Most properties apply to the empty set. Almost everyone in the known Universe claims to be quite smart or, at least, more intelligent than average. Everyone believes what they do is right; otherwise, they will change their beliefs to fit what they do or did, The Ten Immutable, Absolute, and Transcendent Laws of Physics, Psychology, Philosophy, and Reality, Apocalypse, Anawim, #justtothepoint.
Let G be an arbitrary group, and let H be a subgroup of G, H ≤ G . We define an equivalent relation on G, ∀a, b ∈ G, a ≡ b mod H if ∃h ∈ H : a = bh. The equivalent classes are called cosets.
Proof: Reflexivity. a ≡ a mod H if ∃h ∈ H : a = ah, but h = e ∈ H because H is a subgroup, and obviously a = ae.
Symmetry. ∀a, b ∈ G, a ≡ b mod H if ∃h ∈ H : a = bh ⇒ b = ah-1, h-1∈ H because H is a subgroup.
Transitivity. ∀a, b, c ∈ G, a ≡ b mod H, b ≡ c mod H ∃h1, h2 ∈ H : a = bh1, b = ch2 ⇒ a = bh1 = (ch2)h1 = c(h2h1), h2h1 ∈ H because H is a subgroup, so a ≡ c mod H.
Let G be a group, H < G a subgroup. We define two equivalent relations on G, ∀a, b ∈ G, a ~L b if and only if a-1b ∈ H, a ~R b if and only if ab-1 ∈ H
Proof: (~L is an equivalent relation & mutatis mutandis ~R is also an equivalent relation)
Alternative definition. Let G be an arbitrary group, and let H be a subgroup of G, H ≤ G. A left coset of H in G is a subset of the form aH = {ah | h ∈ H} for some a ∈ G. The collection of left cosets is denoted by G/H. Analogously, Ha = {ha | h ∈ H} is the right coset of H in G. The collection of right cosets is denoted by H\G.
We have demonstrated that this is a equivalent relation, we claim gH = [g], and g is the representative of the coset gH.
Proof.
Let x ∈ gH ⇒ ∃h ∈ H: x = gh.
x = gh ⇒[Multiplying by g-1 in both sides of the equation] g-1x = g-1(gh) =[Associative] h ∈ H ⇒[g ~ x ↭ g-1x ∈ H ↭ g-1x = h ↭ ∃h ∈ H: x = gh] g ~ x ⇒ x ∈ [g]∎
Let x ∈ [g] ⇒ g ~ x ⇒ g-1x ∈ H ⇒ ∃h ∈ H: g-1x = h ⇒ x = gh ∈ gH ∎
We use |aH|, |Ha| to express or denote the number of elements in aH or Ha respectively.
a = 0, 0H = 0 + H (additive notation) = 3ℤ.
1 + 3ℤ = {…, -8, -5, -2, 1, 4, 7, 10, …}
2 + 3ℤ = {…, -7, -4, -1, 2, 5, 8, 11, …}
These three left cosets constitute the partition of ℤ into left cosets of 3ℤ. Since ℤ is Abelian, left cosets and right cosets are the same. Besides, a~Rb if and only if ab-1 ∈ nℤ ↭[Additive notation] a-b ∈ nℤ is the same as the relation of congruence modulo n, that is, a ≡ b mod n (a-b divisible by n).
Therefore, the partition of ℤ into cosets of nℤ is the partition of ℤ into residue classes modulo n and we refer it as cosets modulo nℤ.
Example. Let H = 4ℤ ≤ ℤ. Then, ℤ = 4ℤ ∪ (1 + 4ℤ) ∪ (2 + 4ℤ) ∪ (3 + 4ℤ) = {4n: n ∈ ℤ} + {1 + 4n: n ∈ ℤ} + {2 + 4n: n ∈ ℤ} ∪ {3 + 4n: n ∈ ℤ}
ℤ8, ⟨2⟩ = {0, 2, 4, 6} ≤ ℤ8. Then, 0 + ⟨2⟩ = ⟨2⟩. 1 + ⟨2⟩ = {1, 3, 5, 7}. 2 + ⟨2⟩ = ⟨2⟩. 3 + ⟨2⟩ = {3, 5, 7, 1} = 1 + ⟨2⟩. Thus, there are only two cosets and ℤ8 = ⟨2⟩ ∪ (1 + ⟨2⟩.
U12 = {1, 5, 7, 11}, ⟨5⟩ = {1, 5} ≤ U12. Let’s compute the cosets: 1⟨5⟩ = ⟨5⟩ = 5⟨5⟩. 7⟨5⟩ = 11⟨5⟩ =[7·5 = 35 ≡ 1 (mod 12)] {7, 11}. Thus, U12 = ⟨5⟩ ∪ 7⟨5⟩.
H = ⟨3⟩ = {0, 3} ≤ ℤ6. The partition of ℤ6 into cosets of {0, 3} is 0 + H = H + 0 = {0, 3}, 1 + H = H + 1 = {1, 4}, and 2 + H = H + 2 = {2, 5}.
ℤ6 is Abelian ⇒ the left and right cosets are the same.
Let G be the symmetric group of degree three, G = S3 = {1, (12), (13), (23), (123), (132)}, and let H be the alternating group (the subgroup of all even permutations) = A3 = ⟨(123)⟩ ≤ S3. H = {1, (123), (132)}. Let’s compute the left cosets of H in G.
(id)H = (123)H = (132)H = H = {1, (123), (132)},
(12)H = (23)H = (13)H = {(12), (23), (13)}.
(12)(1) = (12), (12)(123) = (23), (12)(132) = (13) ⇒ (12)H = {(12), (23), (13)}. G = H ∪ (12)H.
Futhermore, (id)H = (123)H = (132)H = H(id) = H(123) = H(132)
(12)H = (23)H = (13)H = H(12) = H(23) = H(13) = {(12), (23), (13)}.
(1)H = (12)H = H = {1, (12)},
(123)H = (13)H = {(123), (13)},
(132)H = (23)H = {(132), (23)}.
G = H ∪ (13)H ∪ (23)H
However, H(123) = H(23) = {(123), (23)}, and H(132) = H(13) = {(132), (13)}, and in particular, (123)H ≠ H(123), i.e., right and left cosets are not always identical.
R0K = K = R180K,
R90K = {R90R0, R90R180} = {R90, R270} = R270K,
VK = {V, H} = HK,
DK = {D, D’} = D’K.
G = K ∪ R90K ∪ VK ∪ DK.
R0K = R180K = VK = HK = K
R90K = R270K = DK = D’K = {R90, R270, R90V, R90H} = {R90, R270, D, D'}
G = K ∪ R90K.
Let G be an arbitrary group, let H be a subgroup of G, H ≤ G, and let a, b be two arbitrary elements of G, a, b ∈ G. Then, the following statements hold true:
Proof.
⇒) Suppose aH = H ⇒[H ≤ G ⇒ e ∈ H] a = ae ∈ aH = H ⇒ a ∈ H.
⇐) Suppose a ∈ H ⇒ aH ⊆ H because H is a subgroup, so there is closure under the group operation.
Let’s prove that H ⊆ aH, let h ∈ H, h ∈ aH? Since a, h ∈ H ⇒ a-1h ∈ H ⇒[a, a-1h ∈ H ∈ H, H ≤ G] a(a-1h) = h ∈ aH∎
Proof.
⇒) If aH = bH ⇒ a = ae ∈ aH = bH ⇒ a ∈ bH.
⇐) If a ∈ bH ⇒ ∃h ∈ H: a = bh ⇒ aH = (bh)H = b(hH) = [aH = H iff a ∈ H] bH.
Let’s suppose aH ∩ bH ≠ ∅ ⇒ ∃ c ∈ aH ∩ bH ⇒ c ∈ aH and c ∈ bH ⇒ [aH = bH iff a ∈ bH] cH = aH and cH = bH ⇒ aH = bH ∎
Proof:
aH = bH ↭ b-1(aH) = b-1(bH) ↭ (b-1a)H = (b-1b)H = H ↭[aH = H iff a ∈ H] b-1a ∈ H∎
However, it is not true that aH = bH ↭ ab-1 ∈ H. Counterexample: G = S3, H = ⟨(12)⟩ = {1, (12)}, a = (123), b = (13), a-1b = (132)(13) = (12) ∈ H ⇒ (123)H = (13)H = {(123), (13)}. However, ab-1 = (123)(13) = (23) ∉ H.
Proof.
We define a mapping Φ: H → gH, Φ(h) = gh which is well-defined.
Φ is bijective ⇒ |H| = |gH| ∎ The same reasoning can be used to demonstrate that Φ: H → Hg defined by Φ(h) = hg is bijective, and therefore |H| = |gH| = |Hg|, that is not to say that gH = Hg, but they have the same cardinality (number of elements).
Proof: (for educational purposes) Let G be a group, H < G a subgroup. Let’s define f: aH → bH by f(ah) = bh.
is it well defined? Suppose ah = ah’ ⇒ a-1ah = a-1ah’ ⇒ h = h’ ⇒ bh = bh'.
Likewise, I can define, g: bH → aH by g(bh) = ah. Since f and g are clearly inverses (f(g(bh)) = f(ah) = bh, g(f(ah)) = g(bh) = ah), f and g are bijections, and aH and bH have the same order.
Proof:
aH = Ha ↭ (aH)a-1 = (Ha)a-1 = H(aa-1) = H ↭ aHa-1 = H∎
Notice that these equalities are not that of elements, but that of sets, e.g., aH = {ah: h ∈ H}, (aH)a-1 = {(ah)a-1 : h ∈ H} =Associativity {aha-1 : h ∈ H} and a-1(aH) = {a-1(ah): h ∈ H} =Associativity H = (Ha)a-1 = {(ha)a-1 : h ∈ H}
Proof:
⇒) aH ≤ G ⇒ e ∈ H, e ∈ aH (every group contains the identity) ⇒ [A coset H absorbs an element if and only if the element belongs to it, aH = H iff a ∈ H.] e ∈ aH ∩ eH ⇒ [Two left (or right) cosets are either identical or disjoint, i.e., aH = bH or aH ∩ bH = ∅.] aH = eH = H ⇒ [aH = H iff a ∈ H] a ∈ H.
⇐) Conversely, let a ∈ H ⇒ [aH = H iff a ∈ H] aH = H ≤ G ∎