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I'm a little stumped on the aforementioned question. It's statement is as follows:

Let $M$ Be any class such that $\forall$x (x $\subset$ $M$ $\rightarrow$ x $\in$ $M$).

Show that $WF$ $\subset$ $M$.

$WF$ = well-founded sets defined by recursion as follows:

R(0) = 0

R($\alpha$ + 1) = P(R($\alpha$))

R($\alpha$) = $\bigcup$R($\beta$) (with $\beta$ < $\alpha$) for limit ordinals $\alpha$.

WF is then the Union of all sets which can be obtained by iterating powerset operation.

Am I supposed to define by recursion The class $M$ using $WF$?

Update: thanks to a hint of Brian M Scott, I think I've got the proof.

Claim: R($\alpha$)$\subset$ $M$ for each ordinal $\alpha$.

We proceed by transfinite induction.

Basis: $\alpha$ = 0. Trivial, but we do it anyway. Then R(0) = 0 and of course 0 $\subset$ $M$.

Successor case: $\alpha$ = $\beta$ + 1, and suppose that the claim holds for all ordinals $\beta$ < $\alpha$. Then by induction hypothesis R($\beta$) $\subset$ $M$. We note by the recursive definition of $WF$, asking whether R($\beta$ + 1) $\subset$ $M$ is equivalent to asking whether P(R($\beta$)) $\subset$ $M$. We also know R($\beta$) $\subset$ R($\beta$ + 1) which is just R($\beta$) $\subset$ P(R($\beta$)). Now every x $\in$ P(R($\beta$)) is either R($\beta$) or R($\gamma$) for some $\gamma$ < $\beta$, so that the inductive hypothesis applies and for every such x, x $\subset$ $M$ and in fact x $\in$ $M$. But this means P(R($\beta$)) $\subset$ $M$. So the claim holds here.

Limit case: $\alpha$ is a limit ordinal. Then R($\alpha$) = $\bigcup$ R($\beta$) for $\beta$ < $\alpha$. Suppose the claim does not hold here. Then some $\beta$ < $\alpha$ is such that either R($\beta$) $\subset$ $M$ fails or for some successor R($\beta$ + 1) such that R($\beta$) $\subset$ R($\beta$ + 1) then R($\beta$ + 1) $\subset$ $M$ fails. Either way, this contradicts the fact obtained in the successor case. So the claim hods here as well.

$END PROOF$. Right?

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  • $\begingroup$ Hi Luis, looks like you're on the right track. In the successor case: it's not true that "every $x\in\mathcal{P}(R(\beta))$ is either $R(\beta)$ or $R(\gamma)$ for some $\gamma<\beta$". It is true that "$x\in\mathcal{P}(R(\beta))$ iff $x\subseteq R(\beta)$", and then you can use the transitivity of $\subseteq$. $\endgroup$ – user52534 Apr 21 '15 at 21:54
  • $\begingroup$ Also, in the case when $\alpha$ is a limit ordinal: We assume that $R(\beta)\subseteq M$ for every $\beta<\alpha$, and then show that this assumption implies $R(\alpha)\subseteq M$. $\endgroup$ – user52534 Apr 21 '15 at 22:03
  • $\begingroup$ Thanks! I follow. I'll make adjustments within the next day. $\endgroup$ – لويس العرب Apr 21 '15 at 23:30
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HINT: Prove by induction on $\alpha$ that $R(\alpha)\subseteq M$ for each ordinal $\alpha$.

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