lorzrk(a,time,param)

function deriv = lorzrk(a,time,param)
%  Function to define the Lorenz model equations
%  The vector a = [x y z]
%  The time is not used in this function
%  The vector param = [r s b]
%  The vector deriv = [dx/dt dy/dt dz/dt]
x = a(1); y = a(2); z = a(3);
r = param(1); s = param(2); b = param(3);
deriv(1) = s*(y-x);
deriv(2) = r*x - y - x*z;
deriv(3) = x*y - b*z;
return;