Cardinality

Introduction

The concept of cardinality, in the context of set theory, is a measure of the "number of elements" in a set. This concept is fundamental in mathematics and has wide-ranging applications in various fields such as computer science, statistics, and logic.

An image showing multiple distinct sets with varying numbers of elements.

An image showing multiple distinct sets with varying numbers of elements.

Definition

The cardinality of a set is a measure of the "number of elements" in the set. It is denoted by the symbol |A| or #A for a set A. For example, if A = {1, 2, 3}, then |A| = 3. This is because there are three elements in the set A.

Finite and Infinite Sets

Sets can be classified as either finite or infinite based on their cardinality. A set is said to be finite if its cardinality is a non-negative integer, i.e., it has a finite number of elements. On the other hand, a set is said to be infinite if it does not have a finite cardinality. The concept of infinity plays a crucial role in the study of infinite sets.

Cardinality of Finite Sets

The cardinality of a finite set is simply the number of elements in the set. For example, the set A = {1, 2, 3, 4, 5} has a cardinality of 5 because there are five elements in the set.

Cardinality of Infinite Sets

The cardinality of infinite sets is a more complex concept. Not all infinite sets have the same cardinality. For example, the set of all integers has a different cardinality than the set of all real numbers. This is because there are more real numbers than integers, even though both sets are infinite.

The cardinality of the set of all integers is denoted by ℵ0 (pronounced "aleph null" or "aleph zero"). This is the smallest infinite cardinal number. The cardinality of the set of all real numbers is denoted by c (for "continuum"). It can be shown that c > ℵ0, i.e., there are more real numbers than integers.

Comparing Cardinalities

The cardinality of one set can be compared to the cardinality of another set. If there exists a one-to-one correspondence (also known as a bijection) between the elements of two sets, then the two sets are said to have the same cardinality. This concept is used to compare the cardinalities of both finite and infinite sets.

Cantor's Theorem

Cantor's theorem is a fundamental result in set theory that deals with the cardinality of sets. It states that for any set A, the cardinality of the power set of A (the set of all subsets of A) is strictly greater than the cardinality of A. This theorem has profound implications for the study of infinite sets and the concept of cardinality.

See Also

• Set Theory
• Infinity
• Bijection
• Cantor's Theorem