vandermonde takes as argument a vector whose components are
denoted by xj for j=0..n−1.
vandermonde returns the corresponding Vandermonde matrix (the k-th row of the matrix is the vector whose components are xik for i=0..n−1 and k=0..n−1).
The indices of the rows and columns begin at 0 with Xcas.
Output (if a is symbolic else purge(a)) :