[petsc-users] MatAXPY not working as expected

[Jed Brown]

Christoph Pohl <Metal-Gear-Rex at web.de> writes:

> Dear fellow PETSc users,
>
> I've been trying to subtract one matrix from another, i.e. Y = Y - X, using 
> `call MatAXPY(Y, minus_one, X, SAME_NONZERO_PATTERN, ierr)`. 

You can only use SAME_NONZERO_PATTERN pattern if the matrices have the
same nonzero pattern.  These matrices don't even have the same number of
nonzeros and there are quite a few entries in different places (neither
is a subset of the other).  Use DIFFERENT_NONZERO_PATTERN in cases like
this.
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