join irreducibility

An element $a$ in a lattice $L$ is said to be join irreducible iff $a$ is not a bottom element, and, whenever $a=b\vee c$, then $a=b$ or $a=c$. Dually, $a\in L$ is meet irreducible iff $a$ is not a top element, and, whenever $a=b\wedge c$, then $a=b$ or $a=c$. If $a$ is both join and meet irreducible, then $a$ is said to be irreducible. Any atom in a lattice is join irreducible.

Example. In the lattice diagram (Hasse diagram) below,

$a$ is meet irreducible but not join irreducible, $d$ is join irreducible but not meet irreducible, while $b,c$ are irreducible.

From this, we make the observations that in any chain, all the elements except the bottom one are join irreducible. Dually, all the elements except the top one are meet irreducible. An element is join irreducible iff it covers (http://planetmath.org/CoveringRelation) at most one other element. An element is meet irreducible iff it is covered by at most one other element.

Remark. If a lattice satisfies the descending chain condition, then every element can be expressed as a join of join irreducible elements. This statement can be dualized: if a lattice satisfies the ascending chain condition, then every element is the meet of meet irreducible elements.

Title	join irreducibility
Canonical name	JoinIrreducibility
Date of creation	2013-03-22 15:47:29
Last modified on	2013-03-22 15:47:29
Owner	CWoo (3771)
Last modified by	CWoo (3771)
Numerical id	10
Author	CWoo (3771)
Entry type	Definition
Classification	msc 06B99
Synonym	join-irreducible
Synonym	meet-irreducible
Defines	join irreducible
Defines	meet irreducible
Defines	irreducible