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9.5.2. Set S with Single 1
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Today, we are learning about cardinality and how we can prove that two sets have the same cardinality. What can you tell me about cardinality, Student_1?
I think it's about the 'size' of the sets, right?
Exactly! Cardinality measures the size of a set. Now, let's dive into how we can show that two sets—the one with real numbers between (0, 1) and the one with numbers in (0, 1]—have the same cardinality.
How do we do that?
We can use the Schroder-Bernstein theorem! If we can find injective functions mapping one set into the other and vice versa, then the two sets have the same cardinality. Can anyone recall what an injective function is?
It's a function where distinct inputs always map to distinct outputs?
Perfect! That’s the essence of it.
So, we need those functions to prove they have the same size?
Yes! Now let's explore how to define those injective functions.
In the end, you should remember that this theorem is a powerful tool in handling cardinalities.
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Now that we've discussed what cardinality is, let's review the injective functions we've come up with. Student_2, can you explain the function you were thinking of for the mapping from the first set to the second?
I think we can use the identity mapping since each number maps to itself, right?
Right! This mapping is indeed injective as it keeps distinct values distinct. Now, what would be the mapping back from the second set?
We could use the mapping where g(x) = x/2?
Spot on! This mapping g is also injective. So, by having both injective functions, what can we conclude?
That the two sets have the same cardinality!
Exactly! Now remember, through the theorem, we ensured both sets have the same size.
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Let's shift gears to discuss infinite sets. What is one key fact about infinite sets we must remember? Student_4?
That there's a distinction between countable and uncountable sets!
Exactly! Now our goal is to prove there's no infinite set with cardinality less than א₀. What are our two claims regarding the sets of positive integers?
One is that any set A whose cardinality is less than or equal to א₀ has a subset B within the positive integers with the same cardinality.
Correct! And the other claim?
Any subset of positive integers is either finite or countably infinite.
Awesome! Let’s apply these claims. Suppose we assume there's an infinite set A with cardinality less than א₀. What can we deduce from our claims?
By the first claim, there's a subset B of positive integers that's the same size as A, and they must be countably infinite?
Exactly! This creates a contradiction since A cannot have a size less than א₀ if it’s infinite.
Remember this critical insight: you cannot have a ‘size’ smaller than countable infinity!
Overview
Short Summary
This section explores the cardinality of two sets of real numbers and demonstrates the absence of a smaller infinite cardinality.
Medium Summary
The section elaborates on proving that two sets of real numbers between (0, 1) and (0, 1] have the same cardinality using injective mappings. Additionally, it argues the impossibility of having an infinite set with cardinality strictly less than the countable infinity, supported by claims about subsets of positive integers and their relationships.
Detailed Summary
Summary of Section 5.2: Set S with Single 1
In this section, we investigate the cardinality of the sets of real numbers across specific intervals. Specifically, we compare the set of all real numbers in the interval (0, 1) excluding the endpoints with the set of real numbers in the interval (0, 1] where 1 is included. We utilize the Schroder-Bernstein theorem, which states that if there exist injective mappings from both sets into each other, they have the same cardinality. First, we define an injective function from the first set to the second set using the identity function, demonstrating that it meets the criteria for injective mapping. We then establish a reverse mapping showing how to map from the second set back to the first, further confirming their cardinality equivalence.
The next critical claim presented is the impossibility of an infinite set having a cardinality strictly less than that of the set of positive integers, denoted as א₀. This assertion is supported through a proof by contradiction involving two key claims regarding subsets of positive integers. Through this exploration, we conclude that any subset of the positive integers is either finite or countably infinite. We reinforce that regardless of the nature of the infinite set A, a countable infinite subset can always be extracted from it, thus demonstrating uncountability in various scenarios. The section ends with various examples, enhancing our understanding of the provided concepts.
Reference YouTube Videos
Audio Book
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Create a free accountIn 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.
Detailed Explanation
This section introduces the concept of cardinality, which refers to the 'size' of a set in terms of the number of elements it contains. Here, we are comparing two sets: the first set excludes both endpoints (0 and 1), while the second set includes 1. To prove they have the same cardinality, we apply the Schroder-Bernstein theorem.
Examples & Analogies
Think of two different-sized boxes representing the two sets. The first box contains numbered balls from a range, and the second also contains similar balls but includes an extra numbered ball. Even if one appears larger, they can still be filled to the same level, showing they can hold the same amount of items if carefully arranged.
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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 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 provides a method for demonstrating that two sets have the same cardinality. It suggests that if you can find one-way mappings (called injective mappings) that show how elements from one set correspond to a unique element in another set, and vice versa, you can conclude the sets are of equal size.
Examples & Analogies
Imagine you are trying to match students to desks in a classroom. If every student can find a unique desk and every desk can fit only one student, your classroom is efficiently organized, showing the students and desks have a balanced number.
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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.
Detailed Explanation
Here, we define a specific injective mapping, referred to as the identity mapping. This means that each element in the first set directly corresponds to itself in the second set. Since we are assured that no two different elements map to the same element, we verify that this mapping is indeed injective.
Examples & Analogies
Think of a library where each book has a unique barcode. If every barcode corresponds to one and only one book, you can easily track which book is which, ensuring an efficient system.
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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.
Detailed Explanation
To further confirm our sets have the same cardinality, we look for a mapping in the opposite direction from the second set back to the first. The mapping g is defined by dividing each element by 2. If we take real numbers from the second set, they remain defined in the first set, affirming the injective nature of this mapping.
Examples & Analogies
Consider a recipe where for every cup of flour needed, you can use half a cup of flour and still make the same dish. It demonstrates how two proportions can represent the same amount, ensuring that each ingredient still matches perfectly, just like our two sets.
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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, crucial for comparison.
- Injective Functions:
Essential for establishing the equivalence of cardinalities.
- Schroder-Bernstein Theorem:
A tool for proving sets have the same cardinality.
- Countable Infinite:
Relates to sets that can be listed in a sequence.
Examples
Memory aids
Once in a land where sizes were tallied, a wise mathematician used cards to count—setting injective mappings to ensure no numbers were missed, proving two sets the same by following the twist!
Flash Cards
Glossary
Cardinality
The measure of the 'size' or number of elements in a set.
Injective Function
A function that maps distinct inputs to distinct outputs.
Schroder-Bernstein theorem
A theorem stating that if there are injective functions mapping two sets into each other, the sets have the same cardinality.
Countably Infinite
A set that can be put into a one-to-one correspondence with the positive integers.
Positive Integers
The set of all integers greater than zero (1, 2, 3, ...).