function vnew=twogrid(vold,f,nu1,nu2)
% TWOGRID two-grid method with inferior data structures
%
global sigma 
vnew=vold;
for ij=1:nu1
vnew=dampj(vnew,f);
end
nt=length(vnew); nl=nt-1; h=1/nl; hm2=nl*nl;
av=zeros(nt,1);
av(2:nt-1)=-vnew(1:nt-2)+(2+sigma*h*h)*vnew(2:nt-1)-vnew(3:nt);
%
% prepare coarse mesh data
%
r=f-hm2*av; fc=ftoc(r);
%
% call direct solver
%
vc=helmholtz(fc);
vnew=vnew+ctof(vc);
for ij=1:nu2
vnew=dampj(vnew,f);
end