Approximation Theory in Jacobi-weighted Spaces and Its Application to the h-p FEM Benqi Guo Department of Mathematics University of Manitoba, Canada Abstract: Approximation to functions in the Jacobi-weighted Sobolev and Besov spaces are analyzed in uniform and robust way for all dimensions. In the framework of these spaces, the approximability of singular functions, including the upper and lower bounds of approximation error, is derived. The approximability of smooth and singular solutions are applied to the h-p FEM, which yields the optimal convergence of the h-p FEM for elliptic problems on polygonal domains.