[代數] 想請問一題代數
Assume that G is a finite group, and let H be a nonempty subset of G.
Prove that H is closed if and only if H is subgroup of G.
(<=) Since H is subgroup of G, H is closed.
(=>) Since H is closed and nonempty set.
存在一個 a 屬於 H, a*a 屬於 H
接下來就不知道怎麼找出 e 了...
謝謝
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