function [ut]=derv(n,t,u)
%
% Function derv computes the derivative vector 
% of the linear PDE problem
%
% Declare global variables
  global nsteps ndss;
%
% Problem parameters
  xl=0.0;
  xu=1.0;
%
% BC at x = 0
  u(1)=0.0;
%
% BC at x = 1
  u(n)=0.0;
%
% ux
  if ndss == 2 [ux]=dss002(xl,xu,n,u); end
  if ndss == 4 [ux]=dss004(xl,xu,n,u); end
%
% uxx
  if ndss == 2 [uxx]=dss002(xl,xu,n,ux); end
  if ndss == 4 [uxx]=dss004(xl,xu,n,ux); end
  if ndss == 42
    nl=1;
    nu=1;
    ux=zeros(n,1);
    [uxx]=dss042(xl,xu,n,u,ux,nl,nu);
  end
  if ndss == 44
    nl=1;
    nu=1;
    ux=zeros(n,1);
    [uxx]=dss044(xl,xu,n,u,ux,nl,nu);
  end
%
% pdelin
  for i=1:n
     ut(i)=uxx(i);
  end
