CMSC 56 | Lecture 16: Equivalence of Relations & Partial Ordering

Equivalence of Relations
& Partial Ordering
Lecture 16, CMSC 56
Allyn Joy D. Calcaben

Equivalence
Definition 1
A relation on a set A is called an equivalence relation if it is
reflexive, symmetric, and transitive.

Equivalence
Definition 2
Two elements a and b that are related by an equivalence relation
are called equivalent.

Equivalence
Definition 2
Two elements a and b that are related by an equivalence relation
are called equivalent.
The notation a ~ b is often used to denote that a and b are
equivalent elements with respect to a particular equivalence
relation.

Example
Let R be the relation on the set of integers such that aRb if and
only if a = b or a = -b. Is R an equivalence relation?

Solution
Let R be the relation on the set of integers such that aRb if and
only if a = b or a = -b. Is R an equivalence relation?
YES

Example
Let R be the relation on the set of real numbers such that aRb if
and only if a − b is an integer. Is R an equivalence relation?

Solution
and only if a − b is an integer. Is R an equivalence relation?
YES

Example
and only if a − b is a positive integer. Is R an equivalence relation?

Solution
and only if a − b is a positive integer. Is R an equivalence relation?
NO

Example
Is the “divides” relation on the set of positive integers an
equivalence relation?

Solution
Is the “divides” relation on the set of positive integers an
equivalence relation?
NO

Equivalence Class
Definition 3
Let R be an equivalence relation on a set A. The set of all elements
that are related to an element a of A is called the equivalence
class of a.

Equivalence Class
Definition 3
Let R be an equivalence relation on a set A. The set of all elements
that are related to an element a of A is called the equivalence
class of a.
The equivalence class of a with respect to R is denoted by [a]R.
When only one relation is under consideration, we can delete the
subscript R and write [a] for this equivalence class.

Equivalence Class
In other words, if R is an equivalence relation on a set A, the
equivalence class of the element a is [a]R = { b | (a, b) ∈ R }.
If b ∈ [a]R, then b is called a representative of this equivalence
class

Example
Given the equivalence relation R = {(1,1), (1,2), (1,3), (2,1), (2,2),
(2,3), (3,1), (3,2), (3,3), (4,4), (5,5)} on set {1, 2, 3, 4, 5}.
Find [1], [2], [3], [4], and [5].

Solution
Given the equivalence relation R = {(1,1), (1,2), (1,3), (2,1), (2,2),
(2,3), (3,1), (3,2), (3,3), (4,4), (5,5)} on set {1, 2, 3, 4, 5}.
Find [1], [2], [3], [4], and [5].
[1] = {1, 2, 3} [4] = {4}
[2] = {1, 2, 3} [5] = {5}
[3] = {1, 2, 3}

Example
What is the equivalence class of an integer for the equivalence
relation such that aRb if and only if a = b or a = −b?

Solution
[a] = { –a, a }.
This set contains two distinct integers unless a = 0.

Solution
[a] = { –a, a }.
This set contains two distinct integers unless a = 0.
For instance, [ 7 ] = { −7, 7 },
[−5] = {−5, 5 }, and
[0] = {0}

Partial Orderings
Definition 1
A relation R on a set S is called a partial ordering or partial order if
it is reflexive, antisymmetric, and transitive.

Partial Orderings
Definition 1
A relation R on a set S is called a partial ordering or partial order if
it is reflexive, antisymmetric, and transitive.
A set S together with a partial ordering R is called a partially
ordered set, or poset, and is denoted by (S,R). Members of S are
called elements of the poset.

Example
Is the “divides” relation a partial ordering on the set of positive
integers?

Solution
Is the “divides” relation a partial ordering on the set of positive
integers?
(Z+, |) is a poset

Partial Ordering
Definition 2
The elements a and b of a poset (S, ≼ ) are called comparable if
either a ≼ b or b ≼ a.
When a and b are elements of S such that neither a ≼ b nor b ≼ a,
a and b are called incomparable.

Example
Are all the elements in the poset (Z+, |) comparable?

Solution
Are all the elements in the poset (Z+, |) comparable?
NO
For instance: 3 | 7 and 7 | 3

Partial Ordering
The adjective “partial” is used to describe partial orderings
because pairs of elements may be incomparable.
When every two elements in the set are comparable, the relation
is called a total ordering.

Partial Ordering
Definition 3
If (S, ≼ ) is a poset and every two elements of S are comparable, S
is called a totally ordered or linearly ordered set, and ≼ is called a
total order or a linear order. A totally ordered set is also called a
chain.

Example
Are all the elements in the poset (Z, ≤) comparable?

Solution
Are all the elements in the poset (Z, ≤) comparable?
Yes
The poset (Z, ≤) is totally ordered.

Hasse Diagrams
Consider the directed graph for the partial ordering { (a, b) | a ≤ b } on the
set {1, 2, 3, 4}

Hasse Diagrams
set {1, 2, 3, 4}
1. Start with the directed graph for this relation

Hasse Diagrams
set {1, 2, 3, 4}
2. Because a partial ordering is reflexive, a loop (a, a) is present at every
vertex a. Remove these loops.

Hasse Diagrams
set {1, 2, 3, 4}
3. Next, remove all edges that must be in the partial ordering because of
the presence of other edges and transitivity.

Hasse Diagrams
set {1, 2, 3, 4}
3. Next, remove all edges that must be in the partial ordering because of
the presence of other edges and transitivity.
4. Finally, arrange each edge so that its initial vertex is below its terminal
vertex. Remove all the arrows on the directed edges, because all edges
point “upward” toward their terminal vertex.

Example
Draw the Hasse diagram representing the partial ordering { (a, b) |a divides
b } on {1, 2, 3, 4, 6, 8, 12}.

oSet
November 12 – December 05
41
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CMSC 56 | Lecture 16: Equivalence of Relations & Partial Ordering

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