Approximation of Holder continuous homeomorphisms by piecewise affine   homeomorphisms
  This paper is concerned with the problem of approximating a homeomorphism by piecewise affine homeomorphisms. The main result is as follows: every homeomorphism from a planar domain with a polygonal boundary to R^2 that is globally Holder continuous of exponent \alpha, and whose inverse is also globally Holder continuous of exponent \alpha can be approximated in the Holder norm of exponent \beta by piecewise affine homeomorphisms, for some \beta that only depends on \alpha. The proof is constructive. We adapt the proof of simplicial approximation in the supremum norm, and measure the side lengths and angles of the triangulation over which the approximating homeomorphism is piecewise affine. The approximation in the supremum norm, and a control on the minimum angle and on the ratio between the maximum and minimum side lengths of the triangulation suffice to obtain approximation in the Holder norm. 
Jose C. Bellido and Carlos Mora-Corral