Equivalence of Definitions of Lattice

$1$ implies $2$
Let $\struct {S, \vee, \wedge, \preceq}$ adhere to Definition 1.

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


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

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

Hence $\struct {S, \vee, \preceq}$ is a join semilattice.

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


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

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

Thus $\struct {S, \wedge, \preceq}$ is a meet semilattice.

In conclusion, $\struct {S, \vee, \wedge, \preceq}$ satisfies Definition 2.

$2$ implies $1$
Let $\struct {S, \vee, \wedge, \preceq}$ adhere to Definition 2.

We will prove that:


 * $\struct {S, \vee, \preceq}$ is a join semilattice

implies that:


 * $\struct {S, \preceq}$ admits all finite non-empty suprema

From Supremum of Singleton, $\struct {S, \preceq}$ 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 $V = T \cup \set s$ for some $s \in S$ and $T \subseteq S$ with $n$ elements.

By Supremum of Suprema, we have that:


 * $\sup V = \sup \set {\sup T, \sup \set s}$

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

Therefore, since $\struct {S, \vee, \preceq}$ 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:


 * $\struct {S, \wedge, \preceq}$ is a meet semilattice

then it also true that:


 * $\struct {S, \preceq}$ admits all finite non-empty infima

Hence $\struct {S, \vee, \wedge, \preceq}$ satisfies Definition 1.

$2$ implies $3$
Suppose that $\struct {S, \vee, \wedge, \preceq}$ adheres to Definition 2.

By virtue of Join Semilattice is Semilattice and Meet Semilattice is Semilattice, $\struct {S, \vee}$ and $\struct {S, \wedge}$ 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 $\struct {S, \vee, \wedge, \preceq}$ adheres to Definition 3.

$3$ implies $2$
Suppose that $\struct {S, \vee, \wedge, \preceq}$ adheres to Definition 3.

Then $\preceq$ is thus defined by, for all $a, b \in S$:


 * $a \preceq b$ $a \vee b = b$

Suppose now that $a \vee b = b$.

Then by absorption:


 * $a = a \wedge \paren {a \vee b} = a \wedge b$

Conversely, if $a \wedge b = a$, then by the other absorption law:


 * $a \vee b = \paren {a \wedge b} \vee b = b$

We conclude that also $a \preceq b$ $a \wedge b = a$.

Next, to prove $a \vee b$ is a supremum for $\set {a, b}$.

We have by absorption that $a = a \wedge \paren {a \vee b}$ so that $a \preceq a \vee b$, and similarly for $b$.

Thus $a \vee b$ is an upper bound for $\set {a, b}$.

Suppose that $c$ is an upper bound for $\set {a, b}$.

Then:

whence $a \vee b \preceq c$.

Thus $a \vee b \in S$ is a supremum for $\set {a, b}$.

Finally, we will show that $a \wedge b$ is an infimum for $\set {a, b}$.

By Local Duality, $a \wedge b$ must be shown to be a supremum for $\set {a, b}$ with respect to the dual ordering $\succeq$ of $\preceq$.

Since we have established that $a \succeq b$ $a \wedge b = b$, this directly follows from the above by interchanging $\wedge$ and $\vee$.

Thus we have demonstrated $\struct {S, \vee, \wedge, \preceq}$ satisfies Definition 2.