8678e-11
Elapsed time is 0.116907 seconds.
ans =
   1.5276e-09
</pre><p>What if you want to solve multiple systems?  Use a matrix factorization. But which one?  And how do you use it?  Here are
            some alternatives using LU for the sparse west0479 matrix, but some are faster than others.
         </p><pre class="codeinput">tic ; [L,U]     = lu(A) ; x1 = U \ (L \ b)         ; t1=toc ; nz1=nnz(L+U);
tic ; [L,U,P]   = lu(A) ; x2 = U \ (L \ P*b)       ; t2=toc ; nz2=nnz(L+U);
tic ; [L,U,P,Q] = lu(A) ; 