Join and meet

In mathematics, join and meet are dual binary operations on the elements of a partially ordered set. A join on a set is defined as the (necessarily unique) supremum (least upper bound) with respect to a partial order on the set, provided a supremum exists. A meet on a set is defined as the unique infimum (greatest lower bound) with respect to a partial order on the set, provided an infimum exists. If the join of two elements with respect to a given partial order exists then it is always the meet of the two elements in the inverse order, and vice versa.

Usually, the join of two elements x and y is denoted

x ∨ y,

and the meet of x and y is denoted

x ∧ y.

Join and meet can be abstractly defined as commutative and associative binary operations satisfying an idempotency law. The two definitions yield equivalent results, except that in the partial order approach it may be possible directly to define joins and meets of more general sets of elements.

A partially ordered set where the join of any two elements always exists is a join-semilattice. A partially ordered set where the meet of any two elements always exists is a meet-semilattice. A partially ordered set where both the join and the meet of any two elements always exist is a lattice. Lattices provide the most common context in which to find join and meet. In the study of complete lattices, the join and meet operations are extended to return the least upper bound and greatest lower bound of an arbitrary set of elements.

In the following we dispense discussing joins, because they become meet when considering the reverse partial order, thanks to duality.

Partial order approach

Let A be a set with a partial order ≤, and let x and y be two elements in A. An element z of A is the meet (or greatest lower bound or infimum) of x and y, if the following two conditions are satisfied:

1. z ≤ x and z ≤ y (i.e., z is a lower bound of x and y).
2. For any w in A, such that w ≤ x and w ≤ y, we have w ≤ z (i.e., z is greater than or equal to any other lower bound of x and y).

If there is a meet of x and y, then indeed it is unique, since if both z and z′ are greatest lower bounds of x and y, then z ≤ z′ and z′ ≤ z, whence indeed z = z′. If the meet does exist, it is denoted x ∧ y. Some pairs of elements in A may lack a meet, either since they have no lower bound at all, or since none of their lower bounds is greater than all the others. If all pairs of elements have meets, then indeed the meet is a binary operation on A, and it is easy to see that this operation fulfils the following three conditions: For any elements x, y, and z in A,

a. x ∧ y = y ∧ x (commutativity),

b. x ∧ (y ∧ z) = (x ∧ y) ∧ z (associativity), and

c. x ∧ x = x (idempotency).

Universal algebra approach

By definition, a binary operation ∧ on a set A is a meet, if it satisfies the three conditions a, b, and c. The pair (A,∧) then is a meet-semilattice. Moreover, we then may define a binary relation ≤ on A, by stating that x ≤ y if and only if x ∧ y = x. In fact, this relation is a partial order on A. Indeed, for any elements x, y, and z in A,

• x ≤ x, since x ∧ x = x by c;
• if x ≤ y and y ≤ x, then x = x ∧ y = y ∧ x = y by a; and
• if x ≤ y and y ≤ z, then x ≤ z, since then x ∧ z = (x ∧ y) ∧ z = x ∧ (y ∧ z) = x ∧ y = x by b.

Note that both meets and joins equally satisfy this definition: a couple of associated meet and join operations yield partial orders which are the reverse of each other. When choosing one of these orders as the main ones, one also fixes which operation is considered a meet (the one giving the same order) and which is considered a join (the other one).

Equivalence of approaches

If (A,≤) is a partially ordered set, such that each pair of elements in A has a meet, then indeed x ∧ y = x if and only if x ≤ y, since in the latter case indeed x is a lower bound of x and y, and since clearly x is the greatest lower bound if and only if it is a lower bound. Thus, the partial order defined by the meet in the universal algebra approach coincides with the original partial order.

Conversely, if (A,∧) is a meet-semilattice, and the partial order ≤ is defined as in the universal algebra approach, and z = x ∧ y for some elements x and y in A, then z is the greatest lower bound of x and y with respect to ≤, since

z ∧ x = x ∧ z = x ∧ (x ∧ y) = (x ∧ x) ∧ y = x ∧ y = z

and therefore z ≤ x. Similarly, z ≤ y, and if w is another lower bound of x and y, then w ∧ x = w ∧ y = w, whence

w ∧ z = w ∧ (x ∧ y) = (w ∧ x) ∧ y = w ∧ y = w.

Thus, there is a meet defined by the partial order defined by the original meet, and the two meets coincide.

In other words, the two approaches yield essentially equivalent concepts, a set equipped with both a binary relation and a binary operation, such that each one of these structures determines the other, and fulfil the conditions for partial orders or meets, respectively.

Meets of general subsets

If (A,∧) is a meet-semilattice, then the meet may be extended to a well-defined meet of any non-empty finite set, by the technique described in iterated binary operations. Alternatively, if the meet defines or is defined by a partial order, some subsets of A indeed have infima with respect to this, and it is reasonable to consider such an infimum as the meet of the subset. For non-empty finite subsets, the two approaches yield the same result, whence either may be taken as a definition of meet. In the case where each subset of A has a meet, in fact (A,≤) is a complete lattice; for details, see completeness (order theory).

References

• Davey, B.A., and Priestley, H. A. (2002). Introduction to Lattices and Order (Second ed.). Cambridge University Press. ISBN 0-521-78451-4.