4. Module No # 05

Hello everyone welcome to this lecture on examples of countably infinite sets.

So just to recap in the last lecture we introduced the notion of countable and uncountable sets. Countable sets are those sets whose cardinality is either finite or whose cardinality is same as the set of positive integers. So the plan for this lecture is as follows. We will see several examples of countably infinite sets and we will also discuss some properties of countable sets specifically in the context of infinite sets.

In this introductory chunk, the lecturer welcomes everyone and summarizes the previous lecture's discussion on countable and uncountable sets. Countable sets can be finite (having a specific number of elements) or infinite (having the same size as the set of positive integers). This foundation sets the stage for exploring examples of countably infinite sets, which are the focus of the current lecture.

Think of countably infinite sets like a library. A library with a finite number of books can be thought of as a finite set. However, if the library were to have an infinite number of books but you could organize them in a way that allows you to count each book one by one, it would represent a countably infinite set. You can count those books in the same way you can count the number of natural numbers.

So we first prove that the Cartesian product of the set of integers is a countable set. So again this might look non-intuitive, you have many elements in the Cartesian product of the set of integers compared to the set of integers itself because when I say that Cartesian product it is going to consist of all ordered pairs of the form (i, j) where i can be any integer, j can be any integer.

In this chunk, the lecturer introduces the idea of the Cartesian product of the set of integers, which consists of all possible ordered pairs (i, j) where both i and j are integers. Even though it seems counterintuitive that combining two infinite sets (integers x integers) can still be countable, the forthcoming proof demonstrates that the count of these ordered pairs matches that of the set of positive integers.

Imagine a coordinate plane where every point (i, j) represents a combination of two integers. Though there are infinitely many points, like the stars in a night sky, you can systematically list them as you explore in all four quadrants of the plane, similar to how you'd explore each box of a grid.

The idea is very clever here what we do is, So since we are considering the Set ℤ x ℤ it is nothing but the collection of all points in your 2 dimensional plane. So imagine that you have that infinite 2 dimensional plane where you have all the points belonging to the ℤ x ℤ. And our goal is basically to give an enumeration of all the points in that infinite plane such that the enumeration should be well defined and we do not miss any point in the enumeration process.

Here, the lecturer describes a clever method to enumerate all points in the infinite 2D plane of ℤ x ℤ. By starting at the origin (0, 0) and moving in a spiraling manner, the enumerator can effectively label each point without missing any, confirming that the Cartesian product is still countable. This systematic approach of visiting all points is the central idea behind demonstrating the countability of the set.

Visualize a spiral staircase. If you start at the center and walk outward, stopping at each step, you can describe every point around you. Just like how you systematically explore every step on the staircase, this enumeration method allows you to list every possible point in the integer plane.

Now, we will see next whether the set of rational numbers which I denote by this ℚ notation is countable or not. Now intuitively it might look the answer is no because definitely rational numbers is a super set of the set of the integers. And looks like there is no way of sequencing because the fundamental fact about rational numbers is that you take any 2 rational numbers there are infinitely many more rational numbers between the same 2 rational numbers.

In this section, the lecturer poses the question of whether the set of rational numbers (notated as ℚ) is countable. Initially, it seems logical to believe the answer is no, as rational numbers include every integer and infinitely many others. The lecture sets the stage for exploring methods to enumerate these rational numbers, implying a more nuanced understanding of infinity.

Imagine making a cake where you can keep slicing it into smaller and smaller pieces. Even between two slices of cake, you can always find more tiny slices (like rational numbers). Just as you wouldn't reach a limit on how many slices you could create, rational numbers between any two values are infinite. But it turns out, with the right approach, you can still list them in an organized way.

And the sequencing that we are going to see here will be based on the sequencing of the elements of the point in the 2 dimensional integer plane based on enumerating all the points along the spiral that we had been seen in the last slide. So just to recall, this was the enumeration of the set of all elements or points in the set ℤ x ℤ. And based on this enumeration we will get an enumeration of the set of all rational numbers.

This part explains how to enumerate rational numbers by applying the enumeration process already established for integer pairs (ℤ x ℤ). The key is to use the same spiral-like traversal of integer points to eventually reveal rational numbers, as rational numbers can be expressed as the ratio of integers (p/q). This clever method shows that the set of rationals can also be counted systematically.

Think of fishing in a lake using a net. Each time you pull the net in, you have a chance of catching different fish (representing rational numbers). If you systematically comb through the entire lake (like enumerating the integer pairs), you will eventually catch all the different types of fish (rational numbers) present in that lake.

Now let us consider the set of binary strings of finite length. What exactly that means? So, imagine a set Π consisting of 2 elements namely the element 0 and 1 and why 0 and 1? Because I am considering binary strings so, binary strings will be just a string of 0’s and 1’s. And I used this notation Π* to denote the set of all binary strings of finite length.

In this segment, the lecturer introduces binary strings, defined as combinations of the digits 0 and 1. The set Π* is the collection of all finite-length strings formed from these two digits. The lecturer notes that even though this set may seem large, we can still show that it is countable by establishing a method to enumerate these binary strings based on their lengths.

Consider a simple light switch that can either be ON (1) or OFF (0). Every time you flip the switch, you create a new 'state' or configuration (a binary string). If you can only flip the switch a limited number of times, all possible arrangements of those ON/OFF states form a countable set of binary strings.

The idea behind listing down is to arrange or list down all the elements of Π* according to their length. So we start with the length 0 strings and length 0 string will be the empty string denoted by the special notation ε. Then we will go and enumerate or list down all valid string, binary strings of length 1.

The plan for enumerating binary strings involves organizing them by their lengths. Starting from length 0 (with the empty string), the lecturer explains that we can systematically list binary strings for each length, moving to length 1, then length 2, and so on. This process ensures that every binary string will eventually be captured in the enumeration.

Imagine you're organizing a set of building blocks by size. You first lay out all the blocks with no length, then the smallest ones, followed by slightly larger ones. This ensures you see every possible block of every size all the way up to the largest one, just like listing binary strings in order by length.

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