MAT301 Term Test 1 Solutions
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MAT301 Term Test 1 Solutions
Exercise 1. Let G be a finite cyclic group of order 120, let H be a subgroup of G of order 24, and let K be a subgroup of G of order 30. Find the number of elements in the intersection H ∩ K . (Justify your answer.)
Solution. Let a be a generator of G. By the classification of subgroups of finite cyclic groups, the distinct subgroups of G are ⟨a ⟩, ford d ∈ Z>0 with d | 120, and |⟨ad ⟩| = 120/d.
To solve this problem, we can find the generators of H and K . As |H| = 24, we know that H = (a5 ) , and as |K| = 30, we have K = (a6 ) . It can be easily seen that ad ∈ H ∩ K if and only if lcm(5, 6) = 30|d, therefore, H ∩ K = (a3 0), and |H ∩ K| = 6.
Exercise 2. Let G be a finite group. For each prime p, define Hp = {g ∈ G : p ∤ o(g)} .
1. Prove that if G is abelian, then Hp ≤ G for every prime p.
2. Prove that if G = S3 , then H3 is not a subgroup of G.
Proof of 1 . Assume that G is abelian and let p be a prime. Since o(e) = 1 and p ∤ 1, we have e ∈ Hp . Therefore Hp is nonempty. Since Hp is finite, to prove that Hp is a subgroup of G it suffices to show that Hp is closed under the group operation by the finite subgroup test. Let a,b ∈ Hp . Then p ∤ o(a),o(b) and since p is prime we have p ∤ o(a)o(b) by Euclid’s lemma. Since G is abelian, we have o(ab) | o(a)o(b). Therefore
p ∤ o(ab), i.e. ab ∈ Hp , as required.
Proof of 2. If G = S3 , then (12), (23) ∈ H3 but (12)(23) = (123) ̸∈ H3 , so H3 is not a subgroup of G. Exercise 3. Let σ ∈ S9 be the element
σ = )
1. Prove that there does not exist an integer k such that σ 2022k = σ 1001 . (Hint: You can prove this without simplifying σ 2022k or σ1001 .)
2. Prove that there do not exist x,y ∈ S9 such that σ = xy33 x5 y79 .
Proof of 1 . First, note that the cycle decomposition of σ is σ = (148)(239)(57). This is an odd permutation,
but σ 2022k is always even but σ 1011 is always odd, so we get a contradiction.
Proof of 2. A similar argument works here:
sgn(xy33 x5 y79 ) = sgn(x)sgn(y)33 sgn(x)5 sgn(y)79 = sgn(x)6 sgn(y)12 = (±1)6 (±1)112 = 1 sgn(σ).
Therefore there do not exist x,y ∈ S9 such that σ = xy33 x5 y79 .
Exercise 4. True or False? If true, provide a proof. If false, provide a counterexample (and prove it is indeed a counterexamples).
1. Let G be a group and g ∈ G. Then there is a unique homomorphism φ : Zn → G such that φ(1) = g .
2. If A ⊆ G, then the intersection of all subgroups of G that contain A is the smallest subgroup of G containing A.
Proof. 1. This statement is false, as we have to require |g||n, otherwise, the respective homomorphism will not be well-defined. For example, there are no homomorphisms φZ/5Z → Z/7Z .
2. This statement is true. First of all, we observe that the intersection of all subgroups of G that contain A is, indeed, a subgroup (assumed known in the course). Denote this subgroup by GA . Suppose that
H is a strictly smaller subgroup of G which also contains A. In other words, we assume H ⊊ GA . Now we need to carefully read the statement of the problem: as H also contains A, it is one of (possibly many) subgroups that we need to intersect in order to get GA . Therefore, GA ⊆ H, which leads us to a contradiction. Therefore, GA is the smallest subgroup containing A.
Exercise 5. Let G be a group and let A be an abelian group. Let Hom(G,A) be the set of all homomorphisms from G to A. For each ϕ,ψ ∈ Hom(G,A), define ϕ · ψ : G → A by (ϕ · ψ)(g) = ϕ(g)ψ(g) for all g ∈ G.
Prove that ϕ · ψ ∈ Hom(G,A) for all ϕ,ψ ∈ Hom(G,A), and that Hom(G,A) is an abelian group under the binary operation Hom(G,A) × Hom(G,A) → Hom(G,A) that maps (ϕ,ψ) to ϕ · ψ .
Proof. Let ϕ,ψ ∈ Hom(G,A). For all g1 ,g2 ∈ G, we have
(ϕ · ψ)(g1g2 ) = ϕ(g1g2 )ψ(g1g2 ) = ϕ(g1 )ϕ(g2 )ψ(g1 )ψ(g2 ) = ϕ(g1 )ψ(g1 )ϕ(g2 )ψ(g2 ) = (ϕ · ψ)(g1 )(ϕ · ψ)(g2 ),
where the third equality follows from the fact that A is abelian. Therefore ϕ · ψ ∈ Hom(G,A).
To prove that the binary operation
· : Hom(G,A) × Hom(G,A) −→ Hom(G,A)
(ϕ,ψ) }−→ ϕ · ψ
is associative, we must prove that for all ϕ,ψ,χ ∈ Hom(G,A) we have (ϕ · ψ) · χ = ϕ · (ψ · χ). Let ϕ,ψ,χ ∈ Hom(G,A). For all g ∈ G we have
((ϕ · ψ) · χ)(g) = (ϕ · ψ)(g)χ(g) = (ϕ(g)ψ(g))χ(g),
and similarly we have
(ϕ · (ψ · χ))(g) = ϕ(g)(ψ · χ)(g) = ϕ(g)(ψ(g)χ(g)),
so ((ϕ · ψ) · χ)(g) = (ϕ · (ψ · χ))(g) by associativity of the group operation of G. Therefore (ϕ · ψ) · χ = ϕ · (ψ · χ). Let ε ∈ Hom(G,A) be the trivial homomorphism. For all ϕ ∈ Hom(G,A) we have ϕ · ε = ϕ = ε · ϕ since
for all g ∈ G we have
(ϕ · ε)(g) = ϕ(g)ε(g) = ϕ(g)eA = ϕ(g)
and
(ε · ϕ)(g) = ε(g)ϕ(g) = eA ϕ(g) = ϕ(g).
Therefore ε is an identity element for the binary operation · .
Let ϕ ∈ Hom(G,A) and define ψ : G → A by ψ(g) = ϕ(g)− 1 for all g ∈ G. Then ψ ∈ Hom(G,A) since A is abelian. We have ϕ · ψ = ε = ψ · ϕ since for all g ∈ G we have
(ϕ · ψ)(g) = ϕ(g)ψ(g) = ϕ(g)ϕ(g)− 1 = eA = ε(g)
and
(ψ · ϕ)(g) = ψ(g)ϕ(g) = ϕ(g)− 1 ϕ(g) = eA = ε(g).
Therefore ψ is an inverse of ϕ with respect to the binary operation · . Therefore every element of Hom(G,A)
has an inverse with respect to the binary operation · , and Hom(G,A) is a group with respect to · .
Bonus problem. The following table is a Cayley table of a group G of order 12, but with most of its entries deleted. The element denoted by e is not assumed to be the identity element.
1. Prove that a6 is the identity element of the group . (Hint: Look at the first row)
2. Prove that c = a2 . (Hint: Use the rows corresponding to a and c .)
3. Prove that b = a6 . (Hint: Use ca = a3 .) Conclude that b is the identity element of the group .
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4. Compute the order of a and b .
5. Is G D6 ?
Proof of 1 . We have ag = h, ah = i, ai = j , aj = k , ak = l, and al = g . Therefore
a6g = a5 h = a4 i = a3j = a2 k = al = g,
and a6 = gg − 1 , which is the identity element of G.
Proof of 2. Since ag = h and ah = i, we have a2g = i. Also, we have cg = i. Therefore cg = a2g, and cancelling g on the right gives c = a2 .
Proof of 3. We have b = d2 and d = ca = a3 . Therefore b = d2 = (a3 )2 = a6 , and b is the identity element
of G.
Proof of 4 . Since b is the identity element of G, we have o(b) = 1. Since a6 = b, we have o(a) | 6, and therefore o(a) ∈ {1, 2, 3, 6} . Since a b, we have o(a) 1. Since a2g = i g and a3g = j g, we have o(a) 2, 3. Therefore o(a) = 6.
Solution to 5. Every reflection in D6 has order 2. The subgroup of rotations in D6 is cyclic of order 6, and therefore the order of every rotation in D6 divides 6. Therefore D6 does not have an element of order 4. However, G does have an element of order 4, so G is not isomorphic to D6 . Indeed, we have g2 = d b and
g4 = d2 = b, so o(g) = 4.
2022-08-17