irreducible case

Improves the theorem “A finite poset is dismantlable if and only if it is dismantlable by irreducibles”, obtains a new theorem “A finite poset is homotopic dismantlable if and only if it is dismantlable by star irreducibles” and proves the equivalence of these theorems.

就Ｆｉｘｅｄ Ｐｏｉｎｔ ｉｎ Ｐｏｓｅｔ中的定理“有限偏序集Ｐ是可拆的充要条件为Ｐ是不可约可拆”进行了改进，得到了Ｐ是同伦可拆的充要条件为Ｐ是星形不可约可拆，并且证明了这两个充要条件等价，极大地简化了不可约可拆的判别