum likelihood
estimation of parameters. A simulation study and the analysis of real data
demonstrate that our approach is able to detect complex spatial patterns that
are ecologically meaningful. The model-based framework allows us to consider
external information (e.g., geographic proximities, covariates) in the analysis
of ecological networks and appears to be an appealing alternative to consider
such data
Deuterated molecules in DM Tau: DCO+, but no HDO
We report the detection of the J=2-1 line of DCO+ in the proto-planetary disk
of DM Tau and re-analyze the spectrum covering the 465 GHz transition of HDO in
this source, recently published by Ceccarelli et al. (2005). A modelling of the
DCO+ line profile with the source parameters derived from high resolution HCO+
observations yields a DCO+/HCO+ abundance ratio of about 0.004, an order of
magnitude smaller than that derived in the low mass cores. The re-analysis of
the 465 GHz spectrum, using the proper continuum flux (0.5 Jy) and source
systemic velocity (6.05 km/s), makes it clear that the absorption features
attributed to HDO and C6H are almost certainly unrelated to these species. We
show that the line-to-continuum ratio of an absorption line in front of a
Keplerian disk can hardly exceed the ratio of the turbulent velocity to the
projected rotation velocity at the disk edge, unless the line is optically very
thick (tau > 10 000). This ratio is typically 0.1-0.3 in proto-planetary disks
and is about 0.15 in DM Tau, much smaller than that for the alleged absorption
features. We also show that the detection of H2D+ in DM Tau, previously
reported by these authors, is only a 2-sigma detection when the proper velocity
is adopted. So far, DCO+ is thus the only deuterated molecule clearly detected
in proto-planetary disks
Simulation of fully resolved finite-size particles in a turbulent flow
In this study we are interested on the statistics of finite sized particles. A sustained homogeneous isotropic turbulence with particles is numerically simulated in order to obtain those statistics
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