[dune-fem] Strange initialisation of tolerance in CG stopping criterion

[Sacconi, Andrea]

Oops,

I thought I read "sqrt", now that I look at it, it says "sqr". My fault, I admit.

Thanks anyway!
Andrea
__________________________________________________________

Andrea Sacconi
PhD student, Applied Mathematics
AMMP Section, Department of Mathematics, Imperial College London,
London SW7 2AZ, UK
a.sacconi11 at imperial.ac.uk

________________________________________
From: Andreas Dedner [a.s.dedner at warwick.ac.uk]
Sent: 28 May 2014 10:42
To: Sacconi, Andrea
Cc: dune-fem at dune-project.org
Subject: Re: [dune-fem] Strange initialisation of tolerance in CG stopping criterion

On 27/05/14 16:44, Sacconi, Andrea wrote:
> ", sqr(Residuum):"
> >><< residuum << std::endl;
Sorry I missed the point here - but I think I still do (only differently):

The variable residuum contains the square and the verbose output states
that:
sqr(Residuum)

Of course the text shouldn't be in German an probably
using ^2 instead of sqr would be more appropriate since it is not so
easy to confuse with sqrt.
But there is not really a mistake here or am I missing something?

Btw: the stopping criteria does make sense in many cases, consider
a Ax = a b
with some constant real number a - you would like the cg method to
compute x with the same
accuracy independent of the value of a. Of course there are cases where
a relative error is
not a good idea and having some switch would be a good thing. Nobody has
really worked on a
good concept for defining stopping criteria which could be usefully
applied to all the different solvers
available in dune-fem. We have discussed it a few times but....
Andreas