[petsc-users] Zero diagonal term for direct solver

[Barry Smith]

  There is also -pc_factor_nonzeros_along_diagonal which reorders to move the zeros off the diagonal before the numerical factorization

  But if having 0 leads to a factorization without error then it will produce the correct answer (this means that zero got filled in during the factorization)

> On Jul 8, 2020, at 2:11 PM, Mark Adams <mfadams at lbl.gov> wrote:
> 
> 
> 
> On Wed, Jul 8, 2020 at 2:43 PM Qin Lu via petsc-users <petsc-users at mcs.anl.gov <mailto:petsc-users at mcs.anl.gov>> wrote:
> Hello,
> 
> I am using the Petsc native direct solver (with KSPPREONLY and PCLU) to solver a small Linear equation system. The matrix has a couple of diagonal terms with value 0, which makes the solver fail. Then I set a very small value (e.g., 1e-10) at these terms and the solver works. Can this cause noticeable errors in the solution?
> 
> Can't say.
>  
> Are there any options in Petsc to make the direct solver do row/column pivoting so that all the diagonal terms are non-zero?
> 
> I see:
> 
> 
> -pc_factor_pivot_in_blocks <true,false>- allow pivoting within the small blocks during factorization (may increase stability of factorization.
> 
> and
> 
> 
> -pc_factor_shift_type <shifttype>- Sets shift type or PETSC_DECIDE for the default; use '-help' for a list of available types
> -pc_factor_shift_amount <shiftamount>- Sets shift amount or PETSC_DECIDE for the default
> 
>  
> 
> Thanks for any suggestions.
> 
> Regards,
> Qin

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