[ Eq(Derivative(x1, t), (1*k16*k17*k28*x3/(k19*k18)/((1 + k28/k19 + k21*x5/k20)*(1 + x3/k18)) + (-1)*k16*k22*k8*x1/k7*x3/k11*(1 + x1/k7 + x3/k11 + k8*x1/k7*x3/k11)/((1 + x1/k7 + x3/k11 + k8*x1/k7*x3/k11)**2 + k10*((1 + k15*x3/k14)/(1 + x3/k14))**2*(1 + k9*x1/k7 + k13*x3/k11 + k12*k9*x1/k7*k13*x3/k11)**2))/k16),
  Eq(Derivative(x2, t), (1*k16*k22*k8*x1/k7*x3/k11*(1 + x1/k7 + x3/k11 + k8*x1/k7*x3/k11)/((1 + x1/k7 + x3/k11 + k8*x1/k7*x3/k11)**2 + k10*((1 + k15*x3/k14)/(1 + x3/k14))**2*(1 + k9*x1/k7 + k13*x3/k11 + k12*k9*x1/k7*k13*x3/k11)**2) + (-1)*k16*k23*x2*x6/(k24*k25)/((1 + x2/k24)*(1 + x6/k25)))/k16),
  Eq(Derivative(x3, t), ((-1)*k16*k17*k28*x3/(k19*k18)/((1 + k28/k19 + k21*x5/k20)*(1 + x3/k18)) + (-1)*k16*k22*k8*x1/k7*x3/k11*(1 + x1/k7 + x3/k11 + k8*x1/k7*x3/k11)/((1 + x1/k7 + x3/k11 + k8*x1/k7*x3/k11)**2 + k10*((1 + k15*x3/k14)/(1 + x3/k14))**2*(1 + k9*x1/k7 + k13*x3/k11 + k12*k9*x1/k7*k13*x3/k11)**2) + 4*k16*k23*x2*x6/(k24*k25)/((1 + x2/k24)*(1 + x6/k25)) + (-1)*k16*k26*x3/(k27 + x3))/k16),
  Eq(Derivative(x4, t), 0) ]