Often one observes a speed of convergence for CGS that is about twice
as fast as for BiCG, which is in agreement with the observation that
the same ``contraction'' operator is applied twice. However, there is
no reason that the ``contraction'' operator, even if it really reduces
the initial residual 
, should also reduce the once reduced
vector 
. This is evidenced by the often
highly irregular convergence behavior of CGS . One should be aware of
the fact that local corrections to the current solution may be so
large that cancelation effects occur. This may lead to a less
accurate solution than suggested by the updated residual
(see Van der Vorst [207]).
The method tends to diverge if the starting guess is close to the solution.