{ diff(x1, t) = -k1*x1*x17,
  diff(x2, t) = k1*x1*x17,
  diff(x3, t) = 4*(x2 - x3)/k12,
  diff(x4, t) = 4*(x3 - x4)/k12,
  diff(x5, t) = 4*(x4 - x5)/k12,
  diff(x6, t) = 4*(x5 - x6)/k12,
  diff(x7, t) = k3*x2*x18/(1 + k11*x16) - k5*x7,
  diff(x8, t) = k5*x7 - k4*x8,
  diff(x9, t) = 4*(x8 - x9)/k13,
  diff(x10, t) = 4*(x9 - x10)/k13,
  diff(x11, t) = 4*(x10 - x11)/k13,
  diff(x12, t) = 4*(x11 - x12)/k13,
  diff(x13, t) = 4*(x20 - x13)/k14,
  diff(x14, t) = 4*(x13 - x14)/k14,
  diff(x15, t) = 4*(x14 - x15)/k14,
  diff(x16, t) = 4*(x15 - x16)/k14,
  diff(x17, t) = -k1*x1*x17,
  diff(x18, t) = k2*x6 + k9*x19 - k8*x18 - k3*x2*x18/(1 + k11*x16),
  diff(x19, t) = k8*x18 - k9*x19 + k4*x8,
  diff(x20, t) = k10 + k6*x12 - k7*x20 }