{ diff(x1, t) = (1*k21*2060/365 + (-1)*k21*1/(2*365)*x1 + (-1)*k21*45685/(439197*30)*x1*x2)/k21,
  diff(x2, t) = (1*k21*45685/(439197*30)*x1*x2 + (-1)*k21*45682/(439197*30)*x2 + (-1)*k21*1/(2*365)*x2)/k21,
  diff(x3, t) = (1*k22*237/10*k20/365000 + (-1)*k22*1/(346/5*365)*x3 + (-1)*16/(108*1000000*1825/2)*x3*x2)/k22,
  diff(x4, t) = (1*16/(108*1000000*1825/2)*x3*x2 + (-1)*k22*6/(108*1000000*1825/2)*x4 + (-1)*k22*1/(346/5*365)*x4)/k22 }