Welcome back, future engineers and mathematicians! In our previous discussions, we laid the groundwork for understanding what a 'relation' truly is β essentially, a specific connection or association between elements of sets. Think of it as a rule that links elements from one set to another (or within the same set). Today, we're going to embark on a deep dive into the fascinating world of
types of relations. This is a fundamental topic, not just for your CBSE exams, but also a cornerstone for advanced concepts frequently tested in JEE Mains & Advanced.
Let's begin by quickly recalling that a relation `R` from a set `A` to a set `B` is a subset of the Cartesian product `A Γ B`. If the relation is defined on a single set `A`, then `R` is a subset of `A Γ A`. We denote that `a` is related to `b` by `(a, b) β R` or `a R b`.
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1. The Boundary Relations: Empty and Universal
Before we jump into the more interesting types, let's look at the two extreme cases of relations.
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1.1. Empty Relation (Void Relation)
Imagine a relation where absolutely no element from set `A` is related to any element in set `A` (or set `B`). Such a relation is called an
Empty Relation.
Definition: A relation `R` on a set `A` is called an
empty relation if no element of `A` is related to any element of `A`, i.e., `R = β
β A Γ A`.
Analogy: Picture a rule that says "students taller than 10 feet are related to students who can fly." In a typical classroom, no such pair would exist. This would be an empty relation.
Example: Let `A = {1, 2, 3}`. Consider the relation `R = {(a, b) β A Γ A : a + b = 10}`.
Let's check for pairs:
* `1 + 1 = 2`
* `1 + 2 = 3`
* `3 + 3 = 6`
No pair `(a, b)` from `A Γ A` satisfies `a + b = 10`. Therefore, `R = β
`. This is an empty relation.
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1.2. Universal Relation
On the other end of the spectrum is the
Universal Relation, where every element of set `A` is related to every element of set `A`.
Definition: A relation `R` on a set `A` is called a
universal relation if each element of `A` is related to every element of `A`, i.e., `R = A Γ A`.
Analogy: Consider a rule "students who breathe are related to students who have a heartbeat." Assuming all students are living, every student is related to every other student (including themselves) by this rule. This is a universal relation.
Example: Let `A = {1, 2, 3}`. Consider the relation `R = {(a, b) β A Γ A : |a - b| β₯ 0}`.
Let's check:
* `|1 - 1| = 0 β₯ 0`
* `|1 - 2| = 1 β₯ 0`
* `|3 - 1| = 2 β₯ 0`
In fact, for any `a, b β A`, `|a - b|` will always be a non-negative number, so the condition `|a - b| β₯ 0` is always true. Thus, `R = A Γ A`. This is a universal relation.
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2. The Core Three: Reflexive, Symmetric, Transitive
These three types are the building blocks for understanding more complex relations, especially equivalence relations which are very important for JEE.
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2.1. Reflexive Relation
Think of looking in a mirror. You see yourself. A reflexive relation is similar β every element is related to itself.
Definition: A relation `R` on a set `A` is called
reflexive if every element of `A` is related to itself. That is, for every `a β A`, `(a, a) β R` or `a R a`.
Important Note: For a relation to be reflexive, *all* elements in the set `A` must satisfy the condition `(a,a) β R`. If even one element `x β A` does not have `(x,x) β R`, then the relation is NOT reflexive.
Analogy: "Is equal to" is a great example. `a = a` is always true. Another is "is a sibling of" (if we consider oneself as a sibling to oneself in a formal sense, though usually not in common parlance). Or "is the same person as".
Examples:
1. Let `A = {1, 2, 3}`. `R = {(1, 1), (2, 2), (3, 3), (1, 2)}` is a reflexive relation because `(1, 1), (2, 2), (3, 3)` are all present. The additional pair `(1, 2)` doesn't violate reflexivity.
2. The relation "is equal to" (`=`) on the set of real numbers `β`. For any `x β β`, `x = x` is true. So, `(x, x) β R`.
3. The relation "is a subset of" (`β`) on the power set `P(S)` of any set `S`. For any set `X β P(S)`, `X β X` is always true. So, `(X, X) β R`.
Non-Example: Let `A = {1, 2, 3}`.
* `R = {(1, 1), (2, 2)}` is NOT reflexive because `(3, 3) β R`.
* `R = {(1, 2), (2, 3)}` is NOT reflexive because `(1, 1), (2, 2), (3, 3)` are all missing.
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2.2. Symmetric Relation
Imagine a two-way street or friendship. If A is friends with B, then B is friends with A.
Definition: A relation `R` on a set `A` is called
symmetric if whenever `(a, b) β R`, then `(b, a)` must also be in `R`. That is, for all `a, b β A`, if `a R b`, then `b R a`.
Analogy: "Is a sibling of". If John is a sibling of Mary, then Mary is a sibling of John. "Is equal to". If `x = y`, then `y = x`. "Is perpendicular to" for lines.
Examples:
1. Let `A = {1, 2, 3}`. `R = {(1, 2), (2, 1), (1, 3), (3, 1), (2, 2)}`.
* If `(1, 2) β R`, then `(2, 1) β R` (present).
* If `(1, 3) β R`, then `(3, 1) β R` (present).
* If `(2, 2) β R`, then `(2, 2) β R` (present).
This is a symmetric relation.
2. The relation "is perpendicular to" (`β₯`) on the set of all lines in a plane. If line `L1 β₯ L2`, then `L2 β₯ L1`.
3. The relation "is a spouse of" on the set of married individuals. If A is a spouse of B, then B is a spouse of A.
Non-Example: Let `A = {1, 2, 3}`.
* `R = {(1, 2), (2, 3)}` is NOT symmetric because `(1, 2) β R` but `(2, 1) β R`.
* The relation "is less than" (`<`) on integers. `1 < 2` is true, but `2 < 1` is false. So, it's not symmetric.
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2.3. Transitive Relation
Think of a domino effect or a chain reaction. If A leads to B, and B leads to C, then A must lead to C.
Definition: A relation `R` on a set `A` is called
transitive if whenever `(a, b) β R` and `(b, c) β R`, then `(a, c)` must also be in `R`. That is, for all `a, b, c β A`, if `a R b` and `b R c`, then `a R c`.
Analogy: "Is less than" (`<`). If `a < b` and `b < c`, then `a < c`. "Is an ancestor of". If A is an ancestor of B, and B is an ancestor of C, then A is an ancestor of C. "Is parallel to" for lines.
Examples:
1. Let `A = {1, 2, 3}`. `R = {(1, 1), (1, 2), (2, 3), (1, 3)}`.
* ` (1, 2) β R` and `(2, 3) β R`. Is `(1, 3) β R`? Yes, it is.
* Other pairs: `(1, 1)` and `(1, 2)` implies `(1, 2)` (present).
This is a transitive relation.
2. The relation "is a subset of" (`β`) on the power set `P(S)` of any set `S`. If `X β Y` and `Y β Z`, then `X β Z`.
3. The relation "is parallel to" (`||`) on the set of all lines in a plane. If line `L1 || L2` and `L2 || L3`, then `L1 || L3`.
Non-Example: Let `A = {1, 2, 3}`.
* `R = {(1, 2), (2, 3), (3, 1)}`.
* ` (1, 2) β R` and `(2, 3) β R`. According to transitivity, `(1, 3)` should be in `R`. But `(1, 3) β R`. Therefore, it is NOT transitive.
* The relation "is a friend of" (in a strict sense where friendship is not always mutual or transitive). If A is friends with B, and B is friends with C, it doesn't automatically mean A is friends with C.
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3. The Pinnacle: Equivalence Relation
This is where the three core types converge to form a particularly powerful kind of relation. Equivalence relations are extremely important in mathematics, as they partition a set into disjoint subsets called
equivalence classes.
Definition: A relation `R` on a set `A` is called an
equivalence relation if it is:
1.
Reflexive: For all `a β A`, `(a, a) β R`.
2.
Symmetric: For all `a, b β A`, if `(a, b) β R`, then `(b, a) β R`.
3.
Transitive: For all `a, b, c β A`, if `(a, b) β R` and `(b, c) β R`, then `(a, c) β R`.
Analogy: Think of sorting objects into groups based on some shared property. For example, sorting students by their birth month. All students born in January are "equivalent" in terms of their birth month. This 'is born in the same month as' relation would be reflexive (you're born in your own birth month), symmetric (if A is born in the same month as B, B is born in the same month as A), and transitive (if A shares a birth month with B, and B with C, then A shares a birth month with C).
Example 1: The 'Equals' Relation
The relation `R = {(a, b) : a = b}` on the set of real numbers `β`.
1.
Reflexive: For any `a β β`, `a = a`. So `(a, a) β R`. (True)
2.
Symmetric: If `(a, b) β R`, then `a = b`. This implies `b = a`, so `(b, a) β R`. (True)
3.
Transitive: If `(a, b) β R` and `(b, c) β R`, then `a = b` and `b = c`. This implies `a = c`, so `(a, c) β R`. (True)
Since R is reflexive, symmetric, and transitive, it is an
equivalence relation.
Example 2: Congruence Modulo n
Let `Z` be the set of integers. For a fixed positive integer `n`, define a relation `R` on `Z` as `(a, b) β R` if and only if `a - b` is divisible by `n` (or `a β‘ b (mod n)`).
Let's check the properties:
1.
Reflexive: For any `a β Z`, `a - a = 0`. Since `0` is divisible by any non-zero integer `n`, `(a, a) β R`. (True)
2.
Symmetric: If `(a, b) β R`, then `a - b` is divisible by `n`. This means `a - b = kn` for some integer `k`. Then `b - a = -(a - b) = -kn`. Since `-k` is also an integer, `b - a` is divisible by `n`. So, `(b, a) β R`. (True)
3.
Transitive: If `(a, b) β R` and `(b, c) β R`, then `a - b = kβn` and `b - c = kβn` for some integers `kβ`, `kβ`.
Adding these equations: `(a - b) + (b - c) = kβn + kβn`
`a - c = (kβ + kβ)n`.
Since `kβ + kβ` is an integer, `a - c` is divisible by `n`. So, `(a, c) β R`. (True)
Thus, `R` is an
equivalence relation.
💬 JEE Mains & Advanced Focus: Equivalence Classes
A crucial concept arising from equivalence relations is that of equivalence classes.
If `R` is an equivalence relation on a set `A`, then for any element `a β A`, the set of all elements in `A` that are related to `a` is called the equivalence class of `a`, denoted by `[a]` or `Cl(a)`.
Mathematically, `[a] = {x β A : (x, a) β R}`.
The collection of all distinct equivalence classes forms a partition of the set `A`. This means:
- Every element of `A` belongs to some equivalence class.
- No equivalence class is empty.
- Any two distinct equivalence classes are disjoint (they have no elements in common).
- The union of all equivalence classes is the entire set `A`.
Example (Continuing Congruence Modulo 3):
Let `A = Z` and `R` be `a β‘ b (mod 3)`. The equivalence classes are:
- `[0] = {..., -6, -3, 0, 3, 6, ...}` (integers that give remainder 0 when divided by 3)
- `[1] = {..., -5, -2, 1, 4, 7, ...}` (integers that give remainder 1 when divided by 3)
- `[2] = {..., -4, -1, 2, 5, 8, ...}` (integers that give remainder 2 when divided by 3)
These three classes `[0], [1], [2]` are disjoint, and their union is `Z`. This partitioning property is a very deep and useful consequence of equivalence relations, often used in abstract algebra and number theory.
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4. Summary Table of Relations
To consolidate your understanding, here's a quick reference:
Type of Relation |
Condition on Set A |
Example |
|---|
Empty Relation |
`R = β
` |
`R = {(a, b) β A Γ A : a + b = 10}` for `A = {1, 2, 3}` |
Universal Relation |
`R = A Γ A` |
`R = {(a, b) β A Γ A : |a - b| β₯ 0}` for `A = {1, 2, 3}` |
Reflexive |
`(a, a) β R` for all `a β A` |
`a = b` (equality), `a β€ b` (less than or equal to) |
Symmetric |
If `(a, b) β R`, then `(b, a) β R` |
`a = b` (equality), `a` is sibling of `b` |
Transitive |
If `(a, b) β R` and `(b, c) β R`, then `(a, c) β R` |
`a = b` (equality), `a < b` (less than), `a β b` (subset) |
Equivalence Relation |
Reflexive AND Symmetric AND Transitive |
`a = b`, `a β‘ b (mod n)`, `a` is parallel to `b` (for lines) |
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5. Practice Problem (JEE Style):
Let `A = {1, 2, 3, 4, 5, 6, 7}`. Define a relation `R` on `A` as `R = {(a, b) : a, b β A and a - b is divisible by 3}`. Determine if `R` is an equivalence relation. If so, find its equivalence classes.
Step-by-step Solution:
1. Check for Reflexivity:
We need to check if `(a, a) β R` for all `a β A`.
For any `a β A`, `a - a = 0`.
Since `0` is divisible by `3` (`0 = 3 Γ 0`), `(a, a) β R` for all `a β A`.
Therefore, `R` is
reflexive.
2. Check for Symmetry:
We need to check if `(a, b) β R β (b, a) β R` for all `a, b β A`.
Assume `(a, b) β R`. This means `a - b` is divisible by `3`.
So, `a - b = 3k` for some integer `k`.
Then `b - a = -(a - b) = -3k = 3(-k)`.
Since `-k` is also an integer, `b - a` is divisible by `3`.
Thus, `(b, a) β R`.
Therefore, `R` is
symmetric.
3. Check for Transitivity:
We need to check if `(a, b) β R` and `(b, c) β R β (a, c) β R` for all `a, b, c β A`.
Assume `(a, b) β R` and `(b, c) β R`.
This means `a - b` is divisible by `3`, so `a - b = 3kβ` for some integer `kβ`.
And `b - c` is divisible by `3`, so `b - c = 3kβ` for some integer `kβ`.
Now, consider `a - c`:
`a - c = (a - b) + (b - c)`
`a - c = 3kβ + 3kβ`
`a - c = 3(kβ + kβ)`.
Since `kβ + kβ` is an integer, `a - c` is divisible by `3`.
Thus, `(a, c) β R`.
Therefore, `R` is
transitive.
Conclusion: Since `R` is reflexive, symmetric, and transitive, it is an
equivalence relation.
Finding Equivalence Classes:
The equivalence classes will group elements that have the same remainder when divided by 3.
*
Class of 1 (`[1]`): Elements `x β A` such that `x - 1` is divisible by 3.
`x - 1` can be `0, 3, 6, -3, ...`
If `x - 1 = 0 β x = 1`
If `x - 1 = 3 β x = 4`
If `x - 1 = 6 β x = 7`
So, `[1] = {1, 4, 7}`.
*
Class of 2 (`[2]`): Elements `x β A` such that `x - 2` is divisible by 3.
`x - 2` can be `0, 3, 6, ...`
If `x - 2 = 0 β x = 2`
If `x - 2 = 3 β x = 5`
So, `[2] = {2, 5}`.
*
Class of 3 (`[3]`): Elements `x β A` such that `x - 3` is divisible by 3.
`x - 3` can be `0, 3, 6, ...`
If `x - 3 = 0 β x = 3`
If `x - 3 = 3 β x = 6`
So, `[3] = {3, 6}`.
Notice that `[4]` would be the same as `[1]` (`{1, 4, 7}`) because `4` is related to `1`. Similarly for `[5]` and `[2]`, `[6]` and `[3]`, `[7]` and `[1]`.
The distinct equivalence classes are `{1, 4, 7}`, `{2, 5}`, and `{3, 6}`. Their union is `A`, and they are pairwise disjoint.
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This detailed exploration should give you a solid understanding of the various types of relations, especially the crucial concept of equivalence relations and their role in partitioning sets. Keep practicing these definitions and examples, as they are foundational for many areas of higher mathematics!