{ diff(x1, t) = (1*k1*k2 + (-1)*k1*x1*k3*t/(100 + t)/(1 + exp((k4 - x2)*k5)) + (-1)*k1*k6*x1 + 1*k1*k14*x2/(1 + exp((k15 - x4)*k16)) + 1*k1*k18*x2)/k1,
  diff(x2, t) = (1*k1*x1*k3*t/(100 + t)/(1 + exp((k4 - x2)*k5)) + (-1)*k1*k14*x2/(1 + exp((k15 - x4)*k16)) + (-1)*k1*k18*x2)/k1,
  diff(x3, t) = (1*k1*k7 + (-1)*k1*x3*k8*x4^k9/(k10 + x4^k9) + (-1)*k1*k11*x3/(1 + exp((k12 - x2)*k13)) + (-1)*k1*k17*x3 + 1*k1*k19*x4)/k1,
  diff(x4, t) = (1*k1*x3*k8*x4^k9/(k10 + x4^k9) + 1*k1*k11*x3/(1 + exp((k12 - x2)*k13)) + (-1)*k1*k19*x4)/k1 }