MAT301 Weeks 3–4 Exercises
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MAT301 Weeks 3–4 Exercises
Exercise 1. Choose some permutations in S15 and compute their cycle decompositions.
Exercise 2.
1. Describe all elements σ ∈ Sn such that σ 2 = id.
2. Determine the number of such σ ∈ Sn that σ 3 = id.
3. Determine the number of such σ ∈ Sn that σ 4 = id.
Exercise 3 (Parity and cycle decompositions). Let σ ∈ Sn and let σ = γ 1 ··· γr , where γi is an ℓi-cycle. Express the parity of σ in terms of ℓ1 , . . . ,ℓr .
Exercise 4. Recall that an inversion of a permutation σ ∈ Sn is an ordered pair (i,j) ∈ {1, . . . ,n}2 such that i < j and σ(j) < σ(i). We denote the set of inversions of σ by inv(σ).
1. Prove that the largest number of inversions a permutation in Sn can have is 2(n) .
2. Prove that there is a unique element σ ∈ Sn with 2(n) inversions. What is this element? Exercise 5 (Alternating groups are non-abelian). Prove that An is non-abelian for n ≥ 4. Exercise 6. Prove that two transpositions commute if and only if they are disjoint.
Exercise 7 (Z(Sn ) and Z(An )). Prove that Z(Sn ) = {id} for all n ≥ 3 and Z(A4 ) = {id} for all n ≥ 4. Exercise 8 (Generating sets of Sn ).
1. Prove that every element of Sn can be written as a (possibly empty) product of elements in {(12), (13), . . . (1n)} . Thus, Sn = ⟨(12), (13), . . . , (1n)⟩ .
2. Recall that a group G is generated by a subset of elements {g1 ,g2 , . . . ,gk } if every element of G can be represented as a product of several copies of gi and g Prove that for n ≥ 2 the group Sn is generated by (12) and (12 . . . n).
Exercise 9 (An is generated by the 3-cycles and by pairs of transpositions) .
1. Prove that the product of two transpositions is always a product of 3-cycles.
2. Prove that An is generated by the 3-cycles in Sn .
3. Prove that An is generated by the pairs of transpositions in Sn for n ≥ 5. (is this true for n < 5?) Exercise 10 (The subgroups of A4 ). In this exercise you will determine all subgroups of A4 .
1. Prove that if a subgroup H of A4 contains two distinct 3-cycles α,β such that β α −1, then H = A4 .
2. Prove that if a subgroup H of A4 contains a 3-cycle and an element of order 2, then H = A4 .
3. Prove that if H is a subgroup of A4 , then either H = {id}, H = A4 , H is generated by a 3-cycle, or H is generated by elements of order 2.
4. Determine all subgroups of A4 .
Exercise 11 (Permutation matrices, sgn, and det). For each σ ∈ Sn , let P(σ) denote the n × n matrix whose ith row is the σ−1(i)th standard basis vector, that is,
⃗eσ − 1 (1)
P(σ) =
⃗eσ − 1 (n)
1. Let σ ∈ Sn and let A ∈ Matn×n(C). What is P(σ)A?
2. Prove that for all σ 1 ,σ2 ∈ Sn we have P(σ1 σ2 ) = P(σ1 )P(σ2 ).
3. Prove that for all every transposition τ ∈ Sn we have det(P(τ)) = −1.
4. Prove that for all σ ∈ Sn we have sgn(σ) = det(P(σ)).
Exercise 12. Let G be a group and let S ⊆ G be a subset that generates G . (See the Week 2 Exercises for the definition of a group generated by a set.) Let S −1 = {s−1 : s ∈ S} . For each g ∈ G, we define the length of g with respect to S to be the smallest n ∈ Z ≥0 such that g is a product of n elements of S ∪ S−1, and we denote it by ℓS (g). Prove that the function ℓ S : G → Z ≥0 satisfies the following properties.
1. For all g ∈ G we have ℓS (g) = 0 if and only if g = e .
2. For all g ∈ G we have ℓS (g−1) = ℓS (g).
3. For all g1 ,g2 ∈ G we have |ℓS (g1 ) − ℓS (g2 )| ≤ ℓS (g1g2 ) ≤ ℓS (g1 ) + ℓS (g2 ).
4. For all g ∈ G and s ∈ S we have ℓS (sg) = ℓS (g) ± 1.
Define dS : G × G → Z ≥0 by dS (g1 ,g2 ) = ℓS (g1g2(−)1 ) for all g1 ,g2 ∈ G .
5. Prove that dS is a metric on G .
Exercise 13. Recall that Sn is generated by the set S = {τi : i = 1, . . . ,n − 1}, where τi = (i(i + 1)) for each i ∈ {1, . . . ,n − 1} . Therefore Sn is also generated by the set S0 ⊆ Sn of all transpositions in Sn . Let ℓ0 : Sn → Z ≥0 be the length function ℓ S0 and let ℓ : Sn → Z ≥0 be the length function ℓ S . (These length functions are defined in the preceding exercise.) We will begin by comparing ℓ0 and ℓ .
1. Prove that for all σ ∈ Sn we have ℓ0 (σ) ≤ ℓ(σ).
2. Find a permutation σ such that ℓ0 (σ) ℓ(σ).
3. For which n ∈ Z>0 is ℓ0 (σ) = ℓ(σ) for all σ ∈ Sn ?
Now we will establish formulas ℓ0 and ℓ that do not reference to the sets S0 and S .
4. Define the length of a k-cycle to be k − 1. Define the length of a permutation σ ∈ Sn to be the sum of the lengths of the cycles that occur in its cycle decomposition, and denote it by len(σ). Prove that sgn(σ) = (−1)len(σ) .
5. Prove that for all σ ∈ Sn and τ ∈ S0 we have(
(Hint: Compute (ci cj )(c1 c2 . . . cn ).)
6. Prove by induction that for all n ∈ Z>0 we have ℓ0 (σ) = len(σ) for all σ ∈ Sn .
7. Let σ ∈ Sn and τi ∈ S . Prove that
|inv(τi σ)| =
8. Prove by induction that for all n ∈ Z>0 we have ℓ(σ) = |inv(σ)| for all σ ∈ Sn .
Parts 6 and 8 can be used to translate results about ℓ0 and ℓ into results about len and |inv |, and vice versa. Here are some examples:
• It follows from Parts 6 and 8 that len and |inv | satisfy the properties in Exercise 11.
• For all σ ∈ Sn we have
(−1)len(σ) = sgn(σ) = (−1)| inv(σ)| ,
so it follows from Parts 6 and 8 that ℓ0 (σ) ≡ ℓ(σ) (mod 2).
• For all σ ∈ Sn we have ℓ0 (σ) ≤ ℓ(σ), so len(σ) ≤ |inv(σ)| by Parts 6 and 8.
• As an application of Part 8, you can prove that an n-cycle in Sn has at least n − 1 inversions.
2022-06-14