Optimally Adapted Meshes for Finite elements of Arbitrary order and <i>W^1,p</i> norms

International audienceGiven a function ƒ defined on a bounded bidimensional domain and a number N > 0, we study the properties of the triangulation Tn that minimizes the distance between ƒ and its interpolation on the associated finite element space, over all triangulations of at most Nelements. The error is studied in the W1,p norm and we consider Lagrange finite elements of arbitrary polynomial order m-1 We establish sharp asymptotic error estimates as n tends to infinity when the optimal anisotropic triangulation is used. This problem has already been treated with the error measured in the Lp. norm. The extension of this analysis to the W1p norm is crucial in order to match more closely the needs of numerical PDE analysis, and it is not straightforward. In particular, the meshes which satisfy the optimal error estimate are characterized by a metric describing the local aspect ratio of each triangle and by a geometric constraint on their maximal angle, a second feature that does not appear for the Lp. error norm. Our analysis also provides with practical strategies for designing meshes such that the interpolation error satisfies the optimal estimate up to a fixed multiplicative constant