[ Eq(Derivative(x1, t), (1*k1*k2/(k3*k4)*(k5*k21 - k6*x1/k7)/((1 + k8/k9 + k10/k11 + k12/k13)*(1 + k5/k3 + k6/k14)*(1 + k21/k4 + x1/k15)) + (-1)*k1*k16*x1/k17/((1 + x1/k17)*(1 + k18/k19)))/k1),
  Eq(Derivative(x2, t), 0),
  Eq(Derivative(x3, t), 0) ]