{ diff(x1, t) = (1*k26*k5*x1*(1 - x1/k6) + (-1)*k26*k1*k3*x1/(k4 + k2*x1) + (-1)*k26*k7*x2/(1 + (x4/k17)^2)*x1)/k26,
  diff(x2, t) = (1*k26*k10*(k9*x2 + k8*x1 + k13*x3)/(1 + (x4/k17)^2)/(k11 + (k9*x2 + k8*x1 + k13*x3)/(1 + (x4/k17)^2)) + (-1)*k26*k12*x2)/k26,
  diff(x3, t) = (1*k26*k14*(x2/(1 + (x4/k17)^2))^6/(k15^6 + (x2/(1 + (x4/k17)^2))^6) + (-1)*k26*k16*x3)/k26,
  diff(x4, t) = (1*k26*k18 + 1*k26*k19*(x2 + k20*x3)/(1 + (x4/k17)^2)/(1 + (x2 + k20*x3)/(1 + (x4/k17)^2)) + (-1)*k26*k21*x4)/k26 }