{ diff(x1, t) = ((-1)*k13*k5*x1 + (-1)*k13*k11*k10*x3*x1 + 1*k13*k11*x2*x3*x1)/k13,
  diff(x2, t) = ((-1)*k13*k2*x2 + (-1)*k13*piecewise(1, t < 1, piecewise(1, t > 15, 21/5000))*k8*x2*x3 + 1*k13*k1)/k13,
  diff(x3, t) = ((-1)*k13*k3*x3 + 1*k13*piecewise(1, t < 1, piecewise(1, t > 15, 21/5000))*k8*x2*x3 + (-1)*k13*k9*x3*x4)/k13,
  diff(x4, t) = ((-1)*k13*k7*x4 + 1*k13*k11*k10*x3*x1)/k13 }