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