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Section19.1Lattices

SubsectionPartially Ordered Sets

We begin by the study of lattices and Boolean algebras by generalizing the idea of inequality. Recall that a relation on a set \(X\) is a subset of \(X \times X\). A relation \(P\) on \(X\) is called a partial order of \(X\) if it satisfies the following axioms.

  1. The relation is reflexive: \((a, a) \in P\) for all \(a \in X\).

  2. The relation is antisymmetric: if \((a,b) \in P\) and \((b,a) \in P\), then \(a = b\).

  3. The relation is transitive: if \((a, b) \in P\) and \((b, c) \in P\), then \((a, c) \in P\).

We will usually write \(a \preceq b\) to mean \((a, b) \in P\) unless some symbol is naturally associated with a particular partial order, such as \(a \leq b\) with integers \(a\) and \(b\), or \(A \subseteq B\) with sets \(A\) and \(B\). A set \(X\) together with a partial order \(\preceq\) is called a partially ordered set, or poset.

Example19.1

The set of integers (or rationals or reals) is a poset where \(a \leq b\) has the usual meaning for two integers \(a\) and \(b\) in \({\mathbb Z}\).

Example19.2

Let \(X\) be any set. We will define the power set of \(X\) to be the set of all subsets of \(X\). We denote the power set of \(X\) by \({\mathcal P}(X)\). For example, let \(X = \{ a, b, c \}\). Then \({\mathcal P}(X)\) is the set of all subsets of the set \(\{ a, b, c \}\): \begin{align*} & \emptyset & & \{ a \} & & \{ b \} & & \{ c \} &\\ & \{ a, b \} & & \{ a, c\} & &\{ b, c\} & & \{ a, b, c \}. & \end{align*} On any power set of a set \(X\), set inclusion, \(\subseteq\), is a partial order. We can represent the order on \(\{ a, b, c \}\) schematically by a diagram such as the one in Figure 19.3.

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Figure19.3Partial order on \(\mathcal P( \{ a, b, c \})\)
Example19.4

Let \(G\) be a group. The set of subgroups of \(G\) is a poset, where the partial order is set inclusion.

Example19.5

There can be more than one partial order on a particular set. We can form a partial order on \({\mathbb N}\) by \(a \preceq b\) if \(a \mid b\). The relation is certainly reflexive since \(a \mid a\) for all \(a \in {\mathbb N}\). If \(m \mid n\) and \(n \mid m\), then \(m = n\); hence, the relation is also antisymmetric. The relation is transitive, because if \(m \mid n\) and \(n \mid p\), then \(m \mid p\).

Example19.6

Let \(X = \{ 1, 2, 3, 4, 6, 8, 12, 24 \}\) be the set of divisors of 24 with the partial order defined in Example 19.5. Figure 19.7 shows the partial order on \(X\).

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Figure19.7A partial order on the divisors of 24

Let \(Y\) be a subset of a poset \(X\). An element \(u\) in \(X\) is an upper bound of \(Y\) if \(a \preceq u\) for every element \(a \in Y\). If \(u\) is an upper bound of \(Y\) such that \(u \preceq v\) for every other upper bound \(v\) of \(Y\), then \(u\) is called a least upper bound or supremum of \(Y\). An element \(l\) in \(X\) is said to be a lower bound of \(Y\) if \(l \preceq a\) for all \(a \in Y\). If \(l\) is a lower bound of \(Y\) such that \(k \preceq l\) for every other lower bound \(k\) of \(Y\), then \(l\) is called a greatest lower bound or infimum of \(Y\).

Example19.8

Let \(Y = \{ 2, 3, 4, 6 \}\) be contained in the set \(X\) of Example 19.6. Then \(Y\) has upper bounds 12 and 24, with 12 as a least upper bound. The only lower bound is 1; hence, it must be a greatest lower bound.

As it turns out, least upper bounds and greatest lower bounds are unique if they exist.

Proof

On many posets it is possible to define binary operations by using the greatest lower bound and the least upper bound of two elements. A lattice is a poset \(L\) such that every pair of elements in \(L\) has a least upper bound and a greatest lower bound. The least upper bound of \(a, b \in L\) is called the join of \(a\) and \(b\) and is denoted by \(a \vee b\). The greatest lower bound of \(a, b \in L\) is called the meet of \(a\) and \(b\) and is denoted by \(a \wedge b\).

Example19.10

Let \(X\) be a set. Then the power set of \(X\), \({\mathcal P}(X)\), is a lattice. For two sets \(A\) and \(B\) in \({\mathcal P}(X)\), the least upper bound of \(A\) and \(B\) is \(A \cup B\). Certainly \(A \cup B\) is an upper bound of \(A\) and \(B\), since \(A \subseteq A \cup B\) and \(B \subseteq A \cup B\). If \(C\) is some other set containing both \(A\) and \(B\), then \(C\) must contain \(A \cup B\); hence, \(A \cup B\) is the least upper bound of \(A\) and \(B\). Similarly, the greatest lower bound of \(A\) and \(B\) is \(A \cap B\).

Example19.11

Let \(G\) be a group and suppose that \(X\) is the set of subgroups of \(G\). Then \(X\) is a poset ordered by set-theoretic inclusion, \(\subseteq\). The set of subgroups of \(G\) is also a lattice. If \(H\) and \(K\) are subgroups of \(G\), the greatest lower bound of \(H\) and \(K\) is \(H \cap K\). The set \(H \cup K\) may not be a subgroup of \(G\). We leave it as an exercise to show that the least upper bound of \(H\) and \(K\) is the subgroup generated by \(H \cup K\).

In set theory we have certain duality conditions. For example, by De Morgan's laws, any statement about sets that is true about \((A \cup B)'\) must also be true about \(A' \cap B'\). We also have a duality principle for lattices.

The following theorem tells us that a lattice is an algebraic structure with two binary operations that satisfy certain axioms.

Proof

Given any arbitrary set \(L\) with operations \(\vee\) and \(\wedge\), satisfying the conditions of the previous theorem, it is natural to ask whether or not this set comes from some lattice. The following theorem says that this is always the case.

Proof