2020-2ν•™κΈ°, λŒ€ν•™μ—μ„œ β€˜ν˜„λŒ€λŒ€μˆ˜1’ μˆ˜μ—…μ„ λ“£κ³  κ³΅λΆ€ν•œ λ°”λ₯Ό μ •λ¦¬ν•œ κΈ€μž…λ‹ˆλ‹€. 지적은 μ–Έμ œλ‚˜ ν™˜μ˜μž…λ‹ˆλ‹€ :)

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2020-2ν•™κΈ°, λŒ€ν•™μ—μ„œ β€˜ν˜„λŒ€λŒ€μˆ˜1’ μˆ˜μ—…μ„ λ“£κ³  κ³΅λΆ€ν•œ λ°”λ₯Ό μ •λ¦¬ν•œ κΈ€μž…λ‹ˆλ‹€. 지적은 μ–Έμ œλ‚˜ ν™˜μ˜μž…λ‹ˆλ‹€ :)



μ§€κΈˆ λ‹€λ£¨λŠ” Isomorphism Theorem을 μ™„μ „νžˆ μ΄ν•΄ν•˜κ³  μ²΄λ“ν•œλ‹€λ©΄, κ΅°λ‘  자체λ₯Ό μ•„μ£Ό 깊게 이해할 수 μžˆλ‹€ γ…Žγ…Ž



1st Isomorphism Theorem; FHTPermalink

첫번재 Isomorphism Theorem은 μ΄μ „μ˜ ν¬μŠ€νŠΈμ—μ„œ 이미 λ‹€λ£¨μ—ˆλ‹€. FHTκ°€ 곧 1st Isomorphism Theorem이닀!

Theorem. Fundamental Homormophism Theorem (FHT)

Let Ο•:G⟢Gβ€² be a group homo-.

Then,

  1. Ο•[G] is a group.
  2. G/ker⁑ϕ≅ϕ[G]

Image from here



Lemma 34.4Permalink

Lemma.

Let N⊴G, H≀G.

Then,

  1. HN=NH
  2. HN≀G

λ§Œμ•½ H μ—­μ‹œ normal subgroup이라면, HN⊴Gκ°€ λœλ‹€!


증λͺ…이 생각보닀 쉽닀!

proof.

1. HN=NH

Let h∈H, n∈N.

Then,

hnhβˆ’1∈N⟹hn∈Nh⟹hn=nβ€²hfor somenβ€²βˆˆN⟹∴HNβŠ†NH

λ°˜λŒ€λ‘œ hβˆ’1nh∈N둜 μž‘λŠ”λ‹€λ©΄, NHβŠ†HN의 κ²°κ³Όλ₯Ό μ–»λŠ”λ‹€.

λ‘˜μ„ μ’…ν•©ν•˜λ©΄,

HNβŠ†NH∧NHβŠ†HN⟹HN=NH

β—Ό

2. HN≀G

Normal subgpκ³Ό 일반 subgp만 μžˆλ‹€λ©΄, G에 μ†ν•˜λŠ” μƒˆλ‘œμš΄ subgp을 μœ λ„ν•  수 μžˆλ‹€λŠ” λͺ…μ œλ‹€.


κ°„λ‹¨νžˆ HN이 subgp인지 ν™•μΈν•˜λ©΄ λœλ‹€.

(1) closed under opr

(HN)(HN)=H(NH)N=H(HN)N=HN

(2) identity

e∈H∧e∈N⟹eβ‹…e=e∈HN

(3) inverse

(hn)βˆ’1=nβˆ’1hβˆ’1∈NH=HN

β—Ό



Definition. Subgp generated by set S

<S>: the subgroup of G generated by S

<S>=β‹‚SβŠ†H≀GH

μ΄λ•Œ, intersection of subgpsλŠ” μ—¬μ „νžˆ subgpμ΄λΌλŠ” 것이 μ•Œλ €μ Έ μžˆλ‹€.

즉, set Sλ₯Ό ν¬ν•¨ν•˜λŠ” subgroup 쀑 κ°€μž₯ μž‘μ€ subgroup이 <S>이닀.


H join KPermalink

Definition. H join K

H∨K:=<HβˆͺK>

H join KλŠ” subgroup H와 Kλ₯Ό ν¬ν•¨ν•˜λŠ” κ°€μž₯ μž‘μ€ subgroup이닀.

β€» μ΄λ•Œ! λ§Œμ•½ H⊴G라면, H∨K=HKκ°€ λœλ‹€!

β€œλ§Œμ•½ H⊴G라면, H∨K=HKκ°€ λœλ‹€!β€λΌλŠ” λͺ…μ œμ— λŒ€ν•΄ λ³΄μΆ©ν•΄λ³΄κ³ μž ν•œλ‹€.

HβˆͺKλ₯Ό ν¬ν•¨ν•˜λŠ” subgroupμ—λŠ” λ‹Ήμ—°νžˆ H도 ν¬ν•¨ν•˜κ³ , K도 ν¬ν•¨ν•˜κ³ , HK와 KH ν¬ν•¨ν•˜κ³  μžˆμ„ 것이닀.


μ΄λ•Œ, 운이 μ’‹μ•„ HK와 KH μ—°μ‚°μœΌλ‘œ 이미 Group을 이룬닀면, 또 HK=KH라면, Lucky! μš°λ¦¬λŠ” <HβˆͺK>=HK둜 μ°Ύμ•„λƒˆλ‹€!!

ν•˜μ§€λ§Œ, μ•„μ‰½κ²Œλ„ HKκ°€ κΌ­ Group을 μ΄λ£¬λ‹€λŠ” 보μž₯은 μ—†λ‹€ γ… γ…  hkβˆ‰H,K일 μˆ˜λ„ 있기 λ•Œλ¬Έμ΄λ‹€.


μš°λ¦¬κ°€ μ•žμ—μ„œ μ‚΄νŽ΄λ³Έ LemmaλŠ” HKκ°€ Group이 λ˜λŠ” 쑰건을 μ œμ‹œν•œλ‹€.

H⊴G∧K≀G⟹HK≀G

λ”°λΌμ„œ Hκ°€ Normal subgp이라면, H join KλŠ” HKκ°€ λœλ‹€!!



2nd Isomorphism TheoremPermalink

Theorem. 2nd Isomorphism Theorem

Let H≀G, N⊴G.

Then,

HN/Nβ‰…N/(H∩N)

정리 μžμ²΄λŠ” 정말 κ°„κ²°ν•˜λ‹€β€¦ ν•˜μ§€λ§Œ, λ‚΄μš©μ„ ν•œ λ¬Έμž₯으둜 μ••μΆ•ν•΄ 놓은 것이라 정리λ₯Ό μœ λ„ν•˜λŠ” λ°κΉŒμ§€ ν•„μš”ν•œ 뒷배경이 λ§Žμ€ νŽΈμ΄λ‹€ γ… γ… 


proof.

λ¨Όμ € 가정인 H≀G, N⊴Gλ‘œλΆ€ν„° λͺ…μ œμ˜ μž¬λ£Œκ°€ λ˜λŠ” factor group HN/N을 μœ λ„ν•˜μž. 이 κ³Όμ •μ—μ„œ μ•žλΆ€λΆ„μ— λ‚˜μ™”λ˜ Lemmaλ₯Ό μ‚¬μš©ν•œλ‹€.

H≀G, N⊴Gμ΄λ―€λ‘œ Lemma에 μ˜ν•΄ HN≀G이닀.


Normal subgp에 λŒ€ν•΄μ„  μ•„λž˜μ˜ λͺ…μ œκ°€ μ„±λ¦½ν•œλ‹€.

ForN≀K≀G,N⊴G⟹N⊴K

λͺ…μ œμ˜ 증λͺ…은 κ°„λ‹¨ν•˜λ‹ˆ μ—¬κΈ°μ—μ„œλŠ” 생-랡 ν•œλ‹€.

μ΄λ•Œ, N≀HN≀G이고, N⊴Gμ΄λ―€λ‘œ N⊴HN이 λœλ‹€.

N이 HN의 normal subgroupμ΄λ―€λ‘œ
N에 λŒ€ν•œ HN의 Factor Group HN/N을 μ •μ˜ν•  수 μžˆλ‹€!

μ΄λ²ˆμ—λŠ” λ™ν˜•μ‹μ˜ μš°λ³€μΈ H/(H∩N)을 μœ λ„ν•΄λ³΄μž.

λ§Œμ•½ N⊴G라면, H∩N⊴Hκ°€ μ„±λ¦½ν•œλ‹€.

λ”°λΌμ„œ (H∩N)에 λŒ€ν•œ H의 Factor Group H/(H∩N)을 μ •μ˜ν•  수 μžˆλ‹€!

λ“œλ””μ–΄ 증λͺ…μ˜ λ³Έκ²Œμž„μ΄λ‹€!

μ•„λž˜μ™€ 같은 homomorphism Ο•λ₯Ό λ””μžμΈ ν•œλ‹€.

Ο•:H⟢HN⟢HN/Nh⟼h⟼hN

μ΄λ•Œ, Ο•λŠ” homo-와 homo-의 ν•©μ„± μ΄λ―€λ‘œ μ—­μ‹œ homo-이닀.

λ˜ν•œ, Ο•(h)=hN이기 λ•Œλ¬Έμ— Ο•λŠ” onto이닀.

이제 이 homo- Ο•μ˜ kernel을 μƒκ°ν•΄λ³΄μž.
μš°λ¦¬λŠ” ker⁑ϕ=H∩N이 됨을 보일 것이닀.

h∈kerβ‘Ο•βŸΉΟ•(h)=hN=N

λ”°λΌμ„œ h∈N이고, kerβ‘Ο•βŠ†H∩N이닀.

λ°˜λŒ€λ‘œ,

x∈H∩NβŸΉΟ•(x)=xN=N

λ”°λΌμ„œ x∈ker⁑ϕ이고, kerβ‘Ο•βŠ†H∩N이닀.

λ”°λΌμ„œ ker⁑ϕ=H∩N이닀.


FHT에 μ˜ν•΄ H/ker⁑ϕ≅ϕ(H)이닀. μ΄λ•Œ, Ο•κ°€ onto μ˜€μœΌλ―€λ‘œ Ο•(H)=HN/N이닀.

λ”°λΌμ„œ

H/(H∩N)β‰…HN/N

Image from here



3rd Isomorphism TheoremPermalink

Theorem. 3rd Isomorphism Theorem

Let H,K⊴G, K≀H

Then,

G/H≅(G/K)/(H/K)

β—Ό

μœ λ„ κ³Όμ • μžμ²΄λŠ” 2nd iso- theorem에 λΉ„ν•΄μ„  정말 μ‰¬μš΄ νŽΈμ΄λ‹€ γ…Žγ…Ž


proof.

Define a homomoprhism Ο• as

Ο•:G/K⟢G/HgK⟼gH

Then, check properties of Ο•.

(1) well-defined

Supp. gK=gβ€²K, then

gK=gβ€²K⟹g(gβ€²)βˆ’1K=K⟹g(gβ€²)βˆ’1∈K⟹g(gβ€²)βˆ’1∈H(∡K≀H)⟹g(gβ€²)βˆ’1H=H⟹gH=gβ€²H

(2) Ο• is onto

clear

(3) Ο• is a homo-.

Ο•(g1K)Ο•(g2K)=Ο•(g1g2K) (by factor representative opr)

λ”°λΌμ„œ Ο•λŠ” homomorphism이닀.

FHT에 μ˜ν•΄

(G/K)/ker⁑ϕ≅ϕ(G/K)

μ΄λ•Œ, kerβ‘Ο•λŠ” μ•„λž˜μ™€ 같이 μœ λ„ν•  수 μžˆλ‹€.

gK∈kerβ‘Ο•βŸΉΟ•(gK)=gH=H⟹g∈H

λ”°λΌμ„œ gK=hK∈H/K이고, kerβ‘Ο•βŠ†H/Kκ°€ λœλ‹€.

λ°˜λŒ€λ‘œ

hK∈H/KβŸΉΟ•(hK)=hH=H

λ”°λΌμ„œ hK∈ker⁑ϕ이고, H/KβŠ†ker⁑ϕ이닀.

λ”°λΌμ„œ ker⁑ϕ=H/K이닀.


λ‹€μ‹œ FHT에 μ˜ν•΄ (G/K)/ker⁑ϕ≅ϕ(G/K)μ΄λ―€λ‘œ

(G/K)/(H/K)β‰…G/H

β—Ό



λ“œλ””μ–΄ Isomorphism Thm κΉŒμ§€ λ„λ‹¬ν–ˆλ‹€!!

μ•žμœΌλ‘œλ„ ꡰ둠의 λ‹€μ–‘ν•œ 정리듀과 사둀듀이 기닀리고 μžˆμœΌλ‹ˆ! κΈ°λŒ€ν•˜μ‹œλΌ!!!