[petsc-users] MatGetRowMinAbs returns negative number

[Jed Brown]

On Wed, Jan 4, 2012 at 10:40, Matthew Knepley <knepley at gmail.com> wrote:

> I have just noticed that MatGetRowMinAbs (but not the Max variant) in
>> some cases returns a very small negative number like -1.62505e-17
>> (displayed as %g). Is this an epsilon effect or an indication of a
>> problem?
>>
>
> Its not possible that was actually the minimum element?
>

It is supposed to be returning the absolute value anyway, so no. Also, due
to this patch of yours

http://petsc.cs.iit.edu/petsc/petsc-dev/rev/e433a0

it should never return a value smaller than 1e-12 that isn't identically
zero. I don't understand why this shift is being done, but I think that if
you want a shift like this, it should at least be relative to the norm of
the vector or something.
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