Exercise 5.3. Let G be a group and H, K be two subgroups of G. (1) Show that HK := {hk | h € H, k = K} is a disjoint union of some left cosets of K. (2) Let H, K be subgroups of G. Show that we have the following bijection (between coset spaces) K/(H^K) → HK/K defined by к(HK) → KK, Vк(HK) Є K/(H□K), where HK/K := {left cosets of K in HK}. (3) Suppose [G: H] and [G: K] are both finite. Prove or disprove that [G: HK] is finite.

Algebra and Trigonometry (6th Edition)
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ChapterP: Prerequisites: Fundamental Concepts Of Algebra
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Exercise 5.3. Let G be a group and H, K be two subgroups of G.
(1) Show that HK := {hk | h € H, k = K} is a disjoint union of some left cosets of K.
(2) Let H, K be subgroups of G. Show that we have the following bijection (between coset
spaces)
K/(H^K) → HK/K defined by
к(HK) → KK, Vк(HK) Є K/(H□K),
where HK/K := {left cosets of K in HK}.
(3) Suppose [G: H] and [G: K] are both finite. Prove or disprove that [G: HK]
is finite.
Transcribed Image Text:Exercise 5.3. Let G be a group and H, K be two subgroups of G. (1) Show that HK := {hk | h € H, k = K} is a disjoint union of some left cosets of K. (2) Let H, K be subgroups of G. Show that we have the following bijection (between coset spaces) K/(H^K) → HK/K defined by к(HK) → KK, Vк(HK) Є K/(H□K), where HK/K := {left cosets of K in HK}. (3) Suppose [G: H] and [G: K] are both finite. Prove or disprove that [G: HK] is finite.
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