602 research outputs found
Data-driven modelling of biological multi-scale processes
Biological processes involve a variety of spatial and temporal scales. A
holistic understanding of many biological processes therefore requires
multi-scale models which capture the relevant properties on all these scales.
In this manuscript we review mathematical modelling approaches used to describe
the individual spatial scales and how they are integrated into holistic models.
We discuss the relation between spatial and temporal scales and the implication
of that on multi-scale modelling. Based upon this overview over
state-of-the-art modelling approaches, we formulate key challenges in
mathematical and computational modelling of biological multi-scale and
multi-physics processes. In particular, we considered the availability of
analysis tools for multi-scale models and model-based multi-scale data
integration. We provide a compact review of methods for model-based data
integration and model-based hypothesis testing. Furthermore, novel approaches
and recent trends are discussed, including computation time reduction using
reduced order and surrogate models, which contribute to the solution of
inference problems. We conclude the manuscript by providing a few ideas for the
development of tailored multi-scale inference methods.Comment: This manuscript will appear in the Journal of Coupled Systems and
Multiscale Dynamics (American Scientific Publishers
Ten Digit Problems
Most quantitative mathematical problems cannot be solved exactly, but there are powerful algorithms for solving many of them numerically to a specified degree of precision like ten digits or ten thousand. In this article three difficult problems of this kind are presented, and the story is told of the SIAM 100-Dollar, 100-Digit Challenge. The twists and turns along the way illustrate some of the flavor of algorithmic continuous mathematics
Hydrodynamic limit equation for a lozenge tiling Glauber dynamics
We study a reversible continuous-time Markov dynamics on lozenge tilings of
the plane, introduced by Luby et al. Single updates consist in concatenations
of elementary lozenge rotations at adjacent vertices. The dynamics can also
be seen as a reversible stochastic interface evolution. When the update rate is
chosen proportional to , the dynamics is known to enjoy especially nice
features: a certain Hamming distance between configurations contracts with time
on average and the relaxation time of the Markov chain is diffusive, growing
like the square of the diameter of the system. Here, we present another
remarkable feature of this dynamics, namely we derive, in the diffusive time
scale, a fully explicit hydrodynamic limit equation for the height function (in
the form of a non-linear parabolic PDE). While this equation cannot be written
as a gradient flow w.r.t. a surface energy functional, it has nice analytic
properties, for instance it contracts the distance between
solutions. The mobility coefficient in the equation has non-trivial but
explicit dependence on the interface slope and, interestingly, is directly
related to the system's surface free energy. The derivation of the hydrodynamic
limit is not fully rigorous, in that it relies on an unproven assumption of
local equilibrium.Comment: 31 pages, 8 figures. v2: typos corrected, some proofs clarified. To
appear on Annales Henri Poincar
Model simplification by asymptotic order of magnitude reasoning
AbstractOne of the hardest problems in reasoning about a physical system is finding an approximate model that is mathematically tractable and yet captures the essence of the problem. This paper describes an implemented program AOM which automates a powerful simplification method. AOM is based on two domain-independent ideas: self-consistent approximations and asymptotic order of magnitude reasoning. The basic operation of AOM consists of five steps: (1) assign order of magnitude estimates to terms in the equations, (2) find maximal terms of each equation, i.e., terms that are not dominated by any other terms in the same equation, (3) consider all possible n-term dominant balance assumptions, (4) propagate the effects of the balance assumptions, and (5) remove partial models based on inconsistent balance assumptions. AOM also exploits constraints among equations and submodels. We demonstrate its power by showing how the program simplifies difficult fluid models described by coupled nonlinear partial differential equations with several parameters. We believe the derivation given by AOM is more systematic and easily understandable than those given in published papers
Lane formation by side-stepping
In this paper we study a system of nonlinear partial differential equations,
which describes the evolution of two pedestrian groups moving in opposite
direction. The pedestrian dynamics are driven by aversion and cohesion, i.e.
the tendency to follow individuals from the own group and step aside in the
case of contraflow. We start with a 2D lattice based approach, in which the
transition rates reflect the described dynamics, and derive the corresponding
PDE system by formally passing to the limit in the spatial and temporal
discretization. We discuss the existence of special stationary solutions, which
correspond to the formation of directional lanes and prove existence of global
in time bounded weak solutions. The proof is based on an approximation argument
and entropy inequalities. Furthermore we illustrate the behavior of the system
with numerical simulations
Numerical Bifurcation Analysis of PDEs From Lattice Boltzmann Model Simulations: a Parsimonious Machine Learning Approach
We address a three-tier data-driven approach for the numerical solution of the inverse problem in Partial Differential Equations (PDEs) and for their numerical bifurcation analysis from spatio-temporal data produced by Lattice Boltzmann model simulations using machine learning. In the first step, we exploit manifold learning and in particular parsimonious Diffusion Maps using leave-one-out cross-validation (LOOCV) to both identify the intrinsic dimension of the manifold where the emergent dynamics evolve and for feature selection over the parameter space. In the second step, based on the selected features, we learn the right-hand-side of the effective PDEs using two machine learning schemes, namely shallow Feedforward Neural Networks (FNNs) with two hidden layers and single-layer Random Projection Networks (RPNNs), which basis functions are constructed using an appropriate random sampling approach. Finally, based on the learned black-box PDE model, we construct the corresponding bifurcation diagram, thus exploiting the numerical bifurcation analysis toolkit. For our illustrations, we implemented the proposed method to perform numerical bifurcation analysis of the 1D FitzHugh-Nagumo PDEs from data generated by D1Q3 Lattice Boltzmann simulations. The proposed method was quite effective in terms of numerical accuracy regarding the construction of the coarse-scale bifurcation diagram. Furthermore, the proposed RPNN scheme was ∼ 20 to 30 times less costly regarding the training phase than the traditional shallow FNNs, thus arising as a promising alternative to deep learning for the data-driven numerical solution of the inverse problem for high-dimensional PDEs
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