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=bc, then a=b or a=c. Dually, aL is meet irreducible iff a is not a top element, and, whenever a=bc, then a=b or a=c. If a is both join and meet irreduciblePlanetmathPlanetmath, 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 ( 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 conditionMathworldPlanetmathPlanetmathPlanetmathPlanetmath, 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 conditionMathworldPlanetmathPlanetmath, then every element is the meet of meet irreducible elements.


  • 1 B. A. Davey, H. A. Priestley, Introduction to Lattices and Order, 2nd Edition, Cambridge (2003)
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