atom
Let be a poset, partially ordered by . An element is called an atom if it covers some minimal element of . As a result, an atom is never minimal. A poset is called atomic if for every element that is not minimal has an atom such that .
Examples.
- 1.
Let be a set and its power set
. is a poset ordered by with a unique minimal element . Thus, all singleton subsets of are atoms in .
- 2.
is partially ordered if we define to mean that . Then is a minimal element and any prime number is an atom.
Remark. Given a lattice with underlying poset , an element is called an atom (of ) if it is an atom in . A lattice is a called an atomic lattice if its underlying poset is atomic. An atomistic lattice is an atomic lattice such that each element that is not minimal is a join of atoms. If is an atom in a semimodular lattice , and if is not under , then is an atom in any interval lattice where .
Examples.
- 1.
, with the usual intersection
and union as the lattice operations
meet and join, is atomistic: every subset of is the union of all the singleton subsets of .
- 2.
, partially ordered as above, with lattice binary operations
defined by , and , is a lattice that is atomic, as we have seen earlier. But it is not atomistic: is not a join of ’s; is not a join of and are just two counterexamples.