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The set of all vertices of a given polyhedron, the set of all prime numbers less than a given number, and the set of all residents of NYC (at a given time) have a certain property in common, namely, each set has a definite number of elements which can be found in principle, if not in practice. Accordingly, these sets are all said to be .

On the other hand, the set of all positive integers, the set of all points on the line, the set of all circles in the plane, and the set of all polynomials with rational coefficients have a different property in common, namely, . Accordingly, sets of these kind are called sets.

Given two finite sets, we can always decide whether or not they have the same number of elements, and if not, we can always determine which set has more elements than the other. It is natural to ask whether the same is true of infinite sets. In other words, does it make sense to ask, for example, whether there are more circles in the plane than rational points on the line, or more functions defined in the interval [0,1] than lines in space? As will soon be apparent, questions of this kind can indeed be answered.

To compare two finite sets A and B, we can count the number of elements in each set and then compare the two numbers, but alternatively, we can try to establish a between the elements of set A and set B, that is, a correspondence such that each element in A corresponds to one and only element in B, and vice-versa. It is clear that a one-to-one correspondence between two finite sets can be set up if and only if the two sets have the same number of elements. For example, to ascertain if or not the number of students in an assembly is the same as the number of seats in the auditorium, there is no need to count the number of students and the number of seats. We need merely observe whether or not there are empty seats or students with no place to sit down. If the students can all be seated with no empty seats left, that is, if there is a one-to-one correspondence between the set of students and the set of seats, then these two sets obviously have the same number of elements. The important point here is that the first method(counting elements) works only for finite sets, while the second method(setting up a one-to-one correspondence) works for infinite sets as well as for finite sets.

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The simplest infinite set is the set of all positive integers. An infinite set is called if its elements can be put into one-to-one correspondence with those of . In other words, a countable set is a set whose elements can be numbered . By an set we mean, of course, an infinite set which is not countable.

We now give some examples of countable sets:

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The set of all integers, positive, negative, or zero is countable. In fact, we can set up the following one-to-one correspondence between and of all positive integers: (0,1), (-1,2), (1,3), (-2,4), (2,5), and so on. More explicitly, we associate the non-negative integer $n \geq 0$ with the odd number , and the negative integer with the even number , that is,

, if , and
, if , and

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The set of all positive even numbers is countable, as shown by the obvious correspondence .

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The set 2,4,8, is countable as shown by the obvious correspondence .

latex \mathscr{Q}$ of rational numbers is countable. To see this, we first note that every rational number can be written as a fraction , with with a positive denominator. (Of course, p and q are integers). Call the sum as the “height” of the rational number . For example,

is the only rational number of height zero,

, are the only rational numbers of height 2,

, , , are the only rational numbers of height 3, and so on.

We can now arrange all rational numbers in order of increasing “height” (with the numerators increasing in each set of rational numbers of the same height). In other words, we first count the rational numbers of height 1, then those of height 2 (suitably arranged), then those of height 3(suitably arranged), and so on. In this way, we assign every rational number a unique positive integer, that is, we set up a one-to-one correspondence between the set Q of all rational numbers and the set of all positive integers.

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Let set A be countable, with elements , and let set B be a subset of A. Among the elements , let be those in the set B. If the set of numbers has a largest number, then B is finite. Otherwise, B is countable (consider the one-to-one correspondence ).

We can assume that no two of the sets have any elements in common, since otherwise we could consider the sets , , , , instead, which are countable by Theorem 1, and have the same union as the original sets. Suppose we write the elements of in the form of an infinite table

where the elements of the set appear in the first row, the elements of the set appear in the second row, and so on. We now count all the elements in the above array “diagonally”; that is, first we choose , then , then move downwards, diagonally to “left”, picking , then move down vertically picking up , then move across towards right picking up , next pick up and so on ()as per the pattern shown:

It is clear that this procedure associates a unique number to each element in each of the sets thereby establishing a one-to-one correspondence between the union of the sets and the set of all positive integers.

Let M be an infinite set and any element of M. Being infinite, M contains an element distinct from , an element distinct from both and , and so on. Continuing this process, (which can never terminate due to “shortage” of elements, since M is infinite), we get a countable subset of the set .

Theorem 3 shows that countable sets are the “smallest” infinite sets. The question of whether there exist uncountable (infinite) sets will be considered below.

We arrived at the notion of a countable set M by considering one-to-one correspondences between set M and the set of all positive integers. More generally, we can consider one-to-one correspondences between any two sets M and N.

Two sets M and N are said to be (written ) if there is a one-to-one correspondence between the elements of M and the elements of N.

The concept of equivalence is applicable both to finite and infinite sets. Two finite sets are equivalent if and only if they have the same number of elements. We can now define a countable set as a set equivalent to the set of all positive integers. It is clear that two sets are equivalent to a third set are equivalent to each other, and in particular that any two countable sets are equivalent.

The sets of points in any two closed intervals $[a,b]$ and $[c,d]$ are equivalent; you can “see’ a one-to-one correspondence by drawing the following diagram: Step 1: draw cd as a base of a triangle. Let the third vertex of the triangle be O. Draw a line segment “ab” above the base of the triangle; where “a” lies on one side of the triangle and “b” lies on the third side of the third triangle. Note that two points p and q correspond to each other if and only if they lie on the same ray emanating from the point O in which the extensions of the line segments ac and bd intersect.

The set of all points z in the complex plane is equivalent to the set of all points z on a sphere. In fact, a one-to-one correspondence can be established by using stereographic projection. The origin is the North Pole of the sphere.

The set of all points x in the open unit interval is equivalent to the set of all points y on the whole real line. For example, the formula establishes a one-to-one correspondence between these two sets. .

The last example and the examples in Section 2 show that an infinite set is sometimes equivalent to one of its proper subsets. For example, there are “as many” positive integers as integers of arbitrary sign, there are “as many” points in the interval as on the whole real line, and so on. This fact is characteristic of all infinite sets (and can be used to define such sets) as shown by:

According to Theorem 3, every infinite set M contains a countable subset. Let this subset be and partition A into two countable subsets and .

Obviously, we can establish a one-to-one correspondence between the countable subsets A and (merely let ). This correspondence can be extended to a one-to-one correspondence between the sets and by simply assigning x itself to each element . But is a proper subset of M. .

More later, to be continued,

Regards,
Nalin Pithwa