  function ut=pde_1(u,t)
%
% Function pde_1 computes the t derivative vector of the u vector 
%
  global zL D ifd n ncase ncall
%
% Second order spatial derivative
  if(ncase==1)
     [uz]=dss002(0,zL,n,u);
     uz(1)=0; uz(n)=0;
    [uzz]=dss002(0,zL,n,uz);
  end 
  if(ncase==2)
     [uz]=dss004(0,zL,n,u);
     uz(1)=0; uz(n)=0;     
    [uzz]=dss004(0,zL,n,uz);
  end  
  if(ncase==3)
     [uz]=dss006(0,zL,n,u);
     uz(1)=0; uz(n)=0;     
    [uzz]=dss006(0,zL,n,uz);
  end 
  if(ncase==4)
    nl=2; nu=2; uz(1)=0; uz(n)=0;  
    [uzz]=dss042(0,zL,n,u,uz,nl,nu);
  end 
  if(ncase==5)
    nl=2; nu=2; uz(1)=0; uz(n)=0;      
    [uzz]=dss044(0,zL,n,u,uz,nl,nu);
  end 
  if(ncase==6)
    nl=2; nu=2; uz(1)=0; uz(n)=0;  
    [uzz]=dss046(0,zL,n,u,uz,nl,nu);
  end 
%
% Temporal derivative
  for i=1:n
    ut(i)=D*uzz(i);
  end
%
% Increment calls to pde_1
  ncall=ncall+1;
