{ diff(x1, t) = (1*k1*(10*k2 - x1)/(1 + k2 + x1 + x2^2) + (-1)*k1*(2*x1 - x2)/(1 + x1 + x2))/k1,
  diff(x2, t) = (1*k1*(2*x1 - x2)/(1 + x1 + x2) + (-1)*k1*(x2 - x3)/(1 + x2 + x3 + x4^2) + (-1)*k1*(x2 - x5)/(1 + x2 + x5 + x6^2))/k1,
  diff(x3, t) = (1*k1*(x2 - x3)/(1 + x2 + x3 + x4^2) + (-1)*k1*(5*x3 - x4)/(1 + x3 + x4))/k1,
  diff(x4, t) = (1*k1*(5*x3 - x4)/(1 + x3 + x4) + (-1)*k1*(10*x4 - k3)/(1 + x4 + k3))/k1,
  diff(x5, t) = (1*k1*(x2 - x5)/(1 + x2 + x5 + x6^2) + (-1)*k1*(5*x5 - x6)/(1 + x5 + x6))/k1,
  diff(x6, t) = (1*k1*(5*x5 - x6)/(1 + x5 + x6) + (-1)*k1*(10*x6 - k4)/(1 + x6 + k4))/k1,
  diff(x7, t) = 0,
  diff(x8, t) = 0,
  diff(x9, t) = 0 }