The function F(t), 
was a series of rectangular pulses
of a fixed duration 
0.1 tu with amplitude A, 
, varying in different numerical experiments. The time moments 
,
k=0,1, ..., at which the perturbation of amplitude A and duration 
was applied were determined by one of the following procedures:
) 
, where 
was the constant period of
and 
was a time delay;
) 
, where 
was the
k-th moment in time t when the condition 
(mod 
) had been fulfilled, 
was the phase at t=0;
) 
; here 
was the k-th
moment in t when the excitation wavefront, the isoline u=0, reached a
control point interpreted as a recording electrode.