A New Proof of The Strong Subadditivity Theorem
  It is well known that the strong subadditivity theorem is hold for classical system, but it is very difficult to prove that it is hold for quantum system. The first proof of this theorem is due to Lieb by using the Lieb's theorem. Here we use the conditions obtained in our previous work of matrix analysis method to give a new proof of this famous theorem. This new proof is very elementary, it only needs to carefully analyse the minimal value of a function. This proof also shows that the conditions obtained in our previous work are stronger than the strong subadditivity theorem. 
Yong-Jian Han, Yong-Sheng Zhang, Guang-Can Guo