[petsc-users] a question on DMPlexSetAnchors

[Rochan Upadhyay]

I think I sent my previous question (on Dec 28th) to the wrong place
(petsc-users-request at mcs.anl.gov).

To repeat,

I am having bit of a difficulty in understanding the introduction of
constraints in DMPlex. From a quick study of the User Manual I gather
that it is easiest done using DMPlexSetAnchors ? The description of this
routine says that there is an anchorIS that specifies the anchor points
(rows in the
matrix). This is okay and easily understood.

There is also an anchorSection which is described as a map from constraint
points
(columns ?)  to the anchor points listed in the anchorIS. Should this not
be a map between
solution indices (i.e. indices appearing in the vectors and matrices) ?

For example I am completely unable to set up a simple constraint matrix for
the following (say):

Point 1, Field A, B
Point 2-10 Field A
At point 1, Field B depends on Field A at points 1-10

When I set it up it appears to create a matrix where field A depends on
field A values at points 1-10.

How does the mapping work in this case ? Will the DMPlexSetAnchors()
routine work
for this simple scenario ?

If not, is the only recourse to create the constraint matrix oneself
using DMSetDefaultConstraints ?

Also documentation for DMSetDefaultConstraints is incomplete.
The function accepts three arguments (dm, section and Mat) but
what the section is is not described at all.

I don't know if my question makes any sense. If it does not then it is
only a reflection of my utter confusion regarding the routine
DMPlexSetAnchors :-(

Regards,
Rochan
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