A rational number is a number that can be expressed as a fraction where p and q are integers and . A rational number is said to have numerator p and denominator q. Numbers that are not rational are called irrational numbers. The real line consists of the union of the rational and irrational numbers. The set of rational numbers is of measure zero on the real line, so it is "small" compared to the irrationals and the continuum.

The set of all rational numbers is referred to as the "rationals," and forms a field that is denoted Q. Any rational number is trivially also an algebraic number.

Example of rational numbers include -7, 0, 1, 1/2, 22/7, 12345/67, and so on. Farey sequences provide a way of systematically enumerating all rational numbers.

The set of rational numbers is denoted Rationals in Mathematica, and a number x can be tested to see if it is rational using the command Element[x, Rationals].

The elementary algebraic operations for combining rational numbers are exactly the same as for combining fractions.

It is always possible to find another rational number between any two members of the set of rationals. Therefore, rather counterintuitively, the rational numbers are a continuous set, but at the same time countable.

For a, b, and c any different rational numbers, then

is the square of the rational number

(Honsberger 1991).

The probability that a random rational number has an even denominator is 1/3 (Salamin and Gosper 1972).

It is conjectured that if there exists a real number x for which both and are integers, then x is rational. This result would follow from the four exponentials conjecture (Finch 2003).

Algebraic Integer, Algebraic Number, Anomalous Cancellation, Continuum, Denominator, Dirichlet Function, Farey Sequence, Four Exponentials Conjecture, Fraction, Integer, Irrational Number, Numerator, Q, Quotient, Ratio, Rational Polynomial, Rational Spiral, Transcendental Number