A direct fast FFT-based implementation for high order finite element   method on rectangular parallelepipeds for PDE
  We present a new direct logarithmically optimal in theory and fast in practice algorithm to implement the high order finite element method on multi-dimensional rectangular parallelepipeds for solving PDEs of the Poisson kind. The key points are the fast direct and inverse FFT-based algorithms for decomposition in eigenvectors of the 1D eigenvalue problems for the high order FEM. The algorithm can further be used for numerous applications, in particular, to implement the high order finite element methods for various time-dependent PDEs. 
Alexander Zlotnik and Ilya Zlotnik
math.NA