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$$\{f \mid f : \mathbb{N} → \{0, 1\}\} - \{f\mid f : \{0, 1\} → \mathbb{N}\}$$ Where $f$ is a function. Prove that this difference is an uncountable set.

I am pretty stuck on how to start this problem. I understand that the set from $0$ to $1$ is uncountable while the set of natural numbers is countable, but I can't seem to start off the proof.

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    $\begingroup$ Calling the sets $A$ and $B$, it seems that $B \cap A = \varnothing$. None of the elements of $A$ have domain $\{0,1\}$. $\endgroup$
    – While I Am
    Commented Dec 2, 2021 at 20:39
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    $\begingroup$ I agree with @William. Since the sets are disjoint, the difference is $\{f \mid f : \Bbb{N} \to \{0, 1\}\}$, which is uncountable straightforwardly by Cantor (well, it has the usual bijection with the power set of $\Bbb{N}$). In general, if $A$ is uncountable and $B$ is countable, then $A - B$ is uncountable. Why? As always, we have $A \subseteq (A - B) \cup B$. If $A - B$ and $B$ are countable, then so is $(A - B) \cup B$, and hence $A$. This would be a contradiction. $\endgroup$ Commented Dec 2, 2021 at 20:48
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    $\begingroup$ "I understand that the set from 0 to 1 is uncountable": that does not make sense $\endgroup$
    – miracle173
    Commented Dec 2, 2021 at 20:48
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    $\begingroup$ @miracle173 I would guess they mean $[0,1]\subset\mathbb{R}$. $\endgroup$
    – podiki
    Commented Dec 2, 2021 at 20:49
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    $\begingroup$ These sets as disjoint as sets of functions. The domains of the functions in set A and set B intersect. But the functions are distinct. No element of the first set is an element of the second set and vice versa. $\endgroup$ Commented Dec 2, 2021 at 21:21

1 Answer 1

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The comments point out that the two sets we have are, in fact, disjoint, so we have that the cardinality of their difference is just the cardinality of the first set. Even without using this fact, we can still show that we have an uncountable set.

The set $\{f\mid f:\Bbb N\to\{0,1\}\}$ has cardinality $2^{|\mathbb N|}=2^{\aleph_0}$. To see this, notice that each element of $\Bbb N$ can be mapped to one of two elements, either $0$ or $1$.

The set $\{f\mid f:\{0,1\}\to\Bbb N\}$ has cardinality $|\Bbb N|^2=\aleph_0$. To see this, notice that each element of $\{0,1\}$ can be mapped to one of $|\Bbb N|$ elements, either $0,1,2,3,\dots$.

We start with $2^{\aleph_0}$ (uncountably many) elements and remove at most $\aleph_0$ (countably many) elements, so our set difference has at least $2^{\aleph_0}-\aleph_0=2^{\aleph_0}$ elements, i.e., it is uncountable.

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