{ diff(x1, t) = 0,
  diff(x2, t) = (1*k3*k4/(1 + (x3*(1 - 2/(1 + (1 + 8*k6*x3)^(1/2)))/(2*k5))^2) + (-1)*k8*x2*k3)/k3,
  diff(x3, t) = (1*k7*x2*k3 + (-1)*k9*x3*k3 + (-1)*k3*(k10*x3*2/(1 + (1 + 8*k13*x3)^(1/2)) + k11*x3)/(k12 + x3))/k3 }