[petsc-users] forming a matrix from a set of vectors

[Barry Smith]

   Since A is tiny I am assuming you are doing this sequentially only?

   Do you have d stored as a AIJ matrix or is transpose(d) stored as a AIJ matrix?

  


> On Aug 26, 2015, at 3:41 PM, Nicolas Pozin <nicolas.pozin at inria.fr> wrote:
> 
> Actually I want to get the diagonal of the matrix : transpose(d)*A*d where
> -d is a sparse matrix of size (n1,m1)
> -A is a dense symetric matrix of size size (n1,n1)
> with m1 very big compared to n1 (1 million against a few dozens).
> 
> The problem is too big to allow the use of MatMatMult.
> What I planned to do :
> -compute the vectors Vi defined by transpose(d)*Ai where Ai is the i-th column of A : quick since d is sparse and n1 is small
> -deduce the matrix transpose(d)*A = [V1 ... Vn]
> and then get the diagonal of transpose(d)*transpose([V1 ...Vn]) through
> -transpose([V1 ...Vn]) and get its columns C1 ... Cn 
> -conclude on the i-th diagonal value which is the i-th component of tranpose(d)*Ci
> 
> 
> 
> ----- Mail original -----
>> De: "Barry Smith" <bsmith at mcs.anl.gov>
>> À: "Nicolas Pozin" <nicolas.pozin at inria.fr>
>> Cc: "Jed Brown" <jed at jedbrown.org>, petsc-users at mcs.anl.gov
>> Envoyé: Mercredi 26 Août 2015 22:21:04
>> Objet: Re: [petsc-users] forming a matrix from a set of vectors
>> 
>> 
>>> On Aug 26, 2015, at 3:06 PM, Nicolas Pozin <nicolas.pozin at inria.fr> wrote:
>>> 
>>> Thank you for this answer.
>>> 
>>> What I want to do is to get the lines of this matrix and store them in
>>> vectors.
>> 
>>  If you want to treat the columns of the dense matrix as vectors then use
>>  MatDenseGetArray() and call VecCreateMPIWithArray() with a pointer to the
>>  first row of each column of the obtained array (PETSc dense matrices are
>>  stored by column; same as for example LAPACK).
>> 
>>  But if you explained more why you want to treat something sometimes as a
>>  Mat (which is a linear operator on vectors) and sometimes as vectors we
>>  might be able to suggest how to organize your code.
>> 
>>   Barry
>> 
>>> 
>>> 
>>> ----- Mail original -----
>>>> De: "Jed Brown" <jed at jedbrown.org>
>>>> À: "Nicolas Pozin" <nicolas.pozin at inria.fr>, petsc-users at mcs.anl.gov
>>>> Envoyé: Mercredi 26 Août 2015 20:38:37
>>>> Objet: Re: [petsc-users]  forming a matrix from a set of vectors
>>>> 
>>>> Nicolas Pozin <nicolas.pozin at inria.fr> writes:
>>>>> Given a set of vectors V1, V2,...,Vn, is there an efficient way to form
>>>>> the
>>>>> dense matrix [V1 V2 ... Vn]?
>>>> 
>>>> What do you want to do with that matrix?  The vector representation is
>>>> pretty flexible and the memory semantics are similar unless you store
>>>> the dense matrix row-aligned (not the default).
>>>> 
>> 
>>