%  take a n by n matrix a.
%  symmetrize it (take a+a'), and tridiagonalize it (zero out everything
%    outside the three diagonals
%
%  return the answer in c
%
c=a+a';
for i=1:n-2
  for j=i+2:n
    c(i,j)=0;
    c(j,i)=0;
  end
end