[Trilinos-Users] Epetra - Dense Matrix-Vector multiply basics

[Gyorgy Matyasfalvi]

Dear Mike:

Thanks for your help. Indeed I got an error code of -2 and I suspect that,
as you pointed out, the issue is that *this is not a row vector.

My other question then would be: Is it possible to define a "Domain Map"
for Multivectors? i.e I want processors to own columns of the matrix
instead of rows. So basically spread out the columns among different
processors. I know think this is possible for sparse matrices.

Thanks! Best,
Gyorgy



On Thu, Apr 2, 2015 at 9:02 AM, Heroux, Michael A <maherou at sandia.gov>
wrote:

> Gyorgy,
>
> Assuming your x, y and A are dense objects, you are using the correct
> function, Multiply().  Chances are that you have some detail incorrect.
> First you should check for error codes that are being returned.  Multiply()
> will return integer error codes if it runs into issues:
>
> Int ierr = y.Multiply(...)
>
> If (ierr!=0) ...
>
> Regarding your transpose operation, if I understand the situation
> correctly, x should also be transposed to be a row vector.  Multiply() is
> probably complaining about this.  There is no way in the Epetra framework
> to specify that the "this" argument is transposed.
>
> I think it is best to perform the transpose operation as
>
> x = A^T y
>
> This expression keeps the dimensions correct.
>
> Try this and see how it goes.
>
> Mike
>
> From: Gyorgy Matyasfalvi <matyasfalvi at gmail.com<mailto:
> matyasfalvi at gmail.com>>
> Date: Wednesday, April 1, 2015 at 10:41 PM
> To: "trilinos-users at software.sandia.gov<mailto:
> trilinos-users at software.sandia.gov>" <trilinos-users at software.sandia.gov
> <mailto:trilinos-users at software.sandia.gov>>
> Subject: [EXTERNAL] [Trilinos-Users] Epetra - Dense Matrix-Vector multiply
> basics
>
> Dear Developers/Users:
>
> I have a question concerning matrix-vector multiplies:
>
> 1) Is there a document or example file that explains the basics of
> matrix-vector multiplies? Preferably isolated examples of mtx-vec mults.
> instead of being embedded in some algorithm.
>
> If this doesn't exist. Then I'd be interested in finding out:
>
> 2) How to do dense matrix, dense vector multiplies?
> I assume it's done via the Epetra_MultiVector class. However, the only
> operation that seems to have an effect is the following:
>
> y = A x
> using
> y.Multiply('N', 'N', 1.0, A, x, 0.0)
> (x was constructed using an Epetra_LocalMap
> A, and y were constructed using the same Epetra_Map)
>
> The problem is that if I want to compute for example:
>
> x = y^T A
> using
> x.Multiply('T', 'N', 1.0, y, A, 0.0)
>
> it seems the Multiply() operation doesn't do anything x remains unchanged.
>
> What am I doing incorrectly here?
> Thanks a lot! Best,
> Gyorgy
>
>
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://software.sandia.gov/pipermail/trilinos-users/attachments/20150403/3738bc18/attachment.html>