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I'm trying to understand the proof of the Hartogs’ Theorem on page 100 of this book.

My especific question is:

If we have for each set $A$

$$\mathrm{WO}(A)=\{ (U,\leq_{U}) \, | \, U\subseteq A \, \wedge \, (\leq_{U}) \text{ is a wellordering on }U \}.$$

and this relation on $\mathrm{WO}(A)$

$$U \thicksim V \Longleftrightarrow \text{There is an order preserving bijection between } U \text{ and } V$$

Is it true that there is no injection between $\mathrm{WO}(A)/\thicksim$ and $A$?

I'm trying to show that $\mathrm{WO}(A)/\thicksim$ has the same cardinal as $\mathcal{P}(A)$. By the definition I can see that $\mathcal{P}(A)\leq_c \mathrm{WO}(A)$, but when doing the quotient I don't know how to do. I can't see the proof clearly on the book, because it is written that

$$\mathrm{WO}(A)/\thicksim\subseteq \mathcal{P}(\mathrm{WO}(A))$$ and I can't see how this show the result.

Thank you for any help.

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2 Answers 2

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HINT: For $[U],[V]\in\operatorname{WO}(A)/\!\!\sim$ define $[U]\preceq[V]$ iff $\langle U,\le_U\rangle$ is order-isomorphic to an initial segment (not necessarily proper) of $\langle V,\le_V\rangle$. Prove that $\preceq$ is a well-defined well-ordering of $\operatorname{WO}(A)/\!\!\sim$. Now suppose that $h:\operatorname{WO}(A)/\!\!\sim\,\to A$ is an injection. Let $A_0$ be the range of $h$, and define a relation $\sqsubseteq$ on $A_0$ by $a\sqsubseteq b$ iff $h^{-1}(a)\preceq h^{-1}(b)$. Show that $\langle A_0,\sqsubseteq\rangle\in\operatorname{WO}(A)$, and get a contradiction because $\langle A_0,\sqsubseteq\rangle\in\operatorname{WO}(A)$ is ‘too long’.

(Note that it isn’t helpful to think about the cardinality of $\langle A_0,\sqsubseteq\rangle\in\operatorname{WO}(A)$.)

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  • $\begingroup$ What does it mean "too long" in this case? $\endgroup$
    – MaríaCC
    Jan 24, 2015 at 19:44
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    $\begingroup$ @María: Each member of $\operatorname{WO}(A)$ will be order-isomorphic to a proper initial segment of $\langle B,\sqsubseteq\rangle$, so $\langle B,\sqsubseteq\rangle$ can’t be in $\operatorname{WO}(A)$ after all. $\endgroup$ Jan 24, 2015 at 19:46
  • $\begingroup$ But $B$ is not defined. Maybe $B=A_0$? $\endgroup$
    – MaríaCC
    Jan 24, 2015 at 19:49
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    $\begingroup$ @María: Oops! Yes, it was supposed to be $A_0$; I’ve fixed it now. Thanks for catching that. $\endgroup$ Jan 24, 2015 at 19:50
  • $\begingroup$ Haha thank you. My brain is totally frozen today. I cannot see why each member of $\mathrm{WO}(A)$ is isomorphic to a proper initial segment of $A_0$... $\endgroup$
    – MaríaCC
    Jan 24, 2015 at 19:58
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It isn't generally true that $\text{WO}(A)/\sim$ has the same cardinality as $P(A)$; this would be true under the GCH and indeed the assertion that it is true for all infinite $A$ is equivalent to the GCH in ZFC.

Meanwhile, there is no injection from $\text{WO}(A)/\sim$ to $A$, because if there were, you could use the image of that injection to make an order-type that does not occur in $\text{WO}(A)$, which would be a contadiction. Basically, you should show that $\text{WO}(A)/\sim$ is itself well-ordered, and so the injection induces a well-ordering of a subset of $A$, and this order type is strictly larger than anything in $\text{WO}(A)$. This order is the order-type of the smallest well-ordering that does not embed into $A$.

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  • $\begingroup$ Mmmmm... I've already prove that $\mathrm{WO}/\thicksim$ is well ordered but I don't understand the rest of your answer... $\endgroup$
    – MaríaCC
    Jan 24, 2015 at 19:22
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    $\begingroup$ Well, if $\text{WO}/\sim$ is well-ordered, and you've injected it into $A$, then you can copy over that well-ordered relation to get a well-ordering of a subset of $A$. $\endgroup$
    – JDH
    Jan 24, 2015 at 19:32
  • $\begingroup$ OK, I got that. Then, where is the contradiction? $\endgroup$
    – MaríaCC
    Jan 24, 2015 at 19:40
  • $\begingroup$ what is an order type? $\endgroup$
    – MaríaCC
    Jan 24, 2015 at 19:43
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    $\begingroup$ Two order relations have the same order type when they are order isomorphic. The contradiction is that the order type of WO(A)/$\sim$ itself cannot arise as one of the elements of WO(A). $\endgroup$
    – JDH
    Jan 24, 2015 at 20:13

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