4f resid: %e\n', nz2,t2, norm(b-A*x2));
fprintf ('3: nnz(L+U): %5d time: %8.4f resid: %e\n', nz3,t3, norm(b-A*x3));

%% LU and LINSOLVE are fast and accurate but complicated to use
%
% A quick look at ``help lu'' will scroll off your screen.  For full
% matrices, [L,U,p] = lu (A,'vector') is fastest.  Then for the
% forward/backsolves, use LINSOLVE instead of BACKSLASH for even faster
% performance.  But for sparse matrices, use the optional 'Q' output of LU
% so you get a good fill-reducing ordering.  But 