In the proof of the sufficient condition, Hassan used a new method which was developed by Dyn in 1992.The method is different from the direct method used by Dyn in her paper in 1987.

• 其中在充分性条件的证明中，Hassan采用了Dyn在1992年提出的一种新方法，而不是Dyn在1987年发表的四点法中更加直接的办法。