{ diff(x1, t) = (1*k11*(k1 + k2*x1) + (-1)*k11*(k4*x1 + k5*x4*x1 + k2*x1*(x1 + x2 + x3)/k3))/k11,
  diff(x2, t) = (1*k11*k5*x4*x1 + (-1)*k11*(k4*x2 + k6*x2))/k11,
  diff(x3, t) = (1*k11*k6*x2 + (-1)*k11*k7*x3)/k11,
  diff(x4, t) = (1*k11*k9*k7*x3 + (-1)*k11*(k5*x4*x1 + k8*x4))/k11 }