The grid spacing and time step were chosen to reliably create spiral waves
with appropriately smooth profiles and propagation speeds. A criterion of
numerical accuracy was that small changes in the grid or time step should
not give rise to large differences in the solution. With numerical
parameters chosen as in Sect.2.4 neither reducing the steps 
or 
by 
nor reducing the time step by 
changed
the wave period by more than 
. Nevertheless, even in the absence of any
forcing, a radial drift of spiral waves away from the centre was observed.
The radial drift velocity 
su/tu was less than 0.2% of
the plane wave velocity. Numerical experiments evidenced that the mentioned
radial drift velocity decreased as the became smaller, 
su/tu for 
; su/tu for
; 
su/tu for 
; and 
su/tu for 
. In these test computations the time
step 
was used to satisfy the stability criterion for the
case of the smallest . The very slow radial drift should
probably be attributed to the grid anisotropy produced by the summation of
local errors of approximation in integrating the discrete analog of
equations (1)-(5) in polar coordinates. In numerical experiments
described below, this numerical artefact was always much smaller than the
resonant drift.