function df = func_bursts(t,y) 

global r s x1 I 

xx = y(1); 
yy = y(2); 
zz = y(3); 

dx = yy - xx^3 + 3*xx^2 + I - zz; 
dy = 1 - 5*xx^2 - yy; 
dz = r*(s*(xx-x1)-zz); 

df = [dx;dy;dz]; 
