_      function [xz]=diff_1D(n,D,x)
%...
%...  A. Vande Wouwer, P. Saucez and W.E. Schiesser (2002)
%...
%...  function diff_1D computes a first-order derivative over a one-
%...  dimensional domain using either centered or biased upwind 
%...  approximations. 
%...
%...  argument list
%...
%...     n         number of grid points (input)
%...
%...     D         differentiation matrix (input)
%...
%...     x         one-dimensional array containing the dependent vari-
%...               able which is to be differentiated with respect to
%...               independent variable (input)
%...
%...  compute the derivative using the differentiation matrix 
%...  
      xz=D*x;