  From the literature, it is known that the choice of basis functions in hp-FEM
heavily influences the computational cost in order to obtain an approximate
solution. Depending on the choice of the reference element, suitable tensor
product like basis functions of Jacobi polynomials with different weights lead
to optimal properties due to condition number and sparsity. This paper presents
biorthogonal basis functions to the primal basis functions mentioned above. The
authors investigate hypercubes and simplices as reference elements, as well as
the cases of $H^1$ and H(Curl). The functions can be expressed sums of tensor
products of Jacobi polynomials with maximal two summands.
