[petsc-users] How to find the map between the high order coordinates of DMPlex and vertex numbering?

[Zongze Yang]

Hi, if I load a `gmsh` file with second-order elements, the coordinates
will be stored in a DG-P2 space. After obtaining the coordinates of a cell,
how can I map the coordinates to vertex and edge?

Below is some code load the gmsh file, I want to know the relation between
`cl` and `cell_coords`.

```
import firedrake as fd
import numpy as np

# Load gmsh file (2rd)
plex = fd.mesh._from_gmsh('test-fd-load-p2-rect.msh')

cs, ce = plex.getHeightStratum(0)

cdm = plex.getCoordinateDM()
csec = dm.getCoordinateSection()
coords_gvec = dm.getCoordinates()

for i in range(cs, ce):
    cell_coords = cdm.getVecClosure(csec, coords_gvec, i)
    print(f'coordinates for cell {i} :\n{cell_coords.reshape([-1, 3])}')
    cl = dm.getTransitiveClosure(i)
    print('closure:', cl)
    break
```

Best wishes,
Zongze
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