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<p class="MsoNormal"><span lang="EN-US">Dear PETSc developers and users,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I have a question regarding the Fieldsplit preconditioner in PETSc. In particular, I want to know how the submatrices there are created from the parent matrix. The “obvious” way would be to take the DoF indices of the
corresponding split and “renumber” them so that the DoFs in the submatrix have the same order as the ones of the parent matrix. I did not find any documentation on this and as it is at least possible that the DoFs are re-ordered, I wanted to ask this question.
Obviously, in case the DoFs are re-ordered, how can I get the mapping between the DoFs of the parent and the submatrix?
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">The thing I am wanting to work on is implementing a pressure convection diffusion preconditioner with FEniCS for the incompressible Navier-Stokes equations. The parent matrix is assembled via a mixed FEM and then I use
PETSc to solve the system. I want to assemble the corresponding operators on the pressure space from a collapsed (i.e. sub-space of the mixed FEM) function space. However, FEniCS re-orders the DoFs there, but I can get a mapping between the DoFs so this should
not be problematic. However, I am not sure if PETSc also does a re-ordering.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Thanks a lot in advance and best regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Sebastian<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;mso-ligatures:none;mso-fareast-language:DE">--
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;mso-ligatures:none;mso-fareast-language:DE">Dr. Sebastian Blauth<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;mso-ligatures:none;mso-fareast-language:DE">Fraunhofer-Institut für<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;mso-ligatures:none;mso-fareast-language:DE">Techno- und Wirtschaftsmathematik ITWM<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;mso-ligatures:none;mso-fareast-language:DE">Abteilung Transportvorgänge<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;mso-ligatures:none;mso-fareast-language:DE">Fraunhofer-Platz 1, 67663 Kaiserslautern<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;mso-ligatures:none;mso-fareast-language:DE">Telefon: +49 631 31600-4968<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;mso-ligatures:none;mso-fareast-language:DE"><a href="mailto:sebastian.blauth@itwm.fraunhofer.de"><span style="color:blue">sebastian.blauth@itwm.fraunhofer.de</span></a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;mso-ligatures:none;mso-fareast-language:DE"><a href="https://urldefense.us/v3/__https://www.itwm.fraunhofer.de/__;!!G_uCfscf7eWS!f_qaoCRxX3prMgl6ev5fvSFQegVfZo84xW9eJTz7uYmLjZiyJFIlm1tlqYrM3LqjOpkEoMrIJZo6J63-23-atPBnJn4et_4R-UvZoWlBpHM$"><span style="color:blue">https://www.itwm.fraunhofer.de</span></a><o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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