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单词 ProofOfSecondIsomorphismTheoremForGroups
释义

proof of second isomorphism theorem for groups


First, we shall prove that HK is a subgroupMathworldPlanetmathPlanetmath of G:Since eH and eK, clearly e=e2HK.Take h1,h2H,k1,k2K.Clearly h1k1,h2k2HK.Further,

h1k1h2k2=h1(h2h2-1)k1h2k2=h1h2(h2-1k1h2)k2

Since K is a normal subgroupMathworldPlanetmath of G and h2G,then h2-1k1h2K.Therefore h1h2(h2-1k1h2)k2HK,so HK is closed under multiplicationPlanetmathPlanetmath.

Also, (hk)-1HK for hH, kK, since

(hk)-1=k-1h-1=h-1hk-1h-1

and hk-1h-1K since K is a normal subgroup of G.So HK is closed under inversesMathworldPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath, and is thus a subgroup of G.

Since HK is a subgroup of G,the normality of K in HK follows immediately fromthe normality of K in G.

Clearly HK is a subgroup of G,since it is the intersectionMathworldPlanetmathPlanetmath of two subgroups of G.

Finally, define ϕ:HHK/K by ϕ(h)=hK.We claim that ϕ is a surjective homomorphismMathworldPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath from H to HK/K.Let h0k0K be some element of HK/K;since k0K, then h0k0K=h0K, and ϕ(h0)=h0K.Now

ker(ϕ)={hHϕ(h)=K}={hHhK=K}

and if hK=K, then we must have hK. So

ker(ϕ)={hHhK}=HK

Thus, since ϕ(H)=HK/K and kerϕ=HK,by the First Isomorphism TheoremPlanetmathPlanetmath we see thatHK is normal in Hand that there is a canonical isomorphism between H/(HK) and HK/K.

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更新时间:2025/5/4 16:13:28