{ diff(x1, t) = k3 - (k2 + k1*x2)*x1,
  diff(x2, t) = x3*k4*x1^2/(k5 + x1^2) - k6*x2,
  diff(x3, t) = ((-k7) + k8*x1 - k9*x1^2)*x3 }