# Finite Totally Ordered Set is Well-Ordered

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## Theorem

Every finite totally ordered set is well-ordered.

## Proof

Let $\struct {S, \preceq}$ be a finite totally ordered set.

From Condition for Well-Foundedness, $\struct {S, \preceq}$ is well-founded if and only if there is no infinite sequence $\sequence {a_n}$ of elements of $S$ such that $\forall n \in \N: a_{n + 1} \prec a_n$.

If it were the case that $\struct {S, \preceq}$ had such an infinite sequence, then at least some of the terms would be repeated in that sequence.

So there would be, for example:

- $s_i \preceq s_j \preceq s_k \preceq s_i$

and $\preceq$ would therefore not be transitive and so not a totally ordered set.

So $\struct {S, \preceq}$ is well-founded.

The result follows from the definition of well-ordered set.

$\blacksquare$

## Sources

- 1965: Seth Warner:
*Modern Algebra*... (previous) ... (next): $\S 14$ - 1968: A.N. Kolmogorov and S.V. Fomin:
*Introductory Real Analysis*... (previous) ... (next): $\S 3.5$: Well-ordered sets. Ordinal Numbers: Example $1$