On Mon, Mar 3, 2008 at 12:16 PM, li pan <li76pan at yahoo.com> wrote: > hi Matt, > I used to discuss with somebody about SNES for > nonlinear equation. One can use -snes_mf_operator for > matrix free calculation. As far as I know, SNES uses a > Newton-Kryplov Method. In each Newton, one has to > solve a linear equation, which is : > residual = Jacobian * delt_u > If I can solve the linear equation in a iterative way > in Kryplov space, then I only need to optimise > Jacobian*delt_u - residual > The matrix vector multiplication can be expressed > through the gradient calculation for residual. That's > why we can forget the Jacobian matrix. The method is > generally called Jacobian free Newton Kryplov mehtod > (JFNK). You must know it. > Actually, that's the way snes works with -snes_mf. > Here, we define the function SNESSetFunction(). And as > argument, we give the calculation of residual. > I wrote so much only because of the continuity of the > context. > Now my question is, if the SNES method includes a JFNK > method to solve the linear equation, why can't I use > it for my linear equation directly? And how?
The "matrix-free" you refer to above uses a finite difference approximation to the Jacobian of a nonlinear function. If you have a linear equation, the Jacobian is THE EQUATION ITSELF. Thus, what you propose makes no sense to me at all. Again, I recommend you look at MATSHELL since I think this is what you want. Matt > thanx > > pan > > > --- Matthew Knepley <knepley at gmail.com> wrote: > > > On Mon, Mar 3, 2008 at 11:05 AM, li pan > > <li76pan at yahoo.com> wrote: > > > Dear all, > > > If I want to solve a linear equation in matrix > > free > > > scheme, how is the command line argument? I only > > know > > > for nonlinear solver it is _snes_mf > > > > What do you mean here? How do you have linear > > equations without > > a matrix? If you mean that you would only like to > > specify the action > > of the operator, you should use MATSHELL. > > > > Matt > > > > > thanx > > > > > > pan > > > > > > > > > > > > > > > > ____________________________________________________________________________________ > > > Looking for last minute shopping deals? > > > Find them fast with Yahoo! Search. > > > http://tools.search.yahoo.com/newsearch/category.php?category=shopping > > > > > > > > > > > > > > -- > > What most experimenters take for granted before they > > begin their > > experiments is infinitely more interesting than any > > results to which > > their experiments lead. > > -- Norbert Wiener > > > > > > > > > > ____________________________________________________________________________________ > Never miss a thing. Make Yahoo your home page. > http://www.yahoo.com/r/hs > > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener
