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

[Yann Jobic]

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
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