Almost
Term in set theory
In set theory, when dealing with sets of infinite size, the term almost or nearly is used to refer to all but a negligible amount of elements in the set. The notion of "negligible" depends on the context, and may mean "of measure zero" (in a measure space), "finite" (when infinite sets are involved), or "countable" (when uncountably infinite sets are involved).
For example:
- The set is almost for any in , because only finitely many natural numbers are less than .
- The set of prime numbers is not almost , because there are infinitely many natural numbers that are not prime numbers.
- The set of transcendental numbers are almost , because the algebraic real numbers form a countable subset of the set of real numbers (which is uncountable).[1]
- The Cantor set is uncountably infinite, but has Lebesgue measure zero.[2] So almost all real numbers in (0, 1) are members of the complement of the Cantor set.
See also
Look up almost in Wiktionary, the free dictionary.
- Almost periodic function - and Operators
- Almost all
- Almost surely
- Approximation
- List of mathematical jargon
References
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Set theory
- Set (mathematics)
- Cartesian product
- Complement (i.e. set difference)
- De Morgan's laws
- Disjoint union
- Identities
- Intersection
- Power set
- Symmetric difference
- Union
- Concepts
- Methods
- Amorphous
- Countable
- Empty
- Finite (hereditarily)
- Filter
- base
- subbase
- Ultrafilter
- Fuzzy
- Infinite (Dedekind-infinite)
- Recursive
- Singleton
- Subset · Superset
- Transitive
- Uncountable
- Universal
- Paradoxes
- Problems
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