# Trying understand a move in Cohen's proof of the independence of the continuum hypothesis

I've read a few different presentations of Cohen's proof. All of them (that I've seen) eventually make a move where a Cartesian product (call it CP) between the (M-form of) $\aleph_2$ and $\aleph_0$ into {1, 0} is imagined. From what I gather, whether $CP \in M$ is what determines whether $\neg$CH holds in M or not such that if $CP \in M$ then $\neg$CH.

Anyway, my question is: Why does this product, CP, play this role? How does it show us that $\aleph_2 \in M$ (the 'relativized' form of $\aleph_2$, not the $\aleph_2 \in V$)? Could not some other set-theoretical object play the same role?

• You could probably get a faster answer at mathoverflow.net . This is more of a mathematical question than it is a computer science one (not that I would mind a strong logic community in this forum).
– cody
Dec 3 '12 at 16:53
• Didn't take that long :) Dec 3 '12 at 18:45
• That said, it still might make sense to migrate this Q over to math.SE since it's a much more natural fit there than in TCS-world... Dec 6 '12 at 0:53

Our goal is to prove that $\aleph_1 < 2^{\aleph_0}$ in the model $M[G]$, and therefore the Continuum Hypothesis is not true in $M[G]$. This is equivalent to saying that $\aleph_2 \leq 2^{\aleph_0}$. So we need to construct a model $M[G]$ such that there is an injective map $f$ from $\aleph_2$ to $2^{\aleph_0}$ in $M[G]$. Note that each element of $2^{\aleph_0}$ is a function from $\aleph_0$ to $\{0,1\}$ (by definition). Thus $f(x)$ (the value of $f$ at point $x$) is actually a function that maps a natural number to $\{0,1\}$. In the proof, we treat $f$ as a function of two parameters $$h(x,y) = f(x)(y)$$ where $x\in \aleph_2$ and $y\in \aleph_0$. This is the reason why we want to prove that there exist a function $h$ from $\aleph_2 \times \aleph_0$ to $\{0,1\}$ satisfying certain properties.
The choice of $\aleph_2$ here is not arbitrary. Say, if we wanted to prove that $2^{\aleph_0} > \aleph_{19}$, we would consider functions from $\aleph_{20} \times \aleph_0$ to $\{0,1\}$.
BTW, $\aleph_2^M$ is always an element of $M$ and $M[G]$ (for any generic set $G$); on the other hand $\aleph_2$ is usually neither an element of $M$ nor $M[G]$, since $M$ is a countable transitive model of ZFC (or more precisely is a countable model of a large finite fragment of ZFC). So I'm not sure what you mean in the last paragraph.
• Great explanation, Yury. As for the last paragraph, you are right, I skipped the qualification of $\aleph_2$ as the $\aleph_2$ within M (i.e. 'relativized' to M). Dec 3 '12 at 17:14
• In some sense the choice of $\aleph_2$ is arbitrary, isn't it? i.e., there's no special property of $\aleph_2$ that's being used for the argument... Dec 5 '12 at 23:36
• @StevenStadnicki: I agree. We could choose any cardinal $\apha > \aleph_1$ instead and prove that $2^{\aleph_0} \geq \alpha$. That would also show that CH doesn't hold in our model. But at least, the choice of $\aleph_2$ in some sense is very natural.