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lattice
A lattice is any poset in which any two elements and have a least upper bound, , and a greatest lower bound, . The operation is called meet, and the operation is called join. In some literature, is required to be non-empty.
A sublattice of is a subposet of which is a lattice, that is, which is closed under the operations and as defined in .
The operations of meet and join are idempotent, commutative, associative, and absorptive:
Thus a lattice is a commutative band with either operation. The partial order relation can be recovered from meet and join by defining
Once is defined, it is not hard to see that iff as well (one direction goes like: , while the other direction is the dual of the first).
Conspicuously absent from the above list of properties is distributivity. While many nice lattices, such as face lattices of polytopes, are distributive, there are also important classes of lattices, such as partition lattices, that are usually not distributive.
Lattices, like posets, can be visualized by diagrams called Hasse diagrams. Below are two diagrams, both posets. The one on the left is a lattice, while the one on the right is not:
The vertices of a lattice diagram can also be labelled, so the lattice diagram looks like
Remark. Alternatively, a lattice can be defined as an algebraic system. Please see the link below for details.
Mathematics Subject Classification
03G10 Lattices and related structures06B99 None of the above, but in MSC2010 section 06Bxx
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