lattice filter
Let be a lattice. A filter (of ) is the dual concept of an ideal (http://planetmath.org/LatticeIdeal). Specifically, a filter of is a non-empty subset of such that
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is a sublattice of , and
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for any and , .
The first condition can be replaced by a weaker one: for any , .
An equivalent characterization of a filter in a lattice is
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for any , , and
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for any , if , then .
Note that the dualization switches the meet and join operations, as well as reversing the ordering relationship.
Special Filters. Let be a filter of a lattice . Some of the common types of filters are defined below.
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is a proper filter if , and, if contains , .
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is a prime filter if it is proper, and implies that either or .
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is an ultrafilter (or maximal filter) of if is proper and the only filter properly contains is .
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filter generated by a set. Let be a subset of a lattice . Let be the set of all filters of containing . Since (), the intersection of all elements in , is also a filter of that contains . is called the filter generated by , written . If is a singleton , then is said to be a principal filter generated by , written .
Examples.
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Consider the positive integers, with meet and join defined by the greatest common divisor and the least common multiple operations. Then the positive even numbers form a filter, generated by . If we toss in as an additional element, then and consequently any positive integer , since . In general, if are relatively prime, then . In fact, any proper filter in is principal. When the generator is prime, the filter is prime, which is also maximal. So prime filters and ultrafilters coincide in .
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Let be a set and the power set of . If the set inclusion is the ordering defined on , then the definition of a filter here coincides with the ususal definition of a filter (http://planetmath.org/Filter) on a set in general.
Remark. If is both a filter and an ideal of a lattice , then .
Title | lattice filter |
Canonical name | LatticeFilter |
Date of creation | 2013-03-22 15:49:01 |
Last modified on | 2013-03-22 15:49:01 |
Owner | CWoo (3771) |
Last modified by | CWoo (3771) |
Numerical id | 9 |
Author | CWoo (3771) |
Entry type | Definition |
Classification | msc 06B10 |
Synonym | ultra filter |
Synonym | ultra-filter |
Synonym | maximal filter |
Related topic | Ultrafilter |
Related topic | UpperSet |
Related topic | LatticeIdeal |
Related topic | OrderIdeal |
Defines | filter |
Defines | prime filter |
Defines | ultrafilter |
Defines | filter generated by |
Defines | principal filter |