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In set theory, an ordinal number, or ordinal, is a generalization of ordinal numerals (first, second, n th, etc.) aimed to extend enumeration to infinite sets. [ 1 ] A finite set can be enumerated by successively labeling each element with the least natural number that has not been previously used.
Ordinal indicator – Character(s) following an ordinal number (used when writing ordinal numbers, such as a super-script) Ordinal number – Generalization of "n-th" to infinite cases (the related, but more formal and abstract, usage in mathematics) Ordinal data, in statistics; Ordinal date – Date written as number of days since first day of ...
Such a number is algebraic and can be expressed as the sum of a rational number and the square root of a rational number. Constructible number: A number representing a length that can be constructed using a compass and straightedge. Constructible numbers form a subfield of the field of algebraic numbers, and include the quadratic surds.
20: twenty: 3: three: 13: ... Ordinal numbers predate the invention of zero and positional notation. ... Empty numbers are sometimes made up, with obvious meaning ...
Using the definition of ordinal numbers suggested by John von Neumann, ... This is a complete list of all finite transitive sets with up to 20 brackets: [1] {}, ...
In Latin and Greek, the ordinal forms are also used for fractions for amounts higher than 2; only the fraction 1 / 2 has special forms. The same suffix may be used with more than one category of number, as for example the orginary numbers second ary and terti ary and the distributive numbers bi nary and ter nary .
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The ordinal ε 0 is important for various reasons in arithmetic (essentially because it measures the proof-theoretic strength of the first-order Peano arithmetic: that is, Peano's axioms can show transfinite induction up to any ordinal less than ε 0 but not up to ε 0 itself).