Max Operation Yields Supremum of Parameters/General Case

Theorem
Let $\struct {S, \preceq}$ be a totally ordered set. Let $x_1, x_2, \dots ,x_n \in S$ for some $n \in \N_{>0}$.

Then:
 * $\max \set {x_1, x_2, \dots ,x_n} = \sup \set {x_1, x_2, \dots ,x_n}$

where:
 * $\max$ denotes the max operation
 * $\sup$ denotes the supremum.

Proof
We will prove the result by induction on the number of operands $n$.

For all $n \in \Z_{>0}$, let $\map P n$ be the proposition:
 * $\max \set {x_1, x_2, \dots ,x_n} = \sup \set {x_1, x_2, \dots ,x_n}$

Basis for the Induction
$\map P 1$ is the case:
 * $\max \set {x_1} = \sup \set {x_1}$

By definition of the max operation:
 * $\max \set {x_1} = x_1$

From Supremum of Singleton:
 * $\sup \set {x_1} = x_1$

So:
 * $\max \set {x_1} = \sup \set {x_1} = x_1$

Thus $\map P 1$ is seen to hold.

This is the basis for the induction.

Induction Hypothesis
Now it needs to be shown that if $\map P k$ is true, where $k \ge 1$, then it logically follows that $\map P {k + 1}$ is true.

So this is the induction hypothesis:
 * $\max \set {x_1, x_2, \dots ,x_k} = \sup \set {x_1, x_2, \dots ,x_k}$

from which it is to be shown that:
 * $\max \set {x_1, x_2, \dots ,x_k, x_{k+1}} = \sup \set {x_1, x_2, \dots ,x_k, x_{k+1}}$

Induction Step
This is the induction step.

Now:

As $\struct {S, \preceq}$ is a totally ordered set, all elements of $S$ are comparable by $\preceq$.

Therefore there are two cases to consider:

Case 1: $x_{k+1} \preceq \sup \set {x_1, x_2, \dots ,x_k}$
Let $x_{k+1} \preceq \sup \set {x_1, x_2, \dots ,x_k}$.

By definition of the max operation:
 * $\max \set {x_1, x_2, \dots ,x_k, x_{k+1}} = \sup \set {x_1, x_2, \dots ,x_k}$

By definition, $\sup \set {x_1, x_2, \dots ,x_k}$ is an upper bound of $\set {x_1, x_2, \dots ,x_k}$.

That is:
 * $\forall 1 \le i \le k : x_i \preceq \sup \set {x_1, x_2, \dots ,x_k}$

Thus:
 * $\forall 1 \le i \le k + 1 : x_i \preceq \sup \set {x_1, x_2, \dots ,x_k}$

So $\sup \set {x_1, x_2, \dots ,x_k}$ is an upper bound of $\set {x_1, x_2, \dots ,x_k, x_{k+1}}$.

Let $y$ be any other upper bound of $\set {x_1, x_2, \dots ,x_k, x_{k+1}}$.

Then:
 * $y$ is an upper bound of $\set {x_1, x_2, \dots ,x_k}$.

By definition of the supremum:
 * $\sup \set {x_1, x_2, \dots ,x_k} \preceq y$

It has been shown that the supremum of $\set {x_1, x_2, \dots ,x_k, x_{k+1}}$ is:
 * $\sup \set {x_1, x_2, \dots ,x_k, x_{k+1}} = \sup \set {x_1, x_2, \dots ,x_k}$

Thus it follows:
 * $\max \set {x_1, x_2, \dots ,x_k, x_{k+1}} = \sup \set {x_1, x_2, \dots ,x_k, x_{k+1}}$

Case 2: $\sup \set {x_1, x_2, \dots ,x_k} \preceq x_{k+1}$
Let $\sup \set {x_1, x_2, \dots ,x_k} \preceq x_{k+1}$.

By definition of the max operation:
 * $\max \set {x_1, x_2, \dots ,x_k, x_{k+1}} = x_{k+1}$

By definition, $\sup \set {x_1, x_2, \dots ,x_k}$ is an upper bound of $\set {x_1, x_2, \dots ,x_k}$.

That is:
 * $\forall 1 \le i \le k : x_i \preceq \sup \set {x_1, x_2, \dots ,x_k}$

By definition of an ordering:
 * $\preceq$ is transitive.

Thus:
 * $\forall 1 \le i \le k : x_i \preceq x_{k+1}$

By definition of an ordering:
 * $\preceq$ is reflexive.

Thus:
 * $x_{k+1} \preceq x_{k+1}$

It follows that $x_{k+1}$ is an upper bound of $\set {x_1, x_2, \dots ,x_k, x_{k+1}}$.

Let $y$ be any other upper bound of $\set {x_1, x_2, \dots ,x_k, x_{k+1}}$.

Then:
 * $x_{k+1} \preceq y$.

It has been shown that
 * $\sup \set {x_1, x_2, \dots ,x_k, x_{k+1}} = x_{k+1}$.

Thus it follows:
 * $\max \set {x_1, x_2, \dots ,x_k, x_{k+1}} = \sup \set {x_1, x_2, \dots ,x_k, x_{k+1}}$

In either case, the result holds.

So $\map P k \implies \map P {k + 1}$ and the result follows by the Principle of Mathematical Induction.

Therefore:
 * $\max \set {x_1, x_2, \dots ,x_n} = \sup \set {x_1, x_2, \dots ,x_n}$