{ diff(x1, t) = (1*k9*x2/k9 + (-1)*k9*k1*x1/k9)/k9,
  diff(x2, t) = (1*k9*k5*piecewise(sin(t)^2, t < k7*pi, 0)*piecewise(1, t < k8, 0)/k9 + 1*k9*k2*x3^2/(k6^2 + x3^2)/k9 + (-1)*k9*x2/k9)/k9,
  diff(x3, t) = (1*k9*k1*x1/k9 + (-1)*k9*k2*x3/k9 + (-1)*k9*k3*x4*x3/k9)/k9,
  diff(x4, t) = (1*k9*x2/k9 + (-1)*k9*k4*x4/k9)/k9 }