嚇死我，差點證不出來。口語解讀是，如果我在 short exact sequence 中找得到一個反向的 morphism 促使正向的 morphism 是 left(or right) split 的話，那麼這串 sequence 就可以寫成前後兩個 module 的 direct sum。
Splittin Lemma. Let be a short exact sequence of module homomorphism. The the following are equivalent.
- (Right split): There is an module homomorphism with ,
- (Left split): There is an module homomorphism with ,
- (Direct Sum): The given sequence is isomorphic to , and thus .
(1) (3). By the universal property of coproduct, we have a morphism with , and . Moreover, . Apply it by , and then we have . Since by exactness, then . Thus we have proved . Hence, they all commute between these morphisms, and is an isomorphism by the short five lemma.
(2) (3). By the duality of the above statement, we have the dual result. But we could also prove it directly.
By the universal property of product, we have with and . Moreover . Hence . Thus we have . It follows that is an isomorphism by the short five lemma.
(3) (1), (2). Let and . We have and .
Short Five Lemma 比 Five Lemma 簡單多了，用這個會比較好作。另外關於 split 還有
Lemma. If and , then .
Proof . Let and . Sinc . and . We have proved that .
Lemma. If for is a -homomorphism. Then
Proof. Let and . It’s obvious that , and which means . Hence .
Five lemma 好像應該也要打出來，不過這實在太囉唆，找找 LaTeX 的繪圖工具 …