{ diff(x1, t) = ((-1)*k25*(k2*x1*x2 - k3*x3) + 1*k25*k7*x7^2 + (-1)*k25*(k18*x1 - k18*x13))/k25,
  diff(x2, t) = ((-1)*k25*(k2*x1*x2 - k3*x3) + 1*k25*k5*x5*x4 + (-1)*k25*k6*x2*x7)/k25,
  diff(x3, t) = (1*k25*(k2*x1*x2 - k3*x3) + (-1)*k25*k4*x3*x4 + 1*k25*k8*x19*x7)/k25,
  diff(x4, t) = ((-1)*k25*k4*x3*x4 + (-1)*k25*k5*x5*x4 + (-1)*k25*(k19*x4 - k19*x18))/k25,
  diff(x5, t) = (1*k25*k4*x3*x4 + (-1)*k25*k5*x5*x4)/k25,
  diff(x6, t) = (1*k25*k4*x3*x4 + 1*k25*k5*x5*x4 + (-1)*k25*(k20*x6 - k20*x17))/k25,
  diff(x7, t) = ((-1)*k25*k6*x2*x7 + (-2)*k25*k7*x7^2 + (-1)*k25*k8*x19*x7 + (-1)*k25*(k21*x7 - k21*x15) + 2*k25*k22*x10/k1*(1 + x10/k1)*x9/((k23 + (1 + x10/k1)^2)*(k24 + x9)))/k25,
  diff(x8, t) = (-2)*k25*k7*x7^2/k25,
  diff(x9, t) = (1*k25*k7*x7^2 + 1*k25*k8*x19*x7 + (-1)*k25*(k17*x9 - k17*x11) + (-2)*k25*k22*x10/k1*(1 + x10/k1)*x9/((k23 + (1 + x10/k1)^2)*(k24 + x9)))/k25,
  diff(x10, t) = ((-1)*k26*k9*x10*x11 + (-1)*k26*k12*x17*x10 + 1*k26*k14 + (-1)*k25*k22*x10/k1*(1 + x10/k1)*x9/((k23 + (1 + x10/k1)^2)*(k24 + x9)))/k26,
  diff(x11, t) = ((-1)*k26*k9*x10*x11 + (-1)*k26*k10*x14*x11 + 1*k26*k11*x15^2 + 1*k26*k15 + (-1)*k26*k16*x11 + 1*k25*(k17*x9 - k17*x11))/k26,
  diff(x12, t) = (1*k26*k9*x10*x11 + 1*k26*k10*x14*x11 + 1*k25*k22*x10/k1*(1 + x10/k1)*x9/((k23 + (1 + x10/k1)^2)*(k24 + x9)))/k26,
  diff(x13, t) = (1*k26*k9*x10*x11 + 1*k26*k11*x15^2 + 1*k25*(k18*x1 - k18*x13))/k26,
  diff(x14, t) = ((-1)*k26*k10*x14*x11 + 1*k26*k12*x17*x10 + (-2)*k26*k13*x14^2)/k26,
  diff(x15, t) = (1*k26*k10*x14*x11 + (-2)*k26*k11*x15^2 + 1*k25*(k21*x7 - k21*x15))/k26,
  diff(x16, t) = (-2)*k26*k11*x15^2/k26,
  diff(x17, t) = ((-1)*k26*k12*x17*x10 + 1*k25*(k20*x6 - k20*x17))/k26,
  diff(x18, t) = (1*k26*k12*x17*x10 + 1*k25*(k19*x4 - k19*x18))/k26,
  diff(x19, t) = (1*k25*k6*x2*x7 + (-1)*k25*k8*x19*x7)/k25,
  diff(x20, t) = 1*k26*k13*x14^2/k26 }