MAT301 Week 1 Exercises
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MAT301 Week 1 Exercises
Exercise 1. Let F ⊆ R2 be a proper rectangle, i.e. a rectangle that is not a square. Determine all symmetries of F and write down the Cayley table (a.k.a. operation table or multiplication table) of the symmetry group of F .
Exercise 2. Let C be a cube in R3 . How many symmetries of C are there?
Exercise 3. Let G be a group.
1. Let a,b ∈ G with a11 = e. Express the inverses of a,a2 ,a5 ,a7 , and a10 without using negative exponents.
2. Let a,b ∈ G with a6 = e = b7 . Write a −2b −4 and (a2 b4 ) −2 without using negative exponents. Exercise 4. Let G be a group.
1. Prove that for all a ∈ G, we have (a−1) −1 = a.
2. If a1 , . . . ,an ∈ G and n ∈ Z, what is (a1 ··· an ) −1? Prove your formula by induction on n.
Exercise 5. Let G be the set {e,a,b}. Determine all group operations on the set G for which e is the identity element.
Exercise 6. Let S be a reflection in Dn . Which elements x ∈ Dn satisfy x2 = S? Which elements x ∈ Dn satisfy x3 = S?
Exercise 7. List the elements of the group ((Z/8Z)× , · ) and write down its Cayley table. Exercise 8. Define a relation ∼ on R by x ∼ y if and only if x − y ∈ Z, for all x,y ∈ R.
1. Prove that ∼ is an equivalence relation.
2. What are the equivalence classes of ∼?
3. Prove that each equivalence class of ∼ has a unique representative in [0, 1).
4. Prove that for all x1 ,x2 ,y1 ,y2 ∈ R, if x1 ∼ x2 and y1 ∼ y2 , then x1 + y1 ∼ x2 + y2 .
For x ∈ R, let [x] denote the equivalence class of x under the equivalence relation ∼ . Let R/Z = {[x] : x ∈ R}, the set of equivalence classes of ∼ . For α,β ∈ R/Z, define α+β ∈ R/Z as follows: choose x,y ∈ R such that α = [x] and β = [y], and define α + β = [x + y]. This is well-defined, independent of the choices of x and y by Part 4. We have a binary operation R/Z × R/Z → R/Z defined by [x] + [y] = [x + y].
5. Prove that (R/Z, +) is an abelian group.
Exercise 9. Let G be a group and suppose that a2 = e for all a ∈ G. Prove that G is abelian. Exercise 10. Let G be a finite group. Prove that the number of elements x ∈ G such that x3 = e is odd.
Exercise 11. Let G be a finite group whose order is even. Prove that there is an element x ∈ G such that x e and x2 = e. (Hint: x2 = e if and only if x = x −1 .)
Exercise 12. The quaternions H, introduced by Hamilton, is a four-dimensional version of the complex numbers. The quaternions H is the real vector space R4 together with a multiplication defined by
(a,b,c,d)(e,f,g,h) = (ae − bf − cg − dh,af + be + ch − dg,ag + ce − bh + df,ah + de + bg − cf).
for all (a,b,c,d), (e,f,g,h) ∈ R4 . With the operations of addition and multiplication, the quaternions H satisfy all axioms of a field except that multiplication is not commutative. Let 1 = (1 , 0, 0, 0),i = (0, 1, 0, 0),j = (0, 0, 1, 0), and k = (0, 0, 0, 1). Then, the element (a,b,c,d) ∈ H can be written as a1 + bi + cj + dk . It is typical to write a1 as a, so that a1 + bi + cj + dk = a + bi + cj + dk .
Identify the subspace R1 = R(1, 0, 0, 0) = {a1 = (a,0, 0, 0) | a ∈ R} of the quaternions with R via the map a1 7→ a. Identify the subspace Ri + Rj + Rk = {bi + cj + dk = (0,b,c,d) | b,c,d ∈ R} of the quaterions with R3 via the map bi+cj+dk 7→ (b,c,d). For a quaternion x = x0 1+x1 i+x2j+x3 k, we write xs = x0 ∈ R and xv = (x1 ,x2 ,x3 ) ∈ R3 , and then x = xs + xv under our identifications. If x = xs + xv and y = ys + yv are quaternions, then
xy = (xs ys − xv · yv ) + (xs yv + ys xv + xv × yv ),
that is, (xy)s = xs ys − xv · yv and (xy)v = xs yv + ys xv + xv × yv .
Consider the subset Q8 = {±1, ±i,±j,±k} of the quaternions H.
1. Prove that Q8 is a non-abelian group under multiplication. It is called the quaternion group.
2. Write down its Cayley table.
Exercise 13. Define a relation ∼ on the quaternions H as follows: for all x,y ∈ H define a ∼ b to mean x = y or x = −y .
1. Prove that ∼ is an equivalence relation on H and describe its equivalence classes.
2. Prove that for all x1 ,x2 ,y2 ,y2 ∈ H, if x1 ∼ x2 and y1 ∼ y2 , then x1 y1 ∼ x2 y2 .
For x ∈ H, let [x] denote the equivalence class of x, that is, [x] = {x′ ∈ H : x′ ∼ x}. Let = {[x] : x ∈ H× }. If α,β ∈ define α · β ∈ as follows: choose x,y ∈ H such that α = [x] and β = [y], and define α · β = [xy]. (By Part 2, this does not depend on the choice of x and y .) Therefore we have a binary operation
H˜ × H˜ −→ H˜
([x], [y]) 7−→ [x] · [y]
3. Is (, · ) a group? Justify your answer.
4. Let G = {[x] : x ∈ H× }, where H × is the set of invertible quaternions, i.e. H × = H\{0}. Prove that if [x], [y] ∈ G, then [x] · [y] ∈ G. Consequently, we have a binary operation G×G → G,([x], [y]) 7→ [x] · [y].
5. Prove that (G, · ) is a group.
6. Let K = {[1], [i], [j], [k]}. Prove that if α,β ∈ K, then α · β ∈ K, so that we have a binary operation K × K → K,(α,β) 7→ α · β .
7. Prove that (K, · ) is a group and write down its Cayley table.
8. Compare the Cayley table of K with the Cayley table of the symmetry group of a proper rectangle from Exercise 1.
Exercise 14 (x1 ∗ · · · ∗ xn is unambiguous). Let S be a set and let ∗ : S × S → S be an associative binary operation. The pair (S,∗) is called a semigroup. Note that every group is a semigroup. An example of a
semigroup that is not a group is (Z>0 , +).
For each n ∈ Z>0, we will define a function Pn : Sn → 2S such that for all (x1 , . . . ,xn ) ∈ Sn , the set Pn (x1 , . . . ,xn ) is the subset of S consisting of all ways of inserting parentheses in the string of symbols x1 ∗ · · · ∗ xn to obtain a valid product in (S,∗). For example, we will have
P4 (x1 ,x2 ,x4 ) = x1 ∗ (x2 ∗ (x3 ∗ x4 )),x1 ∗ ((x2 ∗ x3 ) ∗ x4 ),
(x1 ∗ (x2 ∗ x3 )) ∗ x4 , ((x1 ∗ x2 ) ∗ x3 ) ∗ x4 , (x1 ∗ x2 ) ∗ (x3 ∗ x4 ) . We now inductively define the functions Pn : Sn → 2S for all n ∈ Z>0 .
(i) Define P1 : S → 2S by P1 (x1 ) = x1 for all x1 ∈ S .
(ii) Suppose that n ∈ Z>0 and that Pm : Sm → 2S is defined for all m ∈ {1, . . . ,n}. We define Pn+1 : Sn+1 → 2S by
n−1
Pn+1(x1 , . . . ,xn+1) = [ Pk (x1 , . . . ,xk ) ∗ Pn−k(k + 1, . . . ,xn )
k=1
for all x1 , . . . ,xn+1 ∈ S .
Prove by strong induction that for all n ∈ Z>0, the set Pn (x1 , . . . ,xn ) is a singleton for all x1 , . . . ,xn ∈ S .
Exercise 15. A monoid is a semigroup (M, · ) such that there exists an element e ∈ M with the property that x · e = x = e · x for all x ∈ M . Let (M, · ) be a monoid.
1. Prove that e is unique.
2. Let x ∈ M and suppose that there exist y,z ∈ M such that y · x = e = x · z . Prove that y = z .
Exercise 16 (Laws of exponents). Let G be a group and let a ∈ G. Recall that for n ∈ Z ≥0 we define an inductively by: a0 = 1 and an+1 = an a. Recall that for n ∈ Z<0, we define an = (a−1) −n, which is defined since −n > 0.
1. Prove that for all n ∈ Z, we have an a = an+1
2. Prove that for all m,n ∈ Z, we have am an = am+n . (Hint: Prove that for any m ∈ Z we have am an = am+n for all n ∈ Z ≥0 by induction on n. For m ∈ Z and n ∈ Z<0, use am an = (a−1) −m(a−1) −n to reduce to a case that has already been proved.)
3. Prove that for all m,n ∈ Z, we have (am )n = amn .
4. Give an example of a group H and elements x,y ∈ H such that (xy)2 x2 y2 .
Exercise 17. Let G be a group and let a,b ∈ G. Prove that for all n ∈ Z, we have (aba−1)n = abn a −1 .
Exercise 18 (Laws of exponents for commuting elements). Let G be a group and let a,b ∈ G with ab = ba.
1. Prove that for all n ∈ Z we have an b = ban . (Note that if n < 0, then an = (a−1) −n, so it suffices to prove the result in the case n ≥ 0.)
2. Prove that for all n ∈ Z we have (ab)n = an bn .
2022-06-14