On 12/13/2017 09:06 AM, Marc Fehling wrote:
thank you for your hint! Indeed, choosing a time step size according to the
/threshold of positivity/ from Theorem 6 of Thomée's paper will lead to
nonnegative results.
However, choosing a step size slightly below this lower bound still yields the
desired nonnegative results, probably because the system matrix is not a
Stieltjes matrix, as I checked the entries of the stiffness matrix. But this
more restrictive condition certainly puts you on the safe side.
I think your observation of negative values is an interesting one (and
surprising one, for many). Would you be interested in writing a couple of
paragraphs about time step choice for the introduction of this program?
Best
W.
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Wolfgang Bangerth email: [email protected]
www: http://www.math.colostate.edu/~bangerth/
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