O Definition 16: Let h be a subgroup of a group G, and let aeG. we define the left coset of H in G containing g, written gH, by gH={g制h∈H. Similarity we define the right coset of H in G containing g, written Hg, by Hg h gl h ehy G a ah a∈G a∈G
Definition 16: Let H be a subgroup of a group G, and let aG. We define the left coset of H in G containing g,written gH, by gH ={g*h| h H}. Similarity we define the right coset of H in G containing g,written Hg, by Hg ={h*g| h H}. a G a G G H a a H = =
E;+ s Example: S3=e, 01, 02, 03, 04,05 H1={e,o1;H2={e,o2;H3={e,o3}; I. H(e, c4, os)o
[E;+] Example:S3={e,1 , 2 , 3 , 4 , 5 } H1={e, 1 }; H2={e, 2 }; H3={e, 3 }; H4={e, 4 , 5 }。 H1
Lemma 2: Let h be a subgroup of the group G. Then gH=H and Hg=H for Vg∈G. Proof: (p: H>Hg, o (h)=h*g
Lemma 2:Let H be a subgroup of the group G. Then |gH|=|H| and |Hg|=|H| for gG. Proof: :H→Hg, (h)=hg
6.4.3 Lagrange's Theorem Theorem 6.19: Let H be a subgroup of the group G. Then ghgeg and Hglgeg have the same cardinal number ◆ Proof:LetS={Hgg∈G}andT=gHg∈G q:S→T,φ(Ha)=alHl 1)p is an everywhere function. for ha=hb a-H?=b-lH a≠blif|a]n[b]= 2)(p is one-to-one For Ha, Hb, suppose that Ha*Hb, and cp(Ha=(p(Hb) oNto
6.4.3 Lagrange's Theorem Theorem 6.19: Let H be a subgroup of the group G. Then {gH|gG} and {Hg|gG} have the same cardinal number Proof:Let S={Hg|gG} and T={gH|gG} : S→T, (Ha)=a-1H。 (1) is an everywhere function. for Ha=Hb, a -1H?=b-1H [a][b] iff [a]∩[b]= (2) is one-to-one。 For Ha,Hb,suppose that HaHb,and (Ha)=(Hb) (3)Onto
Definition 17: Let H is a subgroup of the group G. The number of all right cosets(left cofets) of H is called index of H n g E;+ is a subgroup of z;+ K Es index?? Theorem 6.20: Let g be a finite group and let h be a subgroup of G. Then G is a multiple of H Example: Let g be a finite group and let the order of a in G be n Then n G
Definition 17:Let H is a subgroup of the group G. The number of all right cosets(left cofets) of H is called index of H in G. [E;+] is a subgroup of [Z;+]. E’s index?? Theorem 6.20: Let G be a finite group and let H be a subgroup of G. Then |G| is a multiple of |H|. Example: Let G be a finite group and let the order of a in G be n. Then n| |G|
Example: Let g be a finite group and G=p. If p is prime, then G is a cyclic group
Example: Let G be a finite group and |G|=p. If p is prime, then G is a cyclic group
G ≠0,a,b,c,d∈R C H ≠0,a,b,c2d∈} c d)c a 2a√2b a,b,c,d∈Q 01 √20 a a,b,c,d∈Q 01
{ | 0,a,b,c,d R} c d a b c d a b G = { | 0,a,b,c,d Q} c d a b c d a b H = | a,b, c,d Q} c d 2a 2b H { 0 1 2 0 = | a,b, c,d Q} 2c d 2a b { 0 1 2 0 H =
O 6.4.4 Normal subgroups Definition 18: A subgroup H of a group is a normal subgroup if gh=g for VgEG. 4 Example: Any subgroups of Abelian group are normal subgroups ◆S3={e o5: 90192,03,04905 ◆H1={e,o};H2={e,a2};H3={e,3};H4={e, 4, 05 are subgroups of s3. Hg is a normal subgroup
6.4.4 Normal subgroups Definition 18:A subgroup H of a group is a normal subgroup if gH=Hg for gG. Example: Any subgroups of Abelian group are normal subgroups S3={e,1 , 2 , 3 , 4 , 5 } : H1={e, 1 }; H2={e, 2 }; H3={e, 3 }; H4={e, 4 , 5 } are subgroups of S3 . H4 is a normal subgroup
4(1)If H is a normal subgroup of G, then Hg= gH for Wg∈G (2)H is a subgroup of g. .3)Hg=gH, it does not imply hg=gh. ◆(4)IfHg=gH, then there exists h'∈ H such that hg= gh' for vh∈H
(1) If H is a normal subgroup of G, then Hg=gH for gG (2)H is a subgroup of G. (3)Hg=gH, it does not imply hg=gh. (4) If Hg=gH, then there exists h'H such that hg=gh' for hH
tHeorem 6.21: Let H be a subgroup of G. H .is a normal subgroup of g iff ghgEH for yg∈ G and h∈H Example:LetG={(X;y川x,y∈ R with x≠0}, and consider the binary operation o introduced by (x, y)o(z,w)=(xz, xw+) for (x,y),(z,w)∈G. Let H=l(, y yER. Is H a normal subgroup of g? Why? ◆1. H is a subgroup of c ◆2. normal?
Theorem 6.21: Let H be a subgroup of G. H is a normal subgroup of G iff g-1hgH for gG and hH. Example:Let G ={ (x; y)| x,yR with x 0} , and consider the binary operation ● introduced by (x, y) ● (z,w) = (xz, xw + y) for (x, y), (z, w) G. Let H ={(1, y)| yR}.Is H a normal subgroup of G? Why? 1. H is a subgroup of G 2. normal?