# Chapter 10. Abstract algebra

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1 Chapter 10. Abstract algebra C.O.S. Sorzano Biomedical Engineering December 17, Abstract algebra December 17, / 62

2 Outline 10 Abstract algebra Sets Relations and functions Partitions and equivalence relationships Binary operations Groups and subgroups Homomorphisms and isomorphisms Algebraic structures 10. Abstract algebra December 17, / 62

3 References J.B. Fraleigh. A first course in Abstract Algebra. Pearson, 7th Ed. (2002) 10. Abstract algebra December 17, / 62

4 Outline 10 Abstract algebra Sets Relations and functions Partitions and equivalence relationships Binary operations Groups and subgroups Homomorphisms and isomorphisms Algebraic structures 10. Abstract algebra December 17, / 62

5 Sets Definition 1.1 (Set) A set is a well-defined collection of elements. We denote the different elements as a S. Definition 1.2 (Empty set) The only set without any element is the empty set ( ). Describing sets We may provide the elements of a set: Intensional definition: by giving a property they all meet (e.g., even numbers from 1 to 10) Extensional definition: by listing all the elements in the set (e.g.,{2, 4, 6, 8, 10}). The order in which the different elements are written has no meaning. 10. Abstract algebra December 17, / 62

6 Sets Definition 1.3 (Subset and proper subset) B is subset of A (denoted B A or A B) if all the elements of B are also elements of A. B is a proper subset of A if B is a subset of A and B is different from A (B A or A B). Properties A is an improper subset of A. is a proper subset of A. Definition 1.4 (Power set (Partes de un conjunto)) The set of all subsets of a set A is called the power set of A. Let A = {1, 2, 3} the power set of A is P(A) = {, {1}, {2}, {3}, {1, 2}, {1, 3}, {2, 3}, {1, 2, 3}} 10. Abstract algebra December 17, / 62

7 Sets Definition 1.5 (Cartesian product) The cartesian product of the sets A and B is the set of all ordered pairs in which the first element comes from A and the second element comes from B. A B = {(a, b) a A, b B} Note that because of the ordered nature of the pair A B B A. Let A = {1, 2, 3} and B = {4, 5}. A B = {(1, 4), (1, 5), (2, 4), (2, 5), (3, 4), (3, 5)} Definition 1.6 (Cardinality) The cardinality of a set is the number of elements it has. 10. Abstract algebra December 17, / 62

8 Sets Definition 1.7 (Disjoint sets) Two sets are disjoint if they do not have any element in common. Some useful sets Integer numbers: Z = {..., 2, 1, 0, 1, 2,...}, Z = ℵ 0 Natural numbers, positive integers: N = Z + = {1, 2, 3,...}, N = ℵ 0 Negative integers: Z = {..., 3, 2, 1}, Z = ℵ 0 Non-null integers: Z = Z {0} = {..., 2, 1, 1, 2,...}, Z = ℵ 0 Rational numbers: Q, Q = ℵ 0 Real numbers: R, R = ℵ 1 Interval: [0, 1], [0, 1] = ℵ 1 Complex numbers: C = {a + bi a, b R}, C = ℵ Abstract algebra December 17, / 62

9 Outline 10 Abstract algebra Sets Relations and functions Partitions and equivalence relationships Binary operations Groups and subgroups Homomorphisms and isomorphisms Algebraic structures 10. Abstract algebra December 17, / 62

10 Relations Definition 2.1 (Relation) A relation arb is a subset of the cartesian product A B. 10. Abstract algebra December 17, / 62

11 Functions Definition 2.2 (Function) A function f : X Y is a relation between X and Y in which each x X appears at most in one of the pairs (x, y). We may write (x, y) f or f (x) = y The domain of f is X, the codomain of f is Y. The support of f is the set of all those values in X for which there exists a pair (x, y). The range of f are all values in Y for which there exists at least one pair (x, y). f : R R f (x) = x 3 (2, 8) f f (2) = 8 + : R R R ((2, 3), 5) + +((2, 3)) = = Abstract algebra December 17, / 62

12 Classification of functions Definition 2.3 Functions can be classified as surjective, injective or bijective: Surjective: A function is surjective if every point of the codomain has at least one point of the domain that maps onto it. They are also called onto functions. Injective: A function is injective if every point of the codomain has at most one point in the domain that maps onto it. They are also called one-to-one functions. Bijective: A function is bijective if it is injective and surjective. 10. Abstract algebra December 17, / 62

13 Inverse function Definition 2.4 (Inverse function) Consider an injective function f : X Y. f 1 : Y X is the inverse of f iff (x, y) f (y, x) f 1 f (x) = x + 3 f 1 (y) = y 3 f (x) = x 3 f 1 (y) = y 1 3 f (x) = x 2 is not invertible because it is not injective (f ( 2) = f (2) = 4) 10. Abstract algebra December 17, / 62

14 Inverse function Theorem 2.1 If f is invertible, its inverse is unique. If f is bijective, so is f 1. X and Y have the same cardinality if there exists a bijective function between the two. Consider the following function f : Z N f = {(0, 0), ( 1, 1), (1, 2), ( 2, 3), (2, 4), ( 3, 5), (3, 6),...} f is bijective. Consequently, Z has the same cardinality as N. 10. Abstract algebra December 17, / 62

15 Outline 10 Abstract algebra Sets Relations and functions Partitions and equivalence relationships Binary operations Groups and subgroups Homomorphisms and isomorphisms Algebraic structures 10. Abstract algebra December 17, / 62

16 Partition Definition 3.1 (Partition) A partition of a set S is a collection of non-empty subsets such that each element of S belongs to one and only one subset (cell) of the partition. We denote as x the subset that contains the element x. All cells in a partition are disjoint to any other cell. s We may partition the set of natural numbers into the subset of even numbers ({2, 4, 6,...}) and the subset of odd numbers ({1, 3, 5,...}). We may partition the set of integer numbers into the subset of all multiples of 3 ({..., 6, 3, 0, 3, 6,...}), the subset of all numbers whose remainder after dividing by 3 is 1 ({..., 5, 2, 1, 4, 7,...}), and the subset of all numbers whose remainder after dividing by 3 is 2 ({..., 4, 1, 2, 5, 8,...}). 10. Abstract algebra December 17, / 62

17 Equivalence relation Definition 3.2 (Equivalence relation) R is an equivalence relation in S if it verifies: 1 R is reflexive: xrx 2 R is symmetric: xry yrx 3 R is transitive: xry, yrz xrz s 1 = is an equivalence relation. 2 Congruence modulo n is an equivalence relation (two numbers are related if they have the same remainder after dividing by n) : 1 and 4 have remainder 1 after dividing by 3. We write 1 4(mod3) 3 n, m Z nrm nm 0 is not an equivalence relationship because it is not transitive (e.g., 3R0, 0R5 but 3 R5). 10. Abstract algebra December 17, / 62

18 Partition and equivalence relation Theorem 3.1 Let S be a non-empty set, and R an equivalence relation defined on S. Then R partitions S with the cells ā = {x S xra} Additionally, we may define another equivalence relation a b ā = b 10. Abstract algebra December 17, / 62

19 Partition and equivalence relation Congruence modulo 3 is an equivalence relation in Z (two numbers are related if they have the same remainder after dividing by 3) Additionally and 0 = {..., 6, 3, 0, 3, 6,...} 1 = {..., 5, 2, 1, 4, 7,...} 2 = {..., 4, 1, 2, 5, 8,...}... = 0 = 3 = 6 = = 1 = 4 = 7 = = 2 = 5 = 8 = Z = Abstract algebra December 17, / 62

20 Partition and equivalence relation Consider the cartesian product Z (Z {0}). Let (m 1, n 1 ) and (m 2, n 2 ) be two ordered sets of this cartesian product. Consider now the equivalence relation (m 1, n 1 ) (m 2, n 2 ) m 1 n 2 m 2 n 1 = 0 The set of rational numbers is formally defined Q as the set of equivalence classes of Z (Z {0}) under the relation. 10. Abstract algebra December 17, / 62

21 Outline 10 Abstract algebra Sets Relations and functions Partitions and equivalence relationships Binary operations Groups and subgroups Homomorphisms and isomorphisms Algebraic structures 10. Abstract algebra December 17, / 62

22 Binary operations Introduction What is addition? Let us assume that we arrive to a classroom in Mars, and that martians are learning to add. The teacher says and the students reply: Then, the teacher says: and the students reply: Gloop, poyt Bimt. Ompt, gaft Poyt. We don t know what they do but it seems that when the teacher gives two elements, students respond with another element. 10. Abstract algebra December 17, / 62

23 Binary operations Introduction (continued) What is addition? This is what we do when we say three plus four, seven. And we may not use any two elements ( three plus apples is not defined). We can only use elements on a given set. This is what we formally call a binary operation. Definition 4.1 (Binary operation) A binary operation on a set S is a function: : S S S (a, b) = a b 10. Abstract algebra December 17, / 62

24 Binary operations s The following binary operations are all different: + : R R R + : Z Z Z + : M m n (R) M m n (R) M m n (R) The following is not a binary operation because it is not well defined + : M(R) M(R) M(R) we don t know how to add a 2 2 matrix with a 3 3 one. 10. Abstract algebra December 17, / 62

25 Closed set Definition 4.2 Let S be a set and H a subset of S. H is said to be closed with respect to the operation defined in S iff a, b H a b H Then we may define the binary operation in H: : H H H (a, b) = a b which is called the binary operation induced in H. 10. Abstract algebra December 17, / 62

26 Closed set Let S = Z and H = {n 2 n Z + } = {1, 4, 9, 16, 25, 36,...}. H is not closed with respect to addition. For example: 1 H 4 H but / H Let S = Z and H = {n 2 n Z + } = {1, 4, 9, 16, 25, 36,...}. H is closed with respect to multiplication. For example: n 2 H m 2 H and n2 m 2 = (nm) 2 H 10. Abstract algebra December 17, / 62

27 Closed set Let S be the set of real-valued functions with a single real argument S = {R R}. Let us define the addition of functions as + : (R R) (R R) R R (f + g)(x) = f (x) + g(x) Similarly for the multiplication and subtraction of functions. Let us define the composition of functions as : (R R) (R R) R R (f g)(x) = f (g(x)) S is closed with respect to addition, subtraction, multiplication and composition. 10. Abstract algebra December 17, / 62

28 Definition of a binary operation To define a binary operation either we give the full table (intensional definition) as in a b b = 0 b = 1 b = 2 a = a = a = or a b b = 0 b = 1 b = 2 a = a = a = or we give a rule to compute it (extensional definition) as in a b = (a + b) mod Abstract algebra December 17, / 62

29 Properties of a binary operation Definition 4.3 (Commutativity) A binary operation is commutative iff a b = b a is commutative because its definition table is symmetric with respect to the main diagonal, but is not commutative. 10. Abstract algebra December 17, / 62

30 Properties of a binary operation Definition 4.4 (Associativity) A binary operation is associative iff (a b) c = a (b c) is not associative because But is associative (0 0) 0 = 1 0 = 1 0 (0 0) = 0 1 = 2 (0 0) 0 = 0 0 = 0 0 (0 0) = 0 0 = 0 We would have to test all possible triples, but after a a little bit of work we could show that is associative. 10. Abstract algebra December 17, / 62

31 Properties of a binary operation Function composition is associative although not commutative. Proof Function composition is not commutative Function composition is associative (f g)(x) = f (g(x)) g(f (x)) = (g f )(x) ((f g) h)(x) = (f g)(h(x)) = f (g(h(x))) = f ((g h)(x)) = (f (g h))(x) 10. Abstract algebra December 17, / 62

32 Properties of a binary operation A function may not be well defined. For instance, is not well defined for b = 0 Q / : Q Q Q a/b = a b A function may not be closed in S. For instance, / : Z Z Z a/b = a b is not closed because a = 1 Z, b = 3 Z but 1 3 / Z. 10. Abstract algebra December 17, / 62

33 Properties of a binary operation Definition 4.5 (Existence of a neutral element) A binary operation has a neutral element, e, iff a S a e = e a = a 0 is the neutral element of addition in R because r R r + 0 = 0 + r = r 1 is the neutral element of multiplication in R because r R r 1 = 1 r = r Addition in N has no neutral element since 0 / N. 10. Abstract algebra December 17, / 62

34 Properties of a binary operation Definition 4.6 (Existence of an inverse element) A binary operation has an inverse element iff a S b S a b = b a = e being e the neutral element of. The inverse element of 2 with respect to addition in R is -2 because 2 + ( 2) = ( 2) + 2 = 0 The inverse element of 2 with respect to multiplication in R is 1 2 because = = 1 Multiplication in N has no inverse element since n N 1 n / N. 10. Abstract algebra December 17, / 62

35 Outline 10 Abstract algebra Sets Relations and functions Partitions and equivalence relationships Binary operations Groups and subgroups Homomorphisms and isomorphisms Algebraic structures 10. Abstract algebra December 17, / 62

36 Groups and subgroups Introduction Groups and subgroups are algebraic structures. They are the ones that allow solving equations like and that the equation x + x = a x = a 2 x x = a does not have a solution in R if a < 0. We ll see that defining a group amounts to define the elements belonging to the group as well as the operations that can be used with them. 10. Abstract algebra December 17, / 62

37 Groups Definition 5.1 (Group) Given a set S and a binary operation defined on S, the pair (S, ) is a group if G is closed under and G1. is associative in S G2. has a neutral element in S G3. has an inverse element in S Definition 5.2 (Abelian group) (S, ) is an abelian group if (S, ) is a group and is commutative. Definition 5.3 (Subgroup) Let (S, ) be a group. Let H be a subset of S, H S, and H be the induced operation in H. The pair (H, H ) is a subgroup of (S, ) if it verifies the conditions to be a group. 10. Abstract algebra December 17, / 62

38 Groups Consider S = {z C z = e iϕ ϕ R}. (U, ) is a group. Proof G1. is associative in S z 1 (z 2 z 3 ) = e iϕ1 (e iϕ2 e iϕ3 ) = e iϕ1 (e i(ϕ2+ϕ3) ) = e i(ϕ1+ϕ2+ϕ3) (z 1 z 2 )z 3 = (e iϕ1 e iϕ2 )e iϕ3 = (e iϕ1+ϕ2 )e iϕ3 = e i(ϕ1+ϕ2+ϕ3) 10. Abstract algebra December 17, / 62

39 Groups (continued) Proof G2. has a neutral element in S 1 = e i0 S z 1 = e iϕ e i0 = e i(ϕ+0) = e iϕ = z 1 z = e i0 e iϕ = e i(0+ϕ) = e iϕ = z G3. has an inverse element in S For each z = e iϕ, its inverse element with respect to is z 1 = e iϕ zz 1 = e iϕ e iϕ = e i(ϕ ϕ) = e i0 = 1 z 1 z = e iϕ e iϕ = e i( ϕ+ϕ) = e i0 = Abstract algebra December 17, / 62

40 Groups (N, +) is not a group because it has no neutral element. (N {0}, +) is not a group because it has no inverse element. (Z, +), (Q, +), (R, +), (C, +) and (R n, +) are abelian groups. (M m n, +) is an abelian group. (R, ) is not a group because 0 has no inverse (M n n (R)), ) is not a group because has no inverse Let S M n n (R) be the set of invertible matrices of size n n. (S, ) is a group (although not abelian). It is called the General Linear Group of degree n (GL(n, R)). 10. Abstract algebra December 17, / 62

41 Groups The existence of groups is what allows us to solve equations. For instance, consider the equation and its solution in the group (Z, +) 5 + x = x = 2 [Addition of the inverse of 5 with respect to + in both 5 + (5 + x) = [Addition is associative ] ( 5 + 5) + x = 3 [Definition of inverse] 0 + x = 3 [Definition of neutral element] x = Abstract algebra December 17, / 62

42 Groups Consider the equation 2x = 3 and its solution in the group (Q, ) 2x = 3 [Multiplication by the inverse of 2 in both sides] 1 2 (2x) = 1 3 [Multiplication is associative ] ( 1 2 2)x = 2 3 [Definition of inverse] 1x = 2 3 [Definition of neutral element] x = Abstract algebra December 17, / 62

43 Groups Theorem 5.1 (Cancellation laws) Given any group (S, ), a, b, c S it is verified Left cancellation: a b = a c b = c Right cancellation: b a = c a b = c Theorem 5.2 (Existance of a unique solution of linear equations) Given any group (S, ), a, b S the linear equations a x = b and y a = b always have a unique solution in S. Theorem 5.3 (Properties of the inverse) Given any group (S, ), a S its inverse is unique and a, b S (a b) 1 = (b 1 ) (a 1 ) 10. Abstract algebra December 17, / 62

44 Outline 10 Abstract algebra Sets Relations and functions Partitions and equivalence relationships Binary operations Groups and subgroups Homomorphisms and isomorphisms Algebraic structures 10. Abstract algebra December 17, / 62

45 Homomorphisms Consider the sets S = {a, b, c} and S = {A, B, C} with the operations : S S S and : S S S x y y = a y = b y = c x = a a b c x = b b c a x = c c a b and x y y = A y = B y = C x = A A B C x = B B C A x = C C A B We may construct a function that translates elements in S into elements in S with the same properties. We note that φ : S S φ(a) = A φ(b) = B φ(c) = C b c = a φ(b) φ(c) = φ(a) B C = A 10. Abstract algebra December 17, / 62

46 Homomorphisms Definition 6.1 (Group homomorphism) Given two groups (S, ) and (S, ), the function φ : S S is a group homomorphism iff a, b S φ(a b) = φ(a) φ(b) Definition 6.2 (Group isomorphism) Given two groups (S, ) and (S, ), the function φ : S S is a group isomorphism iff it is a group homomorphism and it is bijective. 10. Abstract algebra December 17, / 62

47 Homomorphisms Consider the two groups (R n, +) and (R m, +) and a matrix A M m n (R). The application is a group homomorphism because φ : R n R m φ(x) = Ax φ(u + v) = A(u + v) = Au + Av = φ(u) + φ(v) Consider the two groups (GL(n, R), ) and (R, ). The application is a group homomorphism because φ : GL(n, R) R φ(a) = det{a} φ(ab) = det{ab} = det{a} det{b} = φ(a) φ(b) 10. Abstract algebra December 17, / 62

48 Homomorphisms Theorem 6.1 Let φ : S S be a group homomogrphism between two groups. Then, φ(e) = e φ(a 1 ) = (φ(a)) 1 Definition 6.3 (Kernel of a group homomorphism) Let φ : S S be a group homomogrphism between two groups. Then, the kernel of φ is the set Ker{φ} = {x S φ(x) = e } Let φ(x) = Ax. Then, Ker{φ} = {x R n Ax = 0} = Nul{A} 10. Abstract algebra December 17, / 62

49 Isomorphisms Theorem 6.2 (Isomorphisms and cardinality) If two groups (S, ) and (S, ) are isomorph (i.e., there exists an isomorphism between the two groups), then S and S have the same cardinality. 10. Abstract algebra December 17, / 62

50 Isomorphisms Q and R cannot be isomorph because the cardinality of Q is ℵ 0 and the cardinality of R is ℵ 1. There are as many natural numbers as natural even numbers. In other words, the cardinality of N and 2N are the same. The reason is that the function φ(n) = 2n is an isomorphism between N and 2N. Consider the set R c = [0, c) R and the operation x + c y = (x + y) mod c. The pair (R c, + c ) is a group. Consider now the two particular cases (R 2π, + 2π ) and (R 1, + 1 ) and the mapping φ : R 2π R 1 φ(x) = x 2π φ is an isomorphism between (R 2π, + 2π ) and (R 1, + 1 ). In fact, all (R c, + c ) groups are isomorph to any other (R c, + c ) group. 10. Abstract algebra December 17, / 62

51 Isomorphisms Cardinality is a group property. The nice things about isomorphisms is that they preserve group properties. Theorem 6.3 If two groups (S, ) and (S, ) are isomorph, then If is commutative, so is. If there is an order relation in S, it can be translated into an order relation in S. If s S there exists a solution in S of the equation x x = s, then s S there exists a solution in S of the equation x x = s. If a, b S there exists a solution in S of the equation a x = b, then a, b S there exists a solution in S of the equation a x = b. The kernel of any isomorphism φ between (S, ) and (S, ) is Ker{φ} = {e} being e the neutral element of in S. 10. Abstract algebra December 17, / 62

52 Isomorphisms ((Z), +) is not isomorph to ((Q), +) because the equation x + x = s has a solution in Q for any s Q (that is x = s 2 ), but it does not have a solution in Z for any s Z (it only has a solution in Z if s is an even number). ((R), ) is not isomorph to ((C), ) because the equation x x = z has two solution in C for any z C (if z = re iθ, then x = ±re i θ 2 are the two solutions), but it does not have a solution in R for any z R (it only has a solution in R if z is a non-negative number). 10. Abstract algebra December 17, / 62

53 Outline 10 Abstract algebra Sets Relations and functions Partitions and equivalence relationships Binary operations Groups and subgroups Homomorphisms and isomorphisms Algebraic structures 10. Abstract algebra December 17, / 62

54 Algebraic structures Algebraic structures Algebraic structures are tools that help us to define operate on numbers and elements within a set, solve equations, etc. 10. Abstract algebra December 17, / 62

55 Algebraic structures Definition 7.1 (Ring) The tuple (S,, ) is a ring iff R1. (S, ) is an abelian group. R2. is associative. R3. is distributive with respect to, i.e., a, b, c S Left-distributive: a (b c) = (a b) (a c) Right-distributive: (a b) c = (a c) (b c) (Z, +, ), (Q, +, ), (R, +, ), (C, +, ) are rings. (M m n (R), +, ) is a ring. (R R, +, ) is a ring. 10. Abstract algebra December 17, / 62

56 Algebraic structures Theorem 7.1 (Properties of rings) Let (S,, ) be a ring and let e be the neutral element of in S. For any a S, let a be the inverse of a with respect to the operation. Then a, b S a e = e a = e. a b = a b = (a b) a b = a b Consider the ring (R, +, ). We are used to the properties a, b R a 0 = 0 a = 0. a ( b) = ( a) b = (a b) ( a) ( b) = a b But, as stated by the previous theorem, these are properties of all rings. 10. Abstract algebra December 17, / 62

57 Algebraic structures Definition 7.2 (Kinds of rings) A ring (S,, ) is commutative iff is commutative. unitary iff has a neutral element (referred as 1). divisive if it is unitary and a S {e}!a 1 S, a a 1 = a 1 a = 1 That is each element has a multiplicative inverse. (P, +, ) the set of polynomials with coefficients from a ring is a ring. 10. Abstract algebra December 17, / 62

58 Algebraic structures Definition 7.3 (Field (cuerpo)) A divisive, commutative ring is called a field. (Q, +, ), (R, +, ), and (C, +, ) are fields. (Z, +, ) is not a field because multiplication has not an inverse in Z. 10. Abstract algebra December 17, / 62

59 Algebraic structures Definition 7.4 (Vector space over a field) Consider a field (K,, ). A vector space over this field is a tuple (V, +, ) so that V is a set whose elements are called vectors, and + : V V V is a binary operation under which V is closed, : K V V is an operation between scalars in the field (K) and vectors in the vector space (V ) such that a, b K, u, v V V1. (V, +) is an abelian group. V2. (a u) V V3. a (b u) = (a b) u V4. (a b) u = a u + b u V5. a (u + v) = a u + a v V6. 1 u = u 10. Abstract algebra December 17, / 62

60 Algebraic structures s (R n, +, ) and (C n, +, ). (M m n (R), +, ): the set of matrices of a given size with coefficients in a field. (P, +, ): the set of polynomials with coefficients in a field. ({X V }, +, ): the set of all functions from an arbitrary set X onto an arbitrary vector space V. The set of all continuous functions is a vector space. The set of all linear maps between two vector spaces is also a vector space. The set of all infinite sequences of values from a field is also a vector space. 10. Abstract algebra December 17, / 62

61 Algebraic structures Definition 7.5 (Algebra) Consider a vector space (V, +, ) over a field (K,, ) and a binary operation : V V V. (V, +,, ) is an algebra iff a, b K, u, v, w V s A1. Left distributivity: (u + v) w = u w + v w A2. Right distributivity: u (v + w) = u v + u w A3. Compatibility with scalars: (a u) (b v) = (a b) (u v) Real numbers (R) are an algebra ( 1D ). Complex numbers (C) are an algebra ( 2D ). Quaternions are an algebra ( 4D ). 10. Abstract algebra December 17, / 62

62 Outline 10 Abstract algebra Sets Relations and functions Partitions and equivalence relationships Binary operations Groups and subgroups Homomorphisms and isomorphisms Algebraic structures 10. Abstract algebra December 17, / 62

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