Equivalence of Definitions of Lattice

Theorem
The following definitions of a lattice are all equivalent:

1 implies 2
Let $\left({S, \vee, \wedge, \preceq}\right)$ adhere to Definition 1.

Then since all finite non-empty suprema exist, we have:


 * $a \vee b = \sup \left\{{a, b}\right\} \in S$

for all $a, b \in S$, where $\vee$ denotes join.

Hence $\left({S, \vee, \preceq}\right)$ is a join semilattice.

Similarly, since all finite non-empty infima exist, we have:


 * $a \wedge b = \inf \left\{{a, b}\right\} \in S$

for all $a, b \in S$, where $\wedge$ denotes meet.

Thus $\left({S, \wedge, \preceq}\right)$ is a meet semilattice.

In conclusion, $\left({S, \vee, \wedge, \preceq}\right)$ satisfies Definition 2.

2 implies 1
Let $\left({S, \vee, \wedge, \preceq}\right)$ adhere to Definition 2.

We will prove that:


 * $\left({S, \vee, \preceq}\right)$ is a join semilattice

implies that:


 * $\left({S, \preceq}\right)$ admits all finite non-empty suprema

From Supremum of Singleton, $\left({S, \preceq}\right)$ admits suprema of singletons.

By definition of join, it also admits suprema of doubletons.

Suppose now inductively that $\sup T \in S$ for all subsets $T$ of $S$ that have $n$ elements, where $n \ge 2$ is a natural number.

Let now $V \subseteq S$ have $n + 1$ elements.

Then $V = T \cup \left\{{s}\right\}$ for some $s \in S$ and $T \subseteq S$ with $n$ elements.

By Supremum of Suprema, we have that:


 * $\sup V = \sup \left\{{ \sup T, \sup \left\{{s}\right\} }\right\}$

By definition of join, this means $\sup V = \sup T \vee s$.

Therefore, since $\left({S, \vee, \preceq}\right)$ is a join semilattice, $\sup V \in S$.

By induction, $\sup T \in S$ for all finite non-empty $T \subseteq S$.

That is, $S$ admits all finite non-empty suprema.

By the Global Duality Principle and Dual Pairs (Order Theory), it follows that if:


 * $\left({S, \wedge, \preceq}\right)$ is a meet semilattice

then it also true that:


 * $\left({S, \preceq}\right)$ admits all finite non-empty infima

Hence $\left({S, \vee, \wedge, \preceq}\right)$ satisfies Definition 1.

2 implies 3
Suppose that $\left({S, \vee, \wedge, \preceq}\right)$ adheres to Definition 2.

By virtue of Join Semilattice is Semilattice and Meet Semilattice is Semilattice, $\left({S, \vee}\right)$ and $\left({S, \wedge}\right)$ are semilattices.

From Ordering Induced by Join Semilattice, the ordering mentioned in Definition 3 coincides with $\preceq$.

The absorption laws stated are proved on Join Absorbs Meet and Meet Absorbs Join.

Hence $\left({S, \vee, \wedge, \preceq}\right)$ adheres to Definition 3.

Also see

 * Lattice