[petsc-users] Using KSPLSQR without explicitly computing U(transpose)U

Jed Brown jed at 59A2.org
Sun Jan 23 08:40:57 CST 2011

On Sun, Jan 23, 2011 at 03:39, Gaurish Telang <gaurish108 at gmail.com> wrote:

> But for this I had to compute U(transpose)U explicitly. The algorithm
> mentioned in the paper does NOT involve an explicit computation of
> U(transpose)U.
> Is there a way to avoid the explicit and expensive computation of
> U(transpose)U and use KSPLSQR?  The details on this page

The need for explicit U^T U is for algebraic preconditioners (like
incomplete factorization).  You can try using cheaper approximations to this
product or use a PCShell if you want to avoid explicitly forming the
product.  Chances are that unpreconditioned LSQR will converge too slowly.
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