[ Diff(x[1], t) = (-x[1]) + k[4]*x[2]^2/(k[3]^2 + x[2]^2),
  Diff(x[2], t) = (-k[5])*x[2] + 1/(1 + x[1]^2)*(k[1] + k[2]*x[2]^2/(1 + x[2]^2)) ]