{ diff(x1, t) = (-x1) + k4*x2^2/(k3^2 + x2^2),
  diff(x2, t) = (-k5)*x2 + 1/(1 + x1^2)*(k1 + (k7 + (k8 - k7)*t^2/(k6^2 + t^2))*x2^2/(1 + x2^2)) }