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单词 Atom
释义

atom


Let P be a poset, partially ordered by . An element aP is called an atom if it covers some minimal element of P. As a result, an atom is never minimalPlanetmathPlanetmath. A poset P is called atomic if for every element pP that is not minimal has an atom a such that ap.

Examples.

  1. 1.

    Let A be a set and P=2A its power setMathworldPlanetmath. P is a poset ordered by with a unique minimal element . Thus, all singleton subsets {a} of A are atoms in P.

  2. 2.

    + is partially ordered if we define ab to mean that ab. Then 1 is a minimal element and any prime number p is an atom.

Remark. Given a latticeMathworldPlanetmath L with underlying poset P, an element aL is called an atom (of L) if it is an atom in P. 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 a is an atom in a semimodular lattice L, and if a is not under x, then ax is an atom in any interval lattice I where x=I.

Examples.

  1. 1.

    P=2A, with the usual intersectionMathworldPlanetmath and union as the lattice operationsMathworldPlanetmath meet and join, is atomistic: every subset B of A is the union of all the singleton subsets of B.

  2. 2.

    +, partially ordered as above, with lattice binary operationsMathworldPlanetmath defined by ab=gcd(a,b), and ab=lcm(a,b), is a lattice that is atomic, as we have seen earlier. But it is not atomistic: 4 is not a join of 2’s; 36 is not a join of 2 and 3 are just two counterexamples.

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更新时间:2025/5/5 2:11:57