Browse Definitions :
Definition

rational number

A rational number is a number determined by the ratio of some integer p to some nonzero natural number q. The set of rational numbers is denoted Q, and represents the set of all possible integer-to-natural-number ratios p/q.In mathematical expressions, unknown or unspecified rational numbers are represented by lowercase, italicized letters from the late middle or end of the alphabet, especially r, s, and t, and occasionally u through z. Rational numbers are primarily of interest to theoreticians.Theoretical mathematics has potentially far-reaching applications in communications and computer science, especially in data encryption and security.

If r and t are rational numbers such that r < t, then there exists a rational number s such that r < s < t. This is true no matter how small the difference between r and t, as long as the two are not equal.In this sense, the set Q is "dense."Nevertheless, Q is a denumerable set.Denumerability refers to the fact that, even though a set might contain an infinite number of elements, and even though those elements might be "densely packed," the elements can be defined by a list that assigns them each a unique number in a sequence corresponding to the set of natural numbers N = {1, 2, 3, ...}..

For the set of natural numbers N and the set of integers Z, neither of which are "dense," denumeration lists are straightforward.For Q, it is less obvious how such a list might be constructed.An example appears below.The matrix includes all possible numbers of the form p/q, where p is an integer and q is a nonzero natural number.Every possible rational number is represented in the array.Following the pink line, think of 0 as the "first stop," 1/1 as the "second stop," -1/1 as the "third stop," 1/2 as the "fourth stop," and so on.This defines a sequential (although redundant) list of the rational numbers.There is a one-to-one correspondence between the elements of the array and the set of natural numbers N.

(2545 bytes)

To demonstrate a true one-to-one correspondence between Q and N, a modification must be added to the algorithm shown in the illustration.Some of the elements in the matrix are repetitions of previous numerical values.For example, 2/4 = 3/6 = 4/8 = 5/10, and so on.These redundancies can be eliminated by imposing the constraint, "If a number represents a value previously encountered, skip over it."In this manner, it can be rigorously proven that the set Q has exactly the same number of elements as the set N.Some people find this hard to believe, but the logic is sound.

In contrast to the natural numbers, integers, and rational numbers, the sets of irrational numbers, real numbers, imaginary numbers, and complex numbers are non-denumerable. They have cardinality greater than that of the set N.This leads to the conclusion that some "infinities" are larger than others!

This was last updated in September 2005

Start the conversation

Send me notifications when other members comment.

Please create a username to comment.

-ADS BY GOOGLE

File Extensions and File Formats

SearchCompliance

  • risk management

    Risk management is the process of identifying, assessing and controlling threats to an organization's capital and earnings.

  • compliance as a service (CaaS)

    Compliance as a Service (CaaS) is a cloud service service level agreement (SLA) that specified how a managed service provider (...

  • data protection impact assessment (DPIA)

    A data protection impact assessment (DPIA) is a process designed to help organizations determine how data processing systems, ...

SearchSecurity

  • cybersecurity insurance (cybersecurity liability insurance)

    Cybersecurity insurance, also called cyber liability insurance or cyber insurance, is a contract that an entity can purchase to ...

  • phishing

    Phishing is a form of fraud in which an attacker masquerades as a reputable entity or person in email or other communication ...

  • cybercrime

    Cybercrime is any criminal activity that involves a computer, networked device or a network.

SearchHealthIT

SearchDisasterRecovery

  • business continuity plan (BCP)

    A business continuity plan (BCP) is a document that consists of the critical information an organization needs to continue ...

  • disaster recovery team

    A disaster recovery team is a group of individuals focused on planning, implementing, maintaining, auditing and testing an ...

  • cloud insurance

    Cloud insurance is any type of financial or data protection obtained by a cloud service provider. 

SearchStorage

  • NVMe over Fabrics (NVMe-oF)

    NVMe over Fabrics, also known as NVMe-oF and non-volatile memory express over fabrics, is a protocol specification designed to ...

  • logical unit number (LUN)

    A logical unit number (LUN) is a unique identifier for designating an individual or collection of physical or virtual storage ...

  • CIFS (Common Internet File System)

    CIFS (Common Internet File System) is a protocol that gained popularity around the year 2000, as vendors worked to establish an ...

Close