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Homogenization of parabolic equations with a continuum of space and time scales. (English) Zbl 1170.34037
The authors consider a linear parabolic equation in divergence form defined via a uniformly elliptic and symmetric matrix $$a$$, with the aim to give some numerical approximation of the solution. In spite of the title the paper is not a paper about some result in homogenization, but deals with numerical approximation of a fixed operator which is defined via a matrix whose coefficients $$a_{ij}$$ belong to $$L^{\infty}(\Omega)$$, $$\Omega \subset {\mathbb R}^n$$ (possibly oscillating and not regular).
The natural space in which a solution of the equation $$u_t-\text{div}(a\cdot\nabla u)=g$$, $$g\in L^2(\Omega\times (0,T))$$, with $$u=0$$ in $$\partial\Omega\times (0,T)$$ lives is $$\{u\in L^2(0,T; H^1_0(\Omega))\mid u_t\in L^2(0,T; H^{-1}(\Omega))\}$$.
The authors consider a change of variables, called caloric coordinates, defined as the solutions
\begin{alignedat}{2}{\partial_t F_i} - \text{div}(a\cdot\nabla F_i)&= 0 &\quad &\text{in }\Omega \times(0,T),\\ F_i&= x_i &\quad &\text{on }\partial\Omega\times(0,T),\\ \text{div}(a\cdot \nabla F_i)&= 0 &\quad &\text{in }\Omega\times\{0\},\end{alignedat} $$i=1,\dots n$$. The authors prove that, if $$\Omega$$ is convex and under some assumption on the trace of $$\langle a \cdot \nabla F, \nabla F \rangle$$, the function $$\tilde u := u \circ F^{-1}$$ belong to the space $${\mathcal Z}= \{v\in L^2(0,T; H^2(\Omega))\mid v_t\in L^2(0,T; L^2(\Omega)) \}$$ and give some estimate in this space depending (only) on the data. Defining finite dimensional spaces of functions of the type $$\varphi \circ F$$ the authors prove several results of convergence of the approximated solutions to the solution of the original problem.

MSC:
 34D10 Perturbations of ordinary differential equations 35B27 Homogenization in context of PDEs; PDEs in media with periodic structure 68P30 Coding and information theory (compaction, compression, models of communication, encoding schemes, etc.) (aspects in computer science) 60F05 Central limit and other weak theorems 35B05 Oscillation, zeros of solutions, mean value theorems, etc. in context of PDEs 35K20 Initial-boundary value problems for second-order parabolic equations 35A35 Theoretical approximation in context of PDEs
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