{ diff(x1, t) = k1*x1,
  diff(x2, t) = k2*x2^k8,
  diff(x3, t) = k3*x3*(1 - x3/k9),
  diff(x4, t) = k4*x4/(x4 + k10),
  diff(x5, t) = k5*x5/(x5 + k11)^(1/3),
  diff(x6, t) = k6*x6*log(k12/(x6 + k14)),
  diff(x7, t) = k7*x7^(2/3) - k13*x7 }