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9.4.2. Listing of Set S
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Today, we are diving into cardinality, focusing on two sets of real numbers: (0, 1) and (0, 1]. Can someone tell me the difference between these two sets?
The first set doesn’t include 0 or 1, and the second one includes 1 but not 0.
Exactly! Now, can we conclude that these sets have the same number of elements, or cardinality?
We could use an example from the Schroder-Bernstein theorem, right?
Correct! The theorem states that if there are injections from one set to another, we can say they have the same cardinality. Let’s define our injections.
How do we represent these injections visually?
Great question! Visual aids like diagrams can clarify how each element in the domain maps to elements in the codomain. Remember, even if we don’t see an overlap, the mappings help support our theorem.
Can each real number still map uniquely even if the sets seem different?
Yes! That’s the power of injective functions. They maintain uniqueness. In summary, although one interval includes 1, both sets have the same cardinality.
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Next, let’s now prove that there exists no infinite set A whose cardinality is strictly less than א₀. What implications does this have?
It implies that א₀ is the smallest infinity!
Exactly! To prove this, we'll use two claims. Let's start with Claim 1: Any set A with cardinality less than or equal to that of the positive integers has a corresponding subset of positive integers with the same cardinality. How can we show that?
Is it by showing an injective mapping?
Yes, great thinking! Now, what’s Claim 2?
It states that any subset of positive integers is either finite or has the cardinality of א₀!
Right! Now let’s put these claims to the test by assuming an infinite set A exists strictly less than א₀.
Then wouldn’t A and its subset B contradict our claims?
Precisely! It's a contradiction, which reinforces our point about א₀ being the 'smallest infinity'.
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Let’s discuss a foundational result: every infinite set contains a countably infinite subset. Let’s start with an infinite set A. What can we conclude about its elements?
We can pick at least one element from A!
Right! What happens when we remove that element?
The remaining set is still infinite.
Correct! And if we repeat this process, what do we obtain?
A sequence of elements from the infinite set!
Yes! The collection we remove will still maintain the same cardinality of A, thus proving there’s a countably infinite subset. Let’s summarize: If A is infinite, it must have infinite countable subsets.
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We now explore if the union of countably many countable sets is also countable. What insights can you provide?
If each set is countable, can we list all their elements?
Exactly! How would we visualize this process?
We can use a grid or pairs of indices to label elements in those sets.
Right! By making a systematic listing of elements, we ensure every element from each countable set appears. So, in what fashion will we proceed with the indices?
We begin with the least index summation and systematically track all inputs.
Exactly! Summarizing our session, we concluded that the union of countable sets is indeed countable since we can enumerate the elements systematically.
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Finally, let's talk about uncountable sets and their intersections. Can we have uncountable sets whose intersection is finite?
Yes, like the sets [0,1] and [1,2], their intersection is {1}, which is finite!
Great example! Now, how about uncountable sets whose intersection is countably infinite?
If we take sets that overlap with some integers, they can still be uncountable!
Correct! To summarize, uncountable sets exhibit diverse properties, providing rich contexts for understanding intersections.
Overview
Short Summary
This section explores the concept of cardinality between two sets, particularly focusing on real numbers and their mappings.
Medium Summary
The section discusses the cardinality of sets, particularly illustrating with the sets of real numbers in the interval (0, 1) and (0, 1]. It uses the Schroder-Bernstein theorem to demonstrate that these sets have the same cardinality. It also addresses infinite sets and their cardinalities, concluding with the properties of unions of countable sets.
Detailed Summary
Detailed Summary
This section provides an overview of set theory related to cardinalities, particularly focusing on the sets of real numbers within specified intervals. We start with two important sets: one consists of all real numbers in the interval (excluding 0 and 1), while the other includes the set (excluding 0 but including 1). The main goal is to demonstrate that these two sets possess the same cardinality through the application of the Schroder-Bernstein theorem, which states that if there exist injections (one-to-one mappings) from set A to set B and from set B to set A, then both sets are of the same cardinality.
The ultimate proofs of the theorem are presented with explicit mappings for each interval. Next, the section explores the cardinality of infinite sets, emphasizing that no infinite set can have a cardinality strictly less than that of the set of positive integers, denoted as א₀. This is achieved using two claims that illustrate any subset of positive integers must be either finite or countably infinite. The implications of these properties culminate in guarantees about the existence of countable infinite subsets within infinite sets, irrespective of their original class (countable or uncountable).
The section concludes by addressing the union of countable sets. It asserts that the union of a countable number of countable sets remains countable, providing necessary proofs to support this claim.
Reference YouTube Videos
Audio Book
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Create a free accountHello everyone welcome to tutorial number 5 so let us start with question number 1 we have to show in question number 1 that these two sets have the same cardinality. So the first set here is the set of all real numbers between (0,1) but excluding 0 as well as 1. Whereas the second collection here it is also the set of all real numbers between (0, 1] but 1 is inclusive that means 1 is allowed. That is why the square bracket here and 0 is not allowed.
Detailed Explanation
This chunk introduces the two sets we will analyze: Set A, which includes all real numbers between 0 and 1 (not including 0 and 1), and Set B, which includes all real numbers between 0 and 1 (including 1 but not 0). The objective is to show that they have the same cardinality, meaning they can be put in a one-to-one correspondence.
Examples & Analogies
Think of Set A as a group of people standing at the edge of a cliff, labeled from just above the edge to just below the cliff, but actually not touching the edge. Set B, on the other hand, would be like a group where one person is allowed to stand at the end of the cliff, while the others remain just above the edge.
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Create a free accountSo recall the Schroder-Bernstein theorem which says that, if you want to prove that two sets have the same cardinality show injective mappings from the first set to the second and from the second set to the first.
Detailed Explanation
The Schroder-Bernstein theorem is a critical concept in set theory. It states that if two sets can each be placed in one-to-one correspondence (through injective mappings) with each other, then the sets have the same cardinality. We will demonstrate this using injective functions from both Set A to Set B and Set B to Set A.
Examples & Analogies
Imagine you are trying to connect two groups of friends (Set A and Set B). If you can find a way to pair each friend in Group A to a unique friend in Group B and vice versa, you can confidently say that both groups have the same number of friends, even if you can’t see all the connections right away.
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Create a free accountLet us consider the injective mapping f which is the identity mapping so clearly this mapping is an injective mapping from this set to this set. Because you take any two different real numbers x and y the corresponding image will be x and y and they will be different and they will be in the range which is allowed as per function f.
Detailed Explanation
This chunk explains an injective mapping defined as the identity function. Here, if we take any two different real numbers from Set A, they map directly to themselves in Set B. Because neither end of the intervals (0 and 1) is included, these mappings remain valid within the specified sets.
Examples & Analogies
Think of this mapping like a classroom where each student (a real number in Set A) can sit on a specific desk (a real number in Set B) without sharing it with anyone else. Each student has their unique desk ensuring that no two students are at the same desk at any given time.
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Create a free accountNow if I want to take the injective mapping in the reverse direction then consider the injective mapping g defined to be x / 2 that means. If you want to find out the value of g(x) the output is (x / 2).
Detailed Explanation
In this step, we define a new mapping g from Set B back to Set A, where each value from Set B is halved. This mapping is also injective because all outputs from this mapping remain distinct and fall within the defined range of Set A. For example, the number 1 (from Set B) will map to 0.5 in Set A.
Examples & Analogies
Consider this mapping like adjusting the volume of music. When the music is too loud (a value in Set B), reducing the volume (halving the value) will still keep the sound in a range where we can fully appreciate it (the allowed values in Set A) without going silent.
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Create a free accountAnd it is again easy to verify that your mapping g is an injective mapping in the sense we have shown here two injective mappings so we can conclude that the cardinality of these two sets are the same.
Detailed Explanation
After showing that both directions have valid injective mappings, we conclude, based on the Schroder-Bernstein theorem, that the two sets A and B indeed have the same cardinality. This insight is fundamental in understanding how different infinite sets can relate to one another.
Examples & Analogies
This can be likened to two different sizes of buckets (Set A and Set B) where we have confirmed that we can fill one bucket from the other without any waste. Therefore, no matter how they appear at first glance, both buckets hold the same amount of water, representing their equal sizes.
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Key concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
- Cardinality:
Refers to the size of a set, which can be finite or infinite.
- Injective Mapping:
A fundamental concept that allows for establishing equivalences between sets.
- Schroder-Bernstein Theorem:
A theorem necessary for comparing the sizes of infinite sets.
- Countable Set:
Any set that can be placed in a one-to-one correspondence with natural numbers.
Examples
Memory aids
When it comes to set size, cardinality's the prize; two sets can share some, if mappings work, then the sums.
Imagine a garden of real numbers where every flower represents a unique number. Some flowers bloom brighter (like in (0, 1]) whereas others hold onto their buds (like (0, 1)). But both gardens, when viewed from afar, hold the same number of blooms, proving their cardinality.
Flash Cards
Glossary
Cardinality
A measure of the 'number of elements' in a set.
Injective Mapping
A function where every element of the range is mapped from a unique element of the domain.
Set of Positive Integers
The set of all natural numbers greater than zero, usually denoted as Z+.
Schroder-Bernstein Theorem
States that if there are injective functions from set A to set B and from B to A, then the two sets have the same cardinality.
Countable Set
A set with the same cardinality as a subset of the natural numbers.