Reflexivity, Symmetry, and Transitivitypeople.hsc.edu/faculty-staff/robbk/Math262/Lectures/Spring...
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Reflexivity, Symmetry, and TransitivityLecture 38Section 8.2
Robb T. Koether
Hampden-Sydney College
Mon, Apr 1, 2013
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1 Three Properties
2 Closures
3 Assignment
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Outline
1 Three Properties
2 Closures
3 Assignment
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Three Properties
Definition (Reflexive, Symmetric, Transitive)Let R be a relation on a set A. Then
R is reflexive if, for all x ∈ A, (x , x) ∈ R.R is symmetric if, for all x , y ∈ A,
(x , y) ∈ R → (y , x) ∈ R.
R is transitive if, for all x , y , z ∈ A,
((x , y) ∈ R and (y , z) ∈ R)→ (x , z) ∈ R.
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Three Properties
Reflexivity: Every element of the set A has the relation to itself.Symmetry: If any element x ∈ A has the relation to some elementy ∈ A, then y has the same relation to x .Transitivity: If any element x ∈ A has the relation to some elementy ∈ A and that element y has the same relation to some elementz ∈ A, then x has that relation to z.
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Examples
Define a relation R on Z by (x , y) ∈ R if 5 | (x − y).Show that R is reflexive, symmetric, and transitive.
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Examples
Which of the following relations are reflexive? symmetric?transitive?
a | b on Z.gcd(a, b) > 1 on Z.x × y < 0 on R.A ⊆ B on P(X ).p → q on a set of statements.p ∧ q ≡ p on a set of statements.
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Outline
1 Three Properties
2 Closures
3 Assignment
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The Reflexive Closure
Definition (Reflexive closure)Let A be a set and let R be a relation on A. The reflexive closure of R,denoted Rr , is
R ∪ {(a, a) | a ∈ A}.
Rr is the smallest relation on A that contains R and is reflexive.
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The Graph of the Reflexive Closure
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Add loops to get the graph of the reflexive closure.
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Examples
What is the reflexive closure of < on R?What is the reflexive closure of ⊂ on P(X )?What is the reflexive closure of ≡ modn on Z.
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The Symmetric Closure
Definition (Symmetric closure)Let A be a set and let R be a relation on A. The symmetric closure ofR, denoted Rs, is
R ∪ {(b, a) | (a, b) ∈ R}.
Rs is the smallest relation on A that contains R and is symmetric.
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The Graph of the Symmetric Closure
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Make every arrow double-ended to get the graph of the symmetricclosure.
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Examples
What is the symmetric closure of < on R?What is the symmetric closure of ⊂ on P(X )?What is the symmetric closure of ≡ modn on Z.
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The Transitive Closure
Definition (Transitive closure)Let A be a set and let R be a relation on A. The transitive closure of R,denoted Rt , is the smallest subset of A× A that contains R and istransitive.
To find the transitive closure of a relation requires an algorithm.It is not enough to define
Rt = R ∪ {(a, c) | (a, b), (b, c) ∈ R}.
Why not?
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The Transitive Closure
Given a relation R on a set A, the following procedure will produceRt .
Let n = 1 and let R0 = ∅ and R1 = R.While Rn 6= Rn−1, do the following.
Let Rn+1 = Rn ∪ {(a, c) | (a, b), (b, c) ∈ Rn}.n← n + 1.
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The Graph of the Transitive Closure
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R1 R2
Connect all back-to-back arrows.
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The Graph of the Transitive Closure
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R2 R3
Connect all back-to-back arrows again.
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Examples
What is the transitive closure of (a, b) ∈ R on Z defined byb = a + 1?What is the transitive closure of (A, B) ∈ R on P({1, 2, 3}) definedby A ∩ B 6= ∅?What is the transitive closure of (A, B) ∈ R on P(Z) defined by|A− B| <∞.
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All Three Closures
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We can do all three closures at the same time.
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Example
Define R on Z by (a, b) ∈ R if a− b = 2.Find the reflexive, symmetric, and transitive closure of R.
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Outline
1 Three Properties
2 Closures
3 Assignment
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Assignment
AssignmentRead Sections 8.2, pages 449 - 457.Exercises 2, 10, 14, 17, 19, 22, 23, 29, 35, 36, 52, 53, page 458.
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Collected Homework 10
Collected Homework 10Exercises 11, 17, page 426.Exercises 9, 12, 15, page 439.Exercises 11, 14, page 448.
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