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Enderton defines the rank of a set $A$ to be the least ordinal $\alpha$ such that $A \subseteq V_{\alpha}$ (equivalently, $A \in V_{\alpha^+}$). He the derives the following identity: $rank(A) = \bigcup \{ (rank(x))^+ : x \in A \}$ for all sets $A$.

In Exercise 30 of Chapter 7, the reader is asked to prove several identities involving rank. For instance, that $rank\{a,b\} = max(rank(a),rank(b))^+$. I am having trouble proving the second identity, that $rank(\wp(x)) = rank(x)^+$ for all sets $x$. I am not sure whether I am missing some elementary identity that would let me prove the identity, or whether I am misunderstanding the definition of rank.

Clearly, $z \in rank(\wp(x)) \Leftrightarrow (\exists y)(y \subseteq x \wedge (z \in rank(y) \vee z = rank(y))$. On the other hand $z \in (rank(x))^+ \Leftrightarrow (\exists y)(y \in x \wedge z \in (rank(y))^+) \vee z = rank(x)$. I'm not sure why the first statement should imply the second, and conversely, however.

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up vote 6 down vote accepted

I assume $\alpha^+$ just means $\alpha+1$, the immediate successor of $\alpha$. (Typically, we use $\alpha^+$ for a different notion.)

The point is this: Suppose that $x\subseteq y$. Then $rank(x)\le rank(y)$. This is immediate from the identity at the end of your first paragraph. It follows that all subsets of $x$ have rank at most $rank(x)$. Say $\beta$ is least such that $x\in V_\beta$. Then ${\mathcal P}(x)\subseteq V_\beta$ (since all subsets of $x$ are also elements of $V_\beta$) and therefore ${\mathcal P}(x)\in V_{\beta+1}$. This shows that $rank({\mathcal P}(x))\le rank(x)^+$.

For the converse: If $rank({\mathcal P}(x))< rank(x)^+$ then $rank({\mathcal P}(x))\le rank(x)$. Use the identity at the end of your first paragraph to conclude that all elements of ${\mathcal P}(x)$ have rank strictly less than $rank(x)$. But then $x$ itself would have rank strictly less than $rank(x)$. This is a contradiction, and we are done.

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Thank you, Andres. I was certainly approaching this problem incorrectly. Your explanation is very clear, and the method helpful. –  danportin May 27 '11 at 6:46
    
Out of curiosity, what does $\alpha^+$ mean? My lecturer used $\alpha^+$ for the successor of $\alpha$ as well... –  Zhen Lin May 27 '11 at 9:00
    
The standard notation is that if $\kappa$ is an infinite cardinal (i.e., an infinite ordinal that is not in bijection with any smaller ordinal; sometimes one says: $\kappa$ is an infinite initial ordinal), then $\kappa^+$ is the successor cardinal of $\kappa$, which is an ordinal much much larger than $\kappa+1$. If $\alpha$ is an ordinal then $|\alpha|$ is its cardinality (the initial ordinal in bijection with $\alpha$), and $\alpha^+$ means $|\alpha|^+$. For $\alpha$ finite sometimes people mean $\alpha^+=\alpha+1$ and sometimes they mean $\alpha^+=\omega$. –  Andres Caicedo May 27 '11 at 14:58
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