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Networks & Heterogeneous Media

June 2011 , Volume 6 , Issue 2

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A central limit theorem for pulled fronts in a random medium
James Nolen
2011, 6(2): 167-194 doi: 10.3934/nhm.2011.6.167 +[Abstract](710) +[PDF](346.5KB)
Abstract:
We consider solutions to a nonlinear reaction diffusion equation when the reaction term varies randomly with respect to the spatial coordinate. The nonlinearity is the KPP type nonlinearity. For a stationary and ergodic medium, and for certain initial condition, the solution develops a moving front that has a deterministic asymptotic speed in the large time limit. The main result of this article is a central limit theorem for the position of the front, in the supercritical regime, if the medium satisfies a mixing condition.
Convergence of discrete duality finite volume schemes for the cardiac bidomain model
Boris Andreianov, Mostafa Bendahmane, Kenneth H. Karlsen and Charles Pierre
2011, 6(2): 195-240 doi: 10.3934/nhm.2011.6.195 +[Abstract](768) +[PDF](1457.9KB)
Abstract:
We prove convergence of discrete duality finite volume (DDFV) schemes on distorted meshes for a class of simplified macroscopic bidomain models of the electrical activity in the heart. Both time-implicit and linearised time-implicit schemes are treated. A short description is given of the 3D DDFV meshes and of some of the associated discrete calculus tools. Several numerical tests are presented.
Perturbation and numerical methods for computing the minimal average energy
Timothy Blass and Rafael de la Llave
2011, 6(2): 241-255 doi: 10.3934/nhm.2011.6.241 +[Abstract](727) +[PDF](209.3KB)
Abstract:
We investigate the differentiability of minimal average energy associated to the functionals $S_\epsilon (u) = \int_{\mathbb{R}^d} \frac{1}{2}|\nabla u|^2 + \epsilon V(x,u)\, dx$, using numerical and perturbative methods. We use the Sobolev gradient descent method as a numerical tool to compute solutions of the Euler-Lagrange equations with some periodicity conditions; this is the cell problem in homogenization. We use these solutions to determine the average minimal energy as a function of the slope. We also obtain a representation of the solutions to the Euler-Lagrange equations as a Lindstedt series in the perturbation parameter $\epsilon$, and use this to confirm our numerical results. Additionally, we prove convergence of the Lindstedt series.
Spectral theory for nonconservative transmission line networks
Robert Carlson
2011, 6(2): 257-277 doi: 10.3934/nhm.2011.6.257 +[Abstract](825) +[PDF](261.8KB)
Abstract:
The global theory of transmission line networks with nonconservative junction conditions is developed from a spectral theoretic viewpoint. The rather general junction conditions lead to spectral problems for nonnormal operators. The theory of analytic functions which are almost periodic in a strip is used to establish the existence of an infinite sequence of eigenvalues and the completeness of generalized eigenfunctions. Simple eigenvalues are generic. The asymptotic behavior of an eigenvalue counting function is determined. Specialized results are developed for rational graphs.
Gaussian estimates on networks with applications to optimal control
Luca Di Persio and Giacomo Ziglio
2011, 6(2): 279-296 doi: 10.3934/nhm.2011.6.279 +[Abstract](816) +[PDF](442.6KB)
Abstract:
We study a class of reaction-diffusion type equations on a finite network with continuity assumptions and a kind of non-local, stationary Kirchhoff's conditions at the nodes. A multiplicative random Gaussian perturbation acting along the edges is also included. For such a problem we prove Gaussian estimates for the semigroup generated by the evolution operator, hence generalizing similar results previously obtained in [21]. In particular our main goal is to extend known results on Gaussian upper bounds for heat equations on networks with local boundary conditions to those with non-local ones. We conclude showing how our results can be used to apply techniques developed in [13] to solve a class of Stochastic Optimal Control Problems inspired by neurological dynamics.
Dynamical behavior of networks of non-uniform Timoshenko beams system with boundary time-delay inputs
Zhong-Jie Han and Gen-Qi Xu
2011, 6(2): 297-327 doi: 10.3934/nhm.2011.6.297 +[Abstract](861) +[PDF](1278.0KB)
Abstract:
The dynamical stability of planar networks of non-uniform Timoshenko beams system is considered. Suppose that the displacement and rotational angle is continuous at the common vertex of this network and the bending moment and shear force satisfies Kirchhoff's laws, respectively. Time-delay terms exist in control inputs at exterior vertices. The feedback control laws are designed to stabilize this kind of networks system. Then it is proved that the corresponding closed loop system is well-posed. Under certain conditions, the asymptotic stability of this system is shown. By a complete spectral analysis, the spectrum-determined-growth condition is proved to be satisfied for this system. Finally, the exponential stability of this system is discussed for a special case and some simulations are given to support these results.
Consensus and synchronization in discrete-time networks of multi-agents with stochastically switching topologies and time delays
Wenlian Lu, Fatihcan M. Atay and Jürgen Jost
2011, 6(2): 329-349 doi: 10.3934/nhm.2011.6.329 +[Abstract](807) +[PDF](312.0KB)
Abstract:
We analyze stability of consensus algorithms in networks of multi-agents with time-varying topologies and delays. The topology and delays are modeled as induced by an adapted process and are rather general, including i.i.d. topology processes, asynchronous consensus algorithms, and Markovian jumping switching. In case the self-links are instantaneous, we prove that the network reaches consensus for all bounded delays if the graph corresponding to the conditional expectation of the coupling matrix sum across a finite time interval has a spanning tree almost surely. Moreover, when self-links are also delayed and when the delays satisfy certain integer patterns, we observe and prove that the algorithm may not reach consensus but instead synchronize at a periodic trajectory, whose period depends on the delay pattern. We also give a brief discussion on the dynamics in the absence of self-links.

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