[petsc-users] Finding off diagonal blocks of matrix

[Eda Oktay]

Hello everyone,

I am trying to find the off diagonal blocks of a matrix. For example, I
partitioned a 10*10 matrix into 4*4 and 6*6 block sub matrices. I need to
find the other blocks in order to find the number of nonzero elements in
them.

I tried to use MatGetLocalSubMatrix but I couldn't figure out how to give
iscol and isrow. Also, from the manual, ı understood that
MatSetLocalToGlobalMapping should be used before this routine but I
couldn't used it properly because before MatGetLocalSuMatrix, the submatrix
doesn't exist. I tried to give zero to all elements so that I can use
Mapping routine, but I got an error saying that it is invalid.

How can I fix this probelm?

Thank you,

Eda
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