# Bài giảng Discrete Mathematics I - Chapter 5: Relations

Relation Definition Let A and B be sets. A binary relation (quan h» hai ngôi) from a set A to a set B is a set R ⊆ A × BRelations Tran Vinh Tan Relation Definition Let A and B be sets. A binary relation (quan h» hai ngôi) from a set A to a set B is a set R ⊆ A × B

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Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.1 Chapter 5 Relations Discrete Mathematics I on 22 March 2012 Tran Vinh Tan Faculty of Computer Science and Engineering University of Technology - VNUHCM Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.2 Contents 1 Properties of Relations 2 Combining Relations 3 Representing Relations 4 Closures of Relations 5 Types of Relations Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.3 Introduction Function? Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.3 Introduction Function? Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.4 Relation Definition Let A and B be sets. A binary relation (quan hệ hai ngôi) from a set A to a set B is a set R ⊆ A×B • Notations: (a, b) ∈ R←→ aRb • n-ary relations? Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.4 Relation Definition Let A and B be sets. A binary relation (quan hệ hai ngôi) from a set A to a set B is a set R ⊆ A×B • Notations: (a, b) ∈ R←→ aRb • n-ary relations? Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.4 Relation Definition Let A and B be sets. A binary relation (quan hệ hai ngôi) from a set A to a set B is a set R ⊆ A×B • Notations: (a, b) ∈ R←→ aRb • n-ary relations? Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.4 Relation Definition Let A and B be sets. A binary relation (quan hệ hai ngôi) from a set A to a set B is a set R ⊆ A×B • Notations: (a, b) ∈ R←→ aRb • n-ary relations? Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.5 Example Example Let A = {a, b, c} be the set of students, B = {l, c, s, d} be the set of the available optional courses. We can have relation R that consists of pairs (a, b), where a is a student enrolled in course b. R = {(a, l), (a, s), (a, g), (b, c), (b, s), (b, g), (c, l), (c, g)} R l c s g a x x x b x x x c x x Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.5 Example Example Let A = {a, b, c} be the set of students, B = {l, c, s, d} be the set of the available optional courses. We can have relation R that consists of pairs (a, b), where a is a student enrolled in course b. R = {(a, l), (a, s), (a, g), (b, c), (b, s), (b, g), (c, l), (c, g)} R l c s g a x x x b x x x c x x Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.5 Example Example Let A = {a, b, c} be the set of students, B = {l, c, s, d} be the set of the available optional courses. We can have relation R that consists of pairs (a, b), where a is a student enrolled in course b. R = {(a, l), (a, s), (a, g), (b, c), (b, s), (b, g), (c, l), (c, g)} R l c s g a x x x b x x x c x x Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.6 Functions as Relations • Is a function a relation? • Yes! • f : A→ B R = {(a, b) | b = f(a)} Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.6 Functions as Relations • Is a function a relation? • Yes! • f : A→ B R = {(a, b) | b = f(a)} Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.6 Functions as Relations • Is a function a relation? • Yes! • f : A→ B R = {(a, b) | b = f(a)} Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.6 Functions as Relations • Is a function a relation? • Yes! • f : A→ B R = {(a, b) | b = f(a)} Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.7 Functions as Relations • Is a relation a function? • No • Relations are a generalization of functions Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.7 Functions as Relations • Is a relation a function? • No • Relations are a generalization of functions Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.7 Functions as Relations • Is a relation a function? • No • Relations are a generalization of functions Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.7 Functions as Relations • Is a relation a function? • No • Relations are a generalization of functions Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.8 Relations on a Set Definition A relation on the set A is a relation from A to A. Example Let A be the set {1, 2, 3, 4}. Which ordered pairs are in the relation R = {(a, b) | a divides b} (a là ước số của b)? Solution: R = {(1, 1), (1, 2), (1, 3), (1, 4), (2, 2), (2, 4), (3, 3), (4, 4)} R 1 2 3 4 1 x x x x 2 x x 3 x 4 x Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.8 Relations on a Set Definition A relation on the set A is a relation from A to A. Example Let A be the set {1, 2, 3, 4}. Which ordered pairs are in the relation R = {(a, b) | a divides b} (a là ước số của b)? Solution: R = {(1, 1), (1, 2), (1, 3), (1, 4), (2, 2), (2, 4), (3, 3), (4, 4)} R 1 2 3 4 1 x x x x 2 x x 3 x 4 x Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.8 Relations on a Set Definition A relation on the set A is a relation from A to A. Example Let A be the set {1, 2, 3, 4}. Which ordered pairs are in the relation R = {(a, b) | a divides b} (a là ước số của b)? Solution: R = {(1, 1), (1, 2), (1, 3), (1, 4), (2, 2), (2, 4), (3, 3), (4, 4)} R 1 2 3 4 1 x x x x 2 x x 3 x 4 x Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.9 Properties of Relations Reflexive xRx,∀x ∈ A (phản xạ) Symmetric xRy → yRx,∀x, y ∈ A (đối xứng) Antisymmetric (xRy ∧ yRx)→ x = y,∀x, y ∈ A (phản đối xứng) Transitive (xRy ∧ yRz)→ xRz,∀x, y, z ∈ A (bắc cầu) Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.9 Properties of Relations Reflexive xRx,∀x ∈ A (phản xạ) Symmetric xRy → yRx,∀x, y ∈ A (đối xứng) Antisymmetric (xRy ∧ yRx)→ x = y,∀x, y ∈ A (phản đối xứng) Transitive (xRy ∧ yRz)→ xRz,∀x, y, z ∈ A (bắc cầu) Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.9 Properties of Relations Reflexive xRx,∀x ∈ A (phản xạ) Symmetric xRy → yRx,∀x, y ∈ A (đối xứng) Antisymmetric (xRy ∧ yRx)→ x = y,∀x, y ∈ A (phản đối xứng) Transitive (xRy ∧ yRz)→ xRz,∀x, y, z ∈ A (bắc cầu) Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.9 Properties of Relations Reflexive xRx,∀x ∈ A (phản xạ) Symmetric xRy → yRx,∀x, y ∈ A (đối xứng) Antisymmetric (xRy ∧ yRx)→ x = y,∀x, y ∈ A (phản đối xứng) Transitive (xRy ∧ yRz)→ xRz,∀x, y, z ∈ A (bắc cầu) Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.9 Properties of Relations Reflexive xRx,∀x ∈ A (phản xạ) Symmetric xRy → yRx,∀x, y ∈ A (đối xứng) Antisymmetric (xRy ∧ yRx)→ x = y,∀x, y ∈ A (phản đối xứng) Transitive (xRy ∧ yRz)→ xRz,∀x, y, z ∈ A (bắc cầu) Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.10 Example Example Consider the following relations on {1, 2, 3, 4}: R1 = {(1, 1), (1, 2), (2, 1), (2, 2), (3, 4), (4, 1), (4, 4)}, R2 = {(1, 1), (1, 2), (2, 1)}, R3 = {(1, 1), (1, 2), (1, 4), (2, 1), (2, 2), (3, 3), (4, 1), (4, 4)}, R4 = {(2, 1), (3, 1), (3, 2), (4, 1), (4, 2), (4, 3)}, R5 = {(3, 4)} Solution: • Reflexive: R3 • Symmetric: R2, R3 • Antisymmetric: R4, R5 • Transitive: R4, R5 Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.10 Example Example Consider the following relations on {1, 2, 3, 4}: R1 = {(1, 1), (1, 2), (2, 1), (2, 2), (3, 4), (4, 1), (4, 4)}, R2 = {(1, 1), (1, 2), (2, 1)}, R3 = {(1, 1), (1, 2), (1, 4), (2, 1), (2, 2), (3, 3), (4, 1), (4, 4)}, R4 = {(2, 1), (3, 1), (3, 2), (4, 1), (4, 2), (4, 3)}, R5 = {(3, 4)} Solution: • Reflexive: R3 • Symmetric: R2, R3 • Antisymmetric: R4, R5 • Transitive: R4, R5 Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.10 Example Example Consider the following relations on {1, 2, 3, 4}: R1 = {(1, 1), (1, 2), (2, 1), (2, 2), (3, 4), (4, 1), (4, 4)}, R2 = {(1, 1), (1, 2), (2, 1)}, R3 = {(1, 1), (1, 2), (1, 4), (2, 1), (2, 2), (3, 3), (4, 1), (4, 4)}, R4 = {(2, 1), (3, 1), (3, 2), (4, 1), (4, 2), (4, 3)}, R5 = {(3, 4)} Solution: • Reflexive: R3 • Symmetric: R2, R3 • Antisymmetric: R4, R5 • Transitive: R4, R5 Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.10 Example Example Consider the following relations on {1, 2, 3, 4}: R1 = {(1, 1), (1, 2), (2, 1), (2, 2), (3, 4), (4, 1), (4, 4)}, R2 = {(1, 1), (1, 2), (2, 1)}, R3 = {(1, 1), (1, 2), (1, 4), (2, 1), (2, 2), (3, 3), (4, 1), (4, 4)}, R4 = {(2, 1), (3, 1), (3, 2), (4, 1), (4, 2), (4, 3)}, R5 = {(3, 4)} Solution: • Reflexive: R3 • Symmetric: R2, R3 • Antisymmetric: R4, R5 • Transitive: R4, R5 Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.10 Example Example Consider the following relations on {1, 2, 3, 4}: R1 = {(1, 1), (1, 2), (2, 1), (2, 2), (3, 4), (4, 1), (4, 4)}, R2 = {(1, 1), (1, 2), (2, 1)}, R3 = {(1, 1), (1, 2), (1, 4), (2, 1), (2, 2), (3, 3), (4, 1), (4, 4)}, R4 = {(2, 1), (3, 1), (3, 2), (4, 1), (4, 2), (4, 3)}, R5 = {(3, 4)} Solution: • Reflexive: R3 • Symmetric: R2, R3 • Antisymmetric: R4, R5 • Transitive: R4, R5 Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.11 Example Example What is the properties of the divides (ước số ) relation on the set of positive integers? Solution: • ∀a ∈ Z+, a | a: reflexive • 1 | 2, but 2 - 1: not symmetric • ∃a, b ∈ Z+, (a | b) ∧ (b | a)→ a = b: antisymmetric • a | b⇒ ∃k ∈ Z+, b = ak; b | c⇒ ∃l ∈ Z+, c = bl. Hence, c = a(kl)⇒ a | c: transitive Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.11 Example Example What is the properties of the divides (ước số ) relation on the set of positive integers? Solution: • ∀a ∈ Z+, a | a: reflexive • 1 | 2, but 2 - 1: not symmetric • ∃a, b ∈ Z+, (a | b) ∧ (b | a)→ a = b: antisymmetric • a | b⇒ ∃k ∈ Z+, b = ak; b | c⇒ ∃l ∈ Z+, c = bl. Hence, c = a(kl)⇒ a | c: transitive Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.12 Example Example What are the properties of these relations on the set of integers: R1 = {(a, b) | a ≤ b} R2 = {(a, b) | a > b} R3 = {(a, b) | a = b or a = −b} Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.13 Combining Relations Because relations from A to B are subsets of A×B, two relations from A to B can be combined in any way two sets can be combined. Example Let A = {1, 2, 3} and B = {1, 2, 3, 4}. List the combinations of relations R1 = {(1, 1), (2, 2), (3, 3)} and R2 = {(1, 1), (1, 2), (1, 3), (1, 4)}. Solution: R1 ∪R2, R1 ∩R2, R1 −R2 and R2 −R1. Example Let A and B be the set of all students and the set of all courses at school, respectively. Suppose R1 = {(a, b) | a has taken the course b} and R2 = {(a, b) | a requires course b to graduate}. What are the relations R1 ∪R2, R1 ∩R2, R1 ⊕R2, R1 −R2, R2 −R1? Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.13 Combining Relations Because relations from A to B are subsets of A×B, two relations from A to B can be combined in any way two sets can be combined. Example Let A = {1, 2, 3} and B = {1, 2, 3, 4}. List the combinations of relations R1 = {(1, 1), (2, 2), (3, 3)} and R2 = {(1, 1), (1, 2), (1, 3), (1, 4)}. Solution: R1 ∪R2, R1 ∩R2, R1 −R2 and R2 −R1. Example Let A and B be the set of all students and the set of all courses at school, respectively. Suppose R1 = {(a, b) | a has taken the course b} and R2 = {(a, b) | a requires course b to graduate}. What are the relations R1 ∪R2, R1 ∩R2, R1 ⊕R2, R1 −R2, R2 −R1? Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.13 Combining Relations Because relations from A to B are subsets of A×B, two relations from A to B can be combined in any way two sets can be combined. Example Let A = {1, 2, 3} and B = {1, 2, 3, 4}. List the combinations of relations R1 = {(1, 1), (2, 2), (3, 3)} and R2 = {(1, 1), (1, 2), (1, 3), (1, 4)}. Solution: R1 ∪R2, R1 ∩R2, R1 −R2 and R2 −R1. Example Let A and B be the set of all students and the set of all courses at school, respectively. Suppose R1 = {(a, b) | a has taken the course b} and R2 = {(a, b) | a requires course b to graduate}. What are the relations R1 ∪R2, R1 ∩R2, R1 ⊕R2, R1 −R2, R2 −R1? Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.13 Combining Relations Because relations from A to B are subsets of A×B, two relations from A to B can be combined in any way two sets can be combined. Example Let A = {1, 2, 3} and B = {1, 2, 3, 4}. List the combinations of relations R1 = {(1, 1), (2, 2), (3, 3)} and R2 = {(1, 1), (1, 2), (1, 3), (1, 4)}. Solution: R1 ∪R2, R1 ∩R2, R1 −R2 and R2 −R1. Example Let A and B be the set of all students and the set of all courses at school, respectively. Suppose R1 = {(a, b) | a has taken the course b} and R2 = {(a, b) | a requires course b to graduate}. What are the relations R1 ∪R2, R1 ∩R2, R1 ⊕R2, R1 −R2, R2 −R1? Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.14 Composition of Relations Definition Let R be relations from A to B and S be from B to C. Then the composite (hợp thành) of S and R is S ◦R = {(a, c) ∈ A× C | ∃b ∈ B (aRb ∧ bSc)} Example R = {(0, 0), (0, 3), (1, 2), (0, 1)} S = {(0, 0), (1, 0), (2, 1), (3, 1)} S ◦R = {(0, 0), (0, 1), (1, 1)} Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.14 Composition of Relations Definition Let R be relations from A to B and S be from B to C. Then the composite (hợp thành) of S and R is S ◦R = {(a, c) ∈ A× C | ∃b ∈ B (aRb ∧ bSc)} Example R = {(0, 0), (0, 3), (1, 2), (0, 1)} S = {(0, 0), (1, 0), (2, 1), (3, 1)} S ◦R = {(0, 0), (0, 1), (1, 1)} Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.14 Composition of Relations Definition Let R be relations from A to B and S be from B to C. Then the composite (hợp thành) of S and R is S ◦R = {(a, c) ∈ A× C | ∃b ∈ B (aRb ∧ bSc)} Example R = {(0, 0), (0, 3), (1, 2), (0, 1)} S = {(0, 0), (1, 0), (2, 1), (3, 1)} S ◦R = {(0, 0), (0, 1), (1, 1)} Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.14 Composition of Relations Definition Let R be relations from A to B and S be from B to C. Then the composite (hợp thành) of S and R is S ◦R = {(a, c) ∈ A× C | ∃b ∈ B (aRb ∧ bSc)} Example R = {(0, 0), (0, 3), (1, 2), (0, 1)} S = {(0, 0), (1, 0), (2, 1), (3, 1)} S ◦R = {(0, 0), (0, 1), (1, 1)} Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.14 Composition of Relations Definition Let R be relations from A to B and S be from B to C. Then the composite (hợp thành) of S and R is S ◦R = {(a, c) ∈ A× C | ∃b ∈ B (aRb ∧ bSc)} Example R = {(0, 0), (0, 3), (1, 2), (0, 1)} S = {(0, 0), (1, 0), (2, 1), (3, 1)} S ◦R = {(0, 0), (0, 1), (1, 1)} Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.15 Power of Relations Definition Let R be a relation on the set A. The powers (lũy thừa) Rn, n = 1, 2, 3, . . . are defined recursively by R1 = R and Rn+1 = Rn ◦R. Example Let R = {(1, 1), (2, 1), (3, 2), (4, 3)}. Find the powers Rn, n = 2, 3, 4, . . .. Solution: R2 = {(1, 1), (2, 1), (3, 1), (4, 2)} R3 = {(1, 1), (2, 1), (3, 1), (4, 1)} R4 = {(1, 1), (2, 1), (3, 1), (4, 1)} · · · Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.15 Power of Relations Definition Let R be a relation on the set A. The powers (lũy thừa) Rn, n = 1, 2, 3, . . . are defined recursively by R1 = R and Rn+1 = Rn ◦R. Example Let R = {(1, 1), (2, 1), (3, 2), (4, 3)}. Find the powers Rn, n = 2, 3, 4, . . .. Solution: R2 = {(1, 1), (2, 1), (3, 1), (4, 2)} R3 = {(1, 1), (2, 1), (3, 1), (4, 1)} R4 = {(1, 1), (2, 1), (3, 1), (4, 1)} · · · Relations Tran Vinh Tan Contents Properties of Relations Combining Relations Representing Relations Closures of Relations Types of Relations 5.15 Power of Relations Definition Let R be a relation on the set A. The powers (lũy thừa) Rn, n = 1, 2, 3, . . . are defined recursively by R1 = R and Rn+1 = Rn ◦R.