[petsc-users] basis/basisDer/Jac in FE

[Yann Jobic]

There is this factor also in front of the rectangle, which should 1/4 
for a unit rectangle of dim 2, but it's also 0.5.
Thanks,
Yann


Le 10/27/2022 à 6:23 PM, Mark Adams a écrit :
> Area of a unit triangle is (1x1)/2 I would guess.
> 
> On Thu, Oct 27, 2022 at 11:32 AM Yann Jobic <yann.jobic at univ-amu.fr 
> <mailto:yann.jobic at univ-amu.fr>> wrote:
> 
>     Hello,
> 
>     I'm trying to understand how to use the lowlevel Petsc Finit Element
>     Framework. I've got few questions about it.
> 
>     1) I'm checking the determinant of the Jacobian transformation from
>     real
>     space to the parametric one. The source code is in :
>     https://petsc.org/main/src/dm/impls/plex/plexgeometry.c.html#DMPlexComputeTriangleGeometry_Internal <https://petsc.org/main/src/dm/impls/plex/plexgeometry.c.html#DMPlexComputeTriangleGeometry_Internal>
>     I don't quite understand the 0.5 factor in front of it.
>     Geometrically, it should be the ratio of the area/volume of the reel
>     element over the referenced one. It's not the case here no ?
>     It seems that, for other elements, this factor still applies.
>     What this factor represent ?
> 
>     2) I only checked triangle elements here. The ordering of the test
>     functions and its derivatives is different it seems, with also the 0.5
>     factor. Am i mistaken ?
> 
>     I could not find any informations about it in the user-doc. Maybe it's
>     written somewhere else ?
> 
>     Many Thanks,
> 
>     Best regards,
> 
>     Yann
>