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18.7.2. Symmetric Closure
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Today, we will discuss the concept of symmetric closure. To start, can anyone tell me what it means for a relation to be symmetric?
Does that mean if (a, b) is in the relation, then (b, a) should also be?
Exactly! That's a perfect definition. Now, what if our original relation R doesn’t satisfy this property?
Then we would need to add pairs to make it symmetric, right?
Correct! This leads us to the idea of symmetric closure. Can anyone think of how we might construct this closure?
We should take the union of R with its inverse, which consists of (b, a) pairs.
Great observation! So, remember the process: Symmetric Closure = R ∪ R⁻¹. This ensures the smallest extensions needed for symmetry.
So the inverse relation is just all the pairs flipped?
Yes, exactly! Let’s summarize: To make R symmetric, we add its inverse pairs ensuring we only add what's necessary.
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Now, let's delve into the inverse of a relation. Can someone explain what that means?
The inverse of a relation R, denoted R⁻¹, contains all pairs (b, a) for each (a, b) in R.
Exactly! Why do we need the inverse when creating the symmetric closure?
Because it helps to add all necessary pairs to ensure the symmetry condition is met.
That's right! And what happens if (a, b) is in R but (b, a) is not?
Then we would add (b, a) from the inverse to create symmetry.
Perfect! The goal is minimal expansion. Hence, we achieve the symmetric closure efficiently.
Is there a notation we could use to denote this closure?
Yes! The symmetric closure of R can often be denoted as Rₛ, which showcases that it includes the subsets plus the inverses.
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Let’s move on to some examples. Suppose we have a relation R = {(1, 2), (3, 4)}. Is it symmetric?
No, because (2, 1) and (4, 3) are not included.
Very good! So what would be the symmetric closure in this case?
It would be R ∪ R⁻¹, so we add {(2, 1), (4, 3)} to get a new relation.
Exactly! So now we have Rₛ = {(1, 2), (2, 1), (3, 4), (4, 3)}. Let's apply this logic to another relation where R = {(A, B), (B, C)}. Can someone check if R is symmetric?
It’s not symmetric either. We would have to add (B, A) and (C, B).
Fantastic! And what’s the new symmetric closure for this relation?
Rₛ = {(A, B), (B, A), (B, C), (C, B)}.
Correct! Remember, identifying the original pairs and their inverses is key to forming symmetric closures.
Overview
Short Summary
This section discusses the concept of symmetric closure of a relation in discrete mathematics.
Medium Summary
The section provides a detailed explanation of what symmetric closure means, how it is constructed, and its significance in the context of relations. It emphasizes the necessity of including both the original relation and its inverse to ensure that the resulting closure is symmetric.
Detailed Summary
Symmetric Closure
The symmetric closure of a relation is a concept in discrete mathematics that involves modifying a given relation to ensure that it satisfies the symmetric property. A relation R on a set A is symmetric if, whenever (a, b) is in R, (b, a) is also in R. The symmetric closure of R is defined as the smallest superset of R that includes all required symmetric pairs. To construct this closure, the operation involves taking the union of the original relation R with its inverse, where the inverse of R consists of ordered pairs (b, a) for every pair (a, b) in R. Therefore, the symmetric closure guarantees that if a relation is not initially symmetric, it can be 'closed' to satisfy this important property while retaining the minimal necessary additions. This section highlights why understanding symmetric closure is essential for further operations on relations and forms a foundation for more complex concepts in discrete mathematics.
Reference YouTube Videos
Audio Book
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Create a free accountNext let us define what we call as closure of a relation. So, what do we mean by this? So imagine you are given a set A that may be finite or infinite and you are given a relation R over the set A that means a relation R is a subset of A x A and I have some abstract property P, it is some abstract property and I am interested to check whether the relation R over the set A satisfies this property P or not?
Detailed Explanation
A symmetric closure of a relation aims to ensure that if a relation includes a pair (a, b), it should also include the pair (b, a). This is defined within the context of a set A, which can have a finite or infinite number of elements. Here, R is a relation defined over set A, meaning R contains ordered pairs of elements from A.
Examples & Analogies
Think of it like a friendship group where you have already established friendships between some people, e.g., if Alice and Bob are friends, you would also want to make sure Bob and Alice acknowledge each other as friends to maintain balance in the group.
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Create a free accountSo, if you recall the property of symmetric relation then the requirement here is that if (a, b) or if (a, a) is present in R, then I need the guarantee that (a, a) should also be present in R. So, I am going to take the union of R with what I call as the inverse of the relation R. So, this is the inverse relation and what is this inverse relation? It is defined to consist of all ordered pairs of the form (a, a) such that a is related to a in the original relation.
Detailed Explanation
To construct the symmetric closure of a relation R, we include both R and its inverse relation, R⁻¹, which consists of pairs (b, a) for every (a, b) in R. Taking the union of R and R⁻¹ ensures that the resulting relation is symmetric — if (a, b) exists, then (b, a) will also exist.
Examples & Analogies
Imagine a game of tag where if Alex tags Maria, it should also mean Maria can tag Alex back. By ensuring that both directions of tagging are accounted for in the 'friendship' network, we create a balanced and fair game.
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Create a free accountIt is easy to see that if you take the union of R with its inverse then the resultant relation will be symmetric and it will have the original relation R and this will be the smallest possible expansion of your relation R which satisfies the symmetric property.
Detailed Explanation
The result of the symmetric closure is a new relation that includes every pair from R plus any missing pairs required to make R symmetric. Importantly, this new relation is the minimal extension needed to satisfy the symmetric property, meaning no unnecessary pairs are added.
Examples & Analogies
Think of a city’s public transport system. If buses can travel from A to B, adding routes for buses returning from B to A ensures that the system operates symmetrically. This addition is the minimal change necessary to maintain balance in public transport availability.
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Key concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
- Symmetric Closure:
The process of creating a new relation that is symmetric by including the original relation and its inverses.
- Inverse Relation:
Necessary for establishing symmetric closure; it contains pairs that are reciprocated from the original relation.
- Set Union:
Technique used to combine the original relation and its inverses to create the symmetric closure.
Examples
Memory aids
Imagine two friends passing notes. If A passes to B, B must pass back to A to keep it fair, thus maintaining symmetry in their communications.
Flash Cards
Glossary
Symmetric Relation
A relation R is symmetric if for every (a, b) in R, (b, a) is also in R.
Symmetric Closure
The smallest superset of a relation R that is symmetric and includes R and its inverse pairs.
Inverse Relation
For a relation R, the inverse R⁻¹ consists of all pairs (b, a) for each (a, b) in R.