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**Discrete Mathematics Relations**

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**What is a relation Relation generalizes the notion of functions.**

Recall: A function takes EACH element from a set and maps it to a UNIQUE element in another set f: X Y x X, y such that f(x) = y Let A and B be sets. A binary relation R from A to B is a subset of A B Recall: A x B = {(a, b) | a A, b B} aRb: (a, b) R. Application Relational database model is based on the concept of relation.

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**What is a relation Example Let A be the students in a the CS major**

A = {Ayşe, Barış, Canan, Davut} Let B be the courses the department offers B = {BİM111, BİM122, BİM124} We specify relation R A B as the set that lists all students a A enrolled in class b B R = {(Ayşe, BİM111), (Barış, BİM122), (Barış, BİM124), (Davut, BİM122), (Davut, BİM124)}

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**More relation examples**

Another relation example: Let A be the cities in Turkey Let B be the districts in Turkey We define R to mean a is a district in city b Thus, the following are in our relation: (Bakırköy, İstanbul) (Keçiören, Ankara) (Nilüfer, Bursa) (Tepebaşı, Eskişehir) etc… Most relations we will see deal with ordered pairs of integers

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**Representing relations**

We can represent relations graphically: We can represent relations in a table: BİM111 BİM122 BİM124 Ayşe X Barış Canan Davut BİM111 BİM122 BİM124 Ayşe Barış Canan Davut Not valid functions!

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**Relations vs. functions**

If R X Y is a relation, then is R a function? If f: X Y is a function, then is f a relation?

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**Relations on a set A relation on the set A is a relation from A to A**

In other words, the domain and co-domain are the same set We will generally be studying relations of this type

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**Relations on a set R 1 2 3 4 X Let A be the set { 1, 2, 3, 4 }**

Which ordered pairs are in the relation R = { (a,b) | a divides b } R = {(1,1), (1,2), (1,3), (1,4), (2,2), (2,4), (3,3), (4,4)} 1 2 3 4 1 2 3 4 R 1 2 3 4 X

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**More examples Consider some relations on the set Z**

Are the following ordered pairs in the relation? (1,1) (1,2) (2,1) (1,-1) (2,2) R1 = { (a,b) | a≤b } R2 = { (a,b) | a>b } R3 = { (a,b) | a=|b| } R4 = { (a,b) | a=b } R5 = { (a,b) | a=b+1 } R6 = { (a,b) | a+b≤3 } X X X X X X X X X X X X X X X

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**Relation properties Six properties of relations we will study:**

Reflexive Irreflexive Symmetric Asymmetric Antisymmetric Transitive

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**Reflexivity vs. Irreflexivity**

Definition: A relation is reflexive if (a,a) R for all a A Irreflexivity Definition: A relation is irreflexive if (a,a) R for all a A Examples Is the “divides” relation on Z+ reflexive? Is the “” (not ) relation on a P(A) irreflexive? = < > reflexive o x x o o irreflexive x o o x x

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**Reflexivity vs. Irreflexivity**

A relation can be neither reflexive nor irreflexive Example? A = {1, 2}, R = {(1, 1)} It is not reflexive, since (2, 2) R, It is not irreflexive, since (1, 1) R.

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**Symmetry, Asymmetry, Antisymmetry**

A relation is symmetric if for all a, b A, (a,b) R (b,a) R A relation is asymmetric if for all a, b A, (a,b) R (b,a) R A relation is antisymmetric if for all a, b A, ((a,b) R (b,a) R) a=b (Second definition) for all a, b A, ((a,b) R a b) (b,a) R) < > = isTwinOf symmetric x x o x x o asymmetric o o x x x x antisymmetric o o o o o x

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**Notes on *symmetric relations**

A relation can be neither symmetric or asymmetric R = { (a,b) | a=|b| } This is not symmetric -4 is not related to itself This is not asymmetric 4 is related to itself Note that it is antisymmetric

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**Transitivity A relation is transitive if**

for all a, b, c A, ((a,b)R (b,c)R) (a,c)R If a < b and b < c, then a < c Thus, < is transitive If a = b and b = c, then a = c Thus, = is transitive

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**Transitivity examples**

Consider isAncestorOf() Let Ayşe be Barış’s ancestor, and Barış be Canan’s ancestor Thus, Ayşe is an ancestor of Barış, and Barış is an ancestor of Canan Thus, Ayşe is an ancestor of Canan Thus, isAncestorOf() is a transitive relation Consider isParentOf() Let Ayşe be Barış’s parent, and Barış be Canan’s parent Thus, Ayşe is a parent of Barış, and Barış is a parent of Canan However, Ayşe is not a parent of Canan Thus, isParentOf() is not a transitive relation

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**Summary of properties of relations**

reflexive a (a, a) R irreflexive a (a, a) R symmetric a, b A, (a,b) R (b,a) R asymmetric a, b A, (a,b) R (b,a) R antisymmetric a, b A, ((a,b) R (b,a) R) a=b (*)for all a, b A, ((a,b) R a b) (b,a) R) transitive a, b, c A, ((a,b) R (b,c) R) (a,c) R (*) Alternative definition…

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**Combining relations There are two ways to combine relations R1 and R2**

Via Set operators Via relation “composition”

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**Combining relations via Set operators**

Consider two relations R≥ and R≤ R≥ U R≤ = all numbers ≥ OR ≤ That’s all the numbers R≥ ∩ R≤ = all numbers ≥ AND ≤ That’s all numbers equal to R≥ R≤ = all numbers ≥ or ≤, but not both That’s all numbers not equal to R≥ - R≤ = all numbers ≥ that are not also ≤ That’s all numbers strictly greater than R≤ - R≥ = all numbers ≤ that are not also ≥ That’s all numbers strictly less than Note that it’s possible the result is the empty set

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**Combining via relational composition**

Similar to function composition Let R be a relation from A to B, and S be a relation from B to C Let a A, b B, and c C Let (a,b) R, and (b,c) S Then the composite of R and S consists of the ordered pairs (a,c) We denote the relation by S ◦ R Note that S comes first when writing the composition! (a, c) S ◦ R if b such that (a, b) R, and (b,c) S

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**Combining via relational composition**

Let M be the relation “is mother of” Let F be the relation “is father of” What is M ◦ F? If (a,b) F, then a is the father of b If (b,c) M, then b is the mother of c Thus, M ◦ F denotes the relation “maternal grandfather” What is F ◦ M? If (a,b) M, then a is the mother of b If (b,c) F, then b is the father of c Thus, F ◦ M denotes the relation “paternal grandmother” What is M ◦ M? Thus, M ◦ M denotes the relation “maternal grandmother”

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**Combining via relational composition**

Given relation R R ◦ R can be denoted by R2 R2 ◦ R = (R ◦ R) ◦ R = R3 Example: M3 is your mother’s mother’s mother

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