% Matlab code mgvrhs1D.m
% For "Applied Numerical Linear Algebra",  Question 6.16
% Written by James Demmel, Apr 22, 1995
%                Modified, Jun  2, 1997
%
% Multigrid on a 1 D grid
% Restrict a matrix from grid n to grid m, where n and m 
% are (powers of 2) minus 1
% used to get right-hand-side for multigrid solve
% Written by J. Demmel, UC Berkeley, April 22, 1995
%
% function nrhs=mgvrhs1D(n,m,rhs)
%
function nrhs=mgvrhs1D(n,m,rhs)
nk=round(log(n-1)/log(2));
mk=round(log(m-1)/log(2));
nrhs=rhs;
r=rhs;
for i=nk-1:-1:mk,
    mi=2^i+1;
    ni=2^(i+1)+1;
    nrhs=zeros(mi,1);
    nrhs(2:mi-1)=.5*r(3:2:ni-2) + .25*(r(2:2:ni-3)+r(4:2:ni-1));
    r=nrhs;
end
return