{ diff(x1, t) = (1*k13*k4/k13 + (-1)*k13*k2*x1/k13 + (-1)*k13*k3*x1*x2/k13 + (-1)*k13*x1*log(2)/k7/k13 + (-1)*k13*k1*x1/k9/k13)/k13,
  diff(x2, t) = (1*k13*k5/k13 + 1*k13*k2*x1/k13 + 1*k13*k3*x1*x2/k13 + (-1)*k13*x2*log(2)/k8/k13)/k13,
  diff(x3, t) = (1*k13*k1*x1/k10/k13 + (-1)*k13*k2*x3/k13 + (-1)*k13*k3*x3*x4/k13 + (-1)*k13*x3*log(2)/k7/k13)/k13,
  diff(x4, t) = (1*k13*k6/k13 + 1*k13*k2*x3/k13 + 1*k13*k3*x3*x4/k13 + (-1)*k13*x4*log(2)/k8/k13)/k13 }