lattice interval
Definition. Let be a lattice. A subset of is called a lattice interval, or simply an if there exist elements such that
The elements are called the endpoints of . Clearly . Also, the endpoints of a lattice interval are unique: if , then and .
Remarks.
- •
It is easy to see that the name is derived from that of an interval on a number line. From this analogy
, one can easily define lattice intervals without one or both endpoints. Whereas an interval on a number line is linearly ordered
, a lattice interval in general is not. Nevertheless, a lattice interval of a lattice is a sublattice of .
- •
A bounded lattice
is itself a lattice interval: .
- •
A prime interval is a lattice interval that contains its endpoints and nothing else. In other words, if is prime, then any implies that either or . Simply put, covers . If a lattice contains , then for any , is a prime interval iff is an atom.
- •
Since no operations
of meet and join are used, all of the above discussion can be generalized to define an interval in a poset.
- •
Given a lattice , let be the collection
of all lattice intervals without endpoints, we can form a topolgy on with as the subbasis. This does not insure that and are continuous
, so that with this topological structure may not be a topological lattice.
- •
Locally Finite Lattice. A lattice that is derived based on the concept of lattice interval is that of a locally finite lattice. A lattice is locally finite
iff every one of its interval is finite. Unless the lattice is finite, a locally finite lattice, if infinite
, is either topless or bottomless.