Difference between revisions of "Rational number"
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Actually, the last property characterizes rationals among all [[real number]]s. | Actually, the last property characterizes rationals among all [[real number]]s. | ||
==Properties== | ==Properties== | ||
− | # Rational numbers form a | + | # Rational numbers form a [[field]]. In plain English it means that you can add, subtract, multiply, and divide them (with the obvious exception of division by <math>0</math>) and the result of each such operation is again a rational number. |
# Rational numbers are [[dense]] in the set of reals. This means that every non-empty open interval on the real line contains at least one (actually, infinitely many) rationals. Alternatively, it means that every real number can be represented as a [[limit]] of a [[sequence]] of rational numbers. | # Rational numbers are [[dense]] in the set of reals. This means that every non-empty open interval on the real line contains at least one (actually, infinitely many) rationals. Alternatively, it means that every real number can be represented as a [[limit]] of a [[sequence]] of rational numbers. | ||
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==See also== | ==See also== | ||
[[rational approximation]] | [[rational approximation]] |
Revision as of 11:44, 24 June 2006
Contents
Definition
A number is called rational if it can be represented as a ratio of two integers.
Examples
- All integers are rational because every integer can be represented as (or , or...)
- All numbers whose decimal expansion or expansion in some other number base is finite are rational (say, )
- All numbers whose decimal expansion is periodic are rationals.
Actually, the last property characterizes rationals among all real numbers.
Properties
- Rational numbers form a field. In plain English it means that you can add, subtract, multiply, and divide them (with the obvious exception of division by ) and the result of each such operation is again a rational number.
- Rational numbers are dense in the set of reals. This means that every non-empty open interval on the real line contains at least one (actually, infinitely many) rationals. Alternatively, it means that every real number can be represented as a limit of a sequence of rational numbers.