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## [Getfem-users] Maximum allowed residuum

 From: Moritz Jena Subject: [Getfem-users] Maximum allowed residuum Date: Tue, 7 May 2019 16:23:00 +0200

Dear GetFEM-Users,

I have written a FEM program, with which you can calculate technical springs numerically.
Therefore I'm using the GetFEM-MATLAB-Interface 4.3

These calculations are highly non-linear due to self-contact issues and large deformations.
That is the reason why technical springs have to be solved iteratively in several substeps.

The calculation by itself works very well. But I think I have a problem with the maximum allowed residuum that is passed to the solve command of getfem.
My theory is that the maximum allowed residuum has to be variable during the different substeps.
By investigating these question online I found a paper of an university in Germany that contains the following for a equation system of
A*x=b : ||delta_x||/||x|| <= Cond(A) * ||delta_b||/||b||
Unfortunatley these _expression_ is not leading to plausible results.

I'm also familiar using ansys. By using this commercial software I know that the maximum allowed residuum is changing by each iteration and by each substep.

All of the delivered getfem examples are using a constant maximum allowed residuum, no matter of high deformation or not.

So my question is, how am I able to find the maximum allowed residuum and by what _expression_? And do I have to define a variable maximum allowed residuum during the substeps to get reliable results out of the solve command?

Best regards,

Moritz Jena