Can we expand “induction principle” to a partial order $(X, \leq)$

elementary-set-theoryorder-theory

We know that every infinite can be made well-ordered with an unknown order.
Also we can expand the induction principle on any infinite set in
the sense that
it can made well ordered.
Now partially ordered set may not be a well ordered set with respect
to the partial order.
Let a partially ordered set $(X, \leq)$ with respect to this particular
order $'\leq'$ and suppose that this partial order $'\leq '$ does not make the
set $X$ well-ordered.

My question is-

Can we expand "induction principle" to the partially order set $(X,\leq)$ keeping
in mind that $(X,\leq)$ is not well ordered?

I have great confusion here.

Best Answer

Induction can be performed over any relation $R$ over a set $X$, provided the relation is well-founded: any subset $S \subseteq X$ must have a minimal element with respect to $R$. A minimal element of $S$ is an element $m \in S$ such that there is no $x \in S$ with $x R m$.

For total orders, being well-founded is equivalent to being well-ordered.

Note: Assuming the axiom of dependent choice (a weaker form of the axiom of choice), one can show that a relation is well-founded if and only if there is no infinite descending chain of elements in $X$ (with respect to the relation $R$).

Note 2: The Axiom of Choice is equivalent to stating that any set can be well-ordered. Thus, one could in principle do (transfinite) induction over any set.

The problem is that the Axiom of Choice is not constructive, which means that no one knows anything about what the well-ordering given by the axiom looks like. Therefore, it is in practice impossible to use that well-ordering.

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