[petsc-users] (no subject)

[Barry Smith]

https://urldefense.us/v3/__https://petsc.org/release/manualpages/KSP/KSPCR/__;!!G_uCfscf7eWS!ZQFXJM_C7qCPyXSN6vUgcQaTYavgEpmvtDdWSpbY1dR7m_6DlTM1CF00dL26gVO4tjPEFFvV79SEmjS1tbaF0nc$ 

The preconditioner must be POSITIVE-DEFINITE and the operator POSITIVE-SEMIDEFINITE.

https://urldefense.us/v3/__https://petsc.org/release/manualpages/KSP/KSPMINRES/__;!!G_uCfscf7eWS!ZQFXJM_C7qCPyXSN6vUgcQaTYavgEpmvtDdWSpbY1dR7m_6DlTM1CF00dL26gVO4tjPEFFvV79SEmjS1rmWgN0k$ 


The operator and the preconditioner must be symmetric and the preconditioner must be positive definite for this method.

https://urldefense.us/v3/__https://petsc.org/release/manualpages/KSP/KSPSYMMLQ/__;!!G_uCfscf7eWS!ZQFXJM_C7qCPyXSN6vUgcQaTYavgEpmvtDdWSpbY1dR7m_6DlTM1CF00dL26gVO4tjPEFFvV79SEmjS1uMjjdeU$ 

The preconditioner must be POSITIVE-DEFINITE.

Of course you can always use KSPGMRES or KSPBCGS


> On Aug 5, 2024, at 2:27 PM, Константин Мурусидзе <konstantin.murusidze at math.msu.ru> wrote:
> 
> I know that matrix is symmetric, but it isn’t positive definite. Is it possible to solve such problem, maybe with another solver or preconditioner?
> 
> --
> Отправлено из мобильной Яндекс Почты
> 
> 05.08.2024, 18:51, "Barry Smith" <bsmith at petsc.dev>:
> PCFactorSetShiftType() only works for certain preconditioners, ILU. 
> 
> If the matrix is not symmetric it also doesn't help.
> 
> Are you sure the matrix is symmetric positive definite?
> 
> 
> 
> 
> On Aug 5, 2024, at 4:43 AM, Константин Мурусидзе <konstantin.murusidze at math.msu.ru <mailto:konstantin.murusidze at math.msu.ru>> wrote:
> 
> After this procedure I saw "Linear solution did not converge due to DIVERGED_INDEFINITE_MAT". Then as shown on https://urldefense.us/v3/__https://petsc.org/main/manualpages/KSP/KSP_DIVERGED_INDEFINITE_PC/__;!!G_uCfscf7eWS!ZQFXJM_C7qCPyXSN6vUgcQaTYavgEpmvtDdWSpbY1dR7m_6DlTM1CF00dL26gVO4tjPEFFvV79SEmjS1OqyHDu0$ ​  I wrote such line PetscCall(PCFactorSetShiftType(pc, MAT_SHIFT_POSITIVE_DEFINITE)); but nothing changed and I still have divergence
> 
> 
> 22.07.2024, 17:22, "Barry Smith" <bsmith at petsc.dev <mailto:bsmith at petsc.dev>>:
> 
>    Run with -ksp_monitor_true_residual -ksp_converged_reason -ksp_view to see why it is stopping at 38 iterations.
> 
>    Barry
> 
> 
> On Jul 22, 2024, at 7:37 AM, Константин Мурусидзе <konstantin.murusidze at math.msu.ru <mailto:konstantin.murusidze at math.msu.ru>> wrote:
> 
> This Message Is From an External Sender
> This message came from outside your organization.
> Good afternoon. I am a student at the Faculty of Mathematics and for my course work I need to solve SLAE with a relative accuracy of 1e-8 or more. To do this, I created the function PetscCall(KSPSetTolerances(ksp, 1.e-8, PETSC_DEFAULT, PETSC_DEFAULT, 100000));. But in the end, only 38 iterations were made and the relative norm ||Ax-b||/||b|| it turns out 4.54011. If you reply to my email, I can give you more information about the solver settings.
> 
> 

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