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Let $A$ and $B$ be nonempty finite sets. Suppose $F: \mathcal{P}(A) \setminus \{\emptyset\} \to \mathcal{P}(B) \setminus \{\emptyset\}$ is a bijection. Then $|A| = |B|$.

I proved what is above with the cardinalities of each power set alone. What would be an explicit bijection for $f: A \to B$?

What I used was that either $A$ or $B$ had more elements than the other. So, WLOG, $|A| \leq |B|$. How can I continue from here?

Edit: My question is to seek an explicit bijection between $A$ and $B$. Is that possible?

  • Even if $A$ and $B$ are finite? – Yoyos Tutoring Aug 11 '24 at 02:23
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    If $|\mathcal{P}(A) \setminus {\emptyset}| = |\mathcal{P}(B) \setminus {\emptyset}|$, then $2^{|A|} = |\mathcal{P}(A)| = |\mathcal{P}(B)| = 2^{|B|}$, and then $|A|=|B|$ by taking $\log_2$ (since they are finite). – azif00 Aug 11 '24 at 02:25
  • I think the way to formalise this question would be to make it about choice functions: does a choice function on a collection of sets of size $(2^n)!$ give you a choice function on a collection of sets of size $n!$? – Zoe Allen Aug 11 '24 at 02:32
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    I am confused as to why this question which is explicitly about the finite case has an accepted answer which is entirely two links discussing the infinite case. – Asaf Karagila Aug 11 '24 at 05:41
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    OP please unaccept it so I can delete it. – Liyang Meng Aug 11 '24 at 07:09
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    If the cardinality of $A$ is $k$, the cardinality of $P(A)$ is $2^k$. There are far more bijections between two sets of size $2^k$ than there are between two sets of size $k$. If $|P(A)| = |P(B)|$ then $A$ and $B$ are of the same cardinality, so a bijection exists. But I doubt it's possible to construct a nice bijection between $A$ and $B$ starting from a random bijection between $P(A)$ and $P(B)$. – user3257842 Aug 11 '24 at 09:31

2 Answers2

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For shorthand, denote: $\mathcal{P}^{*}(A) = \mathcal{P}(A) \setminus \{\emptyset\}$ .
Additionally, use $F_{f}(x)$ as shorthand for $F(f)(x)$.
We have a functor: $$F:(A\to B) \to (\mathcal{P}^{*}(A) \to \mathcal{P}^{*}(B)) $$ defined by: $$\forall f:A\to B,\;\; F_{f}(X) = \{ f(x) \mid x\in X \} $$

This "lifts" bijections between $A$ and $B$ to bijections between $\mathcal{P}^{*}(A)$ and $\mathcal{P}^{*}(B)$ in a "natural" fashion.
We know it is a functor because given $f:A\to B$ and $g: B\to C$, the following $2$ properties hold:

$$ F_{g}\circ F_{f} = F_{g \;\circ \;f}\\ F_{(\text{id}_{X})} = \text{id}_{(\mathcal{P}^{*}(X))}$$

where $\text{id}_{X} : X \to X$ denotes the identity function on a set $X$.
Compositionality makes $F$ a functor.

We would like to find a functor $G$ that "undos" $F$, in a sense. That is:

$$G: (\mathcal{P}^{*}(A) \to \mathcal{P}^{*}(B)) \to (A\to B) \\G_{F_{f}} = f$$

where $G$ is a functor, meaning it obeys:

$$ G_{g}\circ G_{f} = G_{g \;\circ \;f}\\ G_{\text{id}_{(\mathcal{P}^{*}(X))}} = \text{id}_{X}$$

This hypothetical functor would have all the "nice" properties we expect from a way to bring bijections of $ (\mathcal{P}^{*}(A) \to \mathcal{P}^{*}(B))$ down to bijections of $A \to B$, including the "relabeling property":

If you start with a bijection from $(\mathcal{P}^{*}(A) \to \mathcal{P}^{*}(B))$, permute the labels of the elements of $A$ (or the labels of the elements of $B$) in it amongst themselves, then bring it "down" to a bijection from $A \to B$ it should get the same result as bringing it down first, then applying the specified relabelings.

Formally, given $a: A \to A$, $\;b: B \to B$, $\;f:\mathcal{P}^{*}(A) \to \mathcal{P}^{*}(B)$, we have:

$$G_{F_{b} \; \circ \; f \; \circ \; F_{a}} = b \; \circ \; G_{f} \; \circ \; a $$

This follows from the previously stated properties of $G$.


Now that we've formalized what we're looking for, let's prove it does not exist.

Theorem 1
Given $2$ bijections $x: A\to B$, $y : A\to B$, the preimage of $G$ with respect to $x$ has the same cardinality as the preimage of $G$ with respect to $y$. That is, for $I_{x} = \{X \mid G_X = x\}$, $I_{y}= \{Y \mid G_{Y} = y\}$, we have $|I_{x}| = |I_{y}|$.

Proof:
Let: $\psi : B \to B$ have the property of $\psi \circ x = y$. Then $F_{\psi}$ can give us a bijection (through composition), between $I_{x}$ and $I_{y}$. Thus $|I_{x}| = |I_{y}|$. Any two bijections from $A$ to $B$ have an equal number of elements (the elements being specific bijections from $(\mathcal{P}^{*}(A) \to \mathcal{P}^{*}(B))$) in their associated preimages with respect to $G$.

Theorem 2
Let there be a set $A$ such that $|A| \geq 3$.
Then a functor $G: (\mathcal{P}^{*}(A) \to \mathcal{P}^{*}(A)) \to (A\to A)$ cannot exist.

Notes: For $|A| = 2$ you can construct $G$. There is a "nice" mapping $G: (\mathcal{P}^{*}(A) \to \mathcal{P}^{*}(B)) \to (A\to B)$ when $|A| = |B| = 2$. I will first give the impossibility proof for $|A| = 3$, then give a general argument.

Proof:
There are $3! = 6$ bijections from $A$ to $A$.
There are $(2^3-1)! = 5040$ bijections from $(\mathcal{P}^{*}(A)$ to $\mathcal{P}^{*}(A))$.
The inverse image of any $A \to A$ bijection with respect to $G$ is of cardinality $5040/ 6 = 840$.
We will be studying $|I_{\text{id}_{A}}|$, the cardinality of the set of $(\mathcal{P}^{*}(A)$ to $\mathcal{P}^{*}(A))$ bijections which $G$ maps to $\text{id}_{A}$.

Denote: $ f^n = \underbrace{f\;\circ\;f\;\circ \ldots f}_{n\ \text{times}}$.

The bijections of $f:(\mathcal{P}^{*}(A) \to \mathcal{P}^{*}(A))$ can be considered as permutations of $7$ elements.
Consider those which cycle $5$ of the elements and leave the rest unchanged.
Denote one of these as $f_5$.
We have ${f_{5}}^5 = \text{id}_{\mathcal{P}^{*}(A)}$.
So ${G_{f_{5}}}^5 = G_{{f_{5}}^5} = G_{{\text{id}_{{P}^{*}(A)}}} = \text{id}_{A}$ .
But $G_{f_{5}}$ is only a bijection from $A$ to $A$, a $3$ element set.
It cannot have a nontrivial cycle of a length which divides $5$. Therefore $G_{f_{5}} = \text{id}_{A}$.
We can make a similar argument for $G_{f_{7}}$, where $f_{7}:(\mathcal{P}^{*}(A) \to \mathcal{P}^{*}(A))$ denotes a given $7$-cycle of $\mathcal{P}^{*}(A)$. We have $G_{f_{7}} = \text{id}_{A}$.

The number of $5$-cycles of $\mathcal{P}^{*}(A)$ is $\binom{7}{5}*4! = 504$.
The number of $7$-cycles of $\mathcal{P}^{*}(A)$ is $6! = 720$.
From this we get $|I_{\text{id}_{A}}| \geq 504 + 720 > 840$, contradicting theorem $1$.
We cannot construct the functor $G: (\mathcal{P}^{*}(A) \to \mathcal{P}^{*}(A)) \to (A\to A)$ when $|A| = 3$.


Now for the general case. We will rely on $2$ lemmas:

Lemma 1

For $n > 2$, there exists a prime $p$ such that $n < p < 2^n$.
This follows from the Bertrand–Chebyshev theorem.

Lemma 2

Let $n$ be a natural number, and $i > 1$ be an odd number. Then the $i$-cycles $f_{i}$ (on a set of $n$ elements) generate the alternating group $\mathcal{A}_{n}$.

Proof:
Let $f_{i}$ be a $i$-cycle and $\alpha_{k ,f_i(k)}$ be an inversion of $2$ consecutive elements of the cycle $f_{i}$. Then $g_{i} = \alpha_{k ,f_i(k)} \; \circ \; f_{i} \circ \; \alpha_{k ,f_i(k)}$ will also be an $i$-cycle, and $f_{i}\circ g^{-1}_{i}$ will be a $3$-cycle $(f^{-1}(k), \; k, \; f(k))$. Thus we can generate all $3$-cycles, which generate the alternating group $\mathcal{A}_{n}$, which contains $f_{i}$, since odd cycles have even parity.

Now for the proof:

Let $A$ be a set with $|A| = n >3$.
Due to theorem $1$, we have $|I_{\text{id}_{A}}| = \frac{(2^n-1)!}{n!}$.
Due to lemma $1$, there exists a prime $p$ with $n < p < 2^n$. Generalizing the composition argument of the previous proof, we know that $G_{f_{p}} = \text{id}_{A}$ for all $p$-cycles $f_{p}$ of $\mathcal{P}^{*}(A)$.
Due to lemma 2 and the composition property of $G$, we can say: $\forall \alpha\in \mathcal{A}_{(\mathcal{P}^{*}(A))},\; G_{\alpha} = \text{id}_{A}$.
Ie. $G$ will map any permutation which is of the alternating group of $\mathcal{P}^{*}(A)$ to $\text{id}_{A}$.

We have $|I_{\text{id}_A}| \geq |\mathcal{A}_{(\mathcal{P}^{*}(A))}| = \frac{(2^n-1)!}{2} > \frac{(2^n-1)!}{n!}$ for $n > 2$, contradicting theorem $1$. This completes the proof.

user3257842
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  • I wonder if by looking for a functor on category containing $P^(A), P^(B)$ and all isomorphisms one could find a result for $A \ne B$. This is isomorphic to the category of all isomorphisms between two symmetric groups. – Zoe Allen Aug 13 '24 at 07:07
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    I've added an argument that should work for the general case. I think it's unlikely to construct $G$ even if $|A| \neq |B|$, at least not in a general sense. – user3257842 Aug 13 '24 at 08:07
  • Here is the explicit construction for $G$ when $|A| = |B| = 2$ . Let $A = {a,b}, B = {x.y}$. We have: $$G({a}\to {x},{b}\to{y}, {a,b}\to{x,y}) = (a\to x, b \to y)$$ $$G({a}\to {x},{b}\to{x,y}, {a,b}\to{y}) = (a\to y, b \to x)$$ The rest of the cases can be obtained by relabeling. It basically measures how much cardinality the powerset bijection preserves (there exists at least 1 mapping from a set of cardinality $1$ to a set of cardinality $1$), to decide how to construct the function. – user3257842 Aug 13 '24 at 08:22
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I don't have an answer, but I would like to propose a way of making the question rigorous which I think is interesting and amenable to representation theory.

I'll start with an unsatisfying answer: if you pick a way of numbering $B$ from $1$ to $n$ then you have an ordering on $2^B$ (interpret each element as a $\le n$ digit binary number). We can then order $\{a\}$ for each $a \in A$ using the bijection with $P(B)$, and use our ordering of $A$ and $B$ to construct a bijection $A \to B$. However, this is kind of trivial, and what choice of ordering of $B$ you make affects which bijection you end up with. So this doesn't specify a unique bijection if we can make different choices about how to order $B$.

I think the satisfying non-trivial version of this problem is to find a way of doing this which is invariant under relabeling of the elements of $A$ and $B$, so that you specify a unique order without making any additional choice. Specifically, our problem is to find a function $\phi: iso(P(A), P(B)) \to iso(A, B)$ with the appropriate invariances. I'm using $iso(A, B)$ to denote the set of bijections $A \to B$.

We can describe a relabeling of $A$ as a permutation $\sigma_A$ of $A$, and then $\sigma_A$ and $\sigma_B$ act on $iso(A, B)$ by taking the function $f: a_i \mapsto b_i$ to the function $\sigma_B f \sigma_A : \sigma_A^{-1}(a_i) \mapsto \sigma_B(b_i)$.

$\sigma_A$ and $\sigma_B$ also act on $iso(P(A), P(B))$ by taking the function $g: A'_i \mapsto B'_i$ to the function $\bar{\sigma_B} g \bar{\sigma_A} : \sigma_A^{-1}[A'_i] \mapsto \sigma_B[B'_i]$ which can alternatively be viewed as the action of permutations of $P(A)$ and $P(B)$, $\bar{\sigma_A}$ and $\bar{\sigma_B}$, where $\bar{\sigma_A}(A') = \sigma_A[A']$. I will call $\bar{\sigma_A}$ the permutation of $P(A)$ induced by $\sigma_A$.

So the property of $\phi$ that we want is that if $\phi(g) = f$, for any relabelings $\sigma_A$ and $\sigma_B$, $\sigma$, $\bar{\sigma_B} g \bar{\sigma_A}$ gets mapped to $\sigma_B f \sigma_A$.

In summary, we want a $\phi: iso(P(A), P(B)) \to iso(A, B)$ such that for any permutations $\sigma_A, \sigma_B$ of $A, B$,

$$\phi(\bar{\sigma_B} g \bar{\sigma_A}) = \sigma_B \phi(g) \sigma_A$$

Note that this implies something about choice functions. If we have a choice function, $\psi$ on $\{iso(P(A_i), P(B_i)) | i \in I\}$, where $|A_i| = |B_i| = n$, we automatically get another choice function $\psi'$ on $\{iso(A_i, B_i) | i \in I\}$ by simply applying $\phi$ to each choice given by $\psi$. The invariance property means our answer doesn't depend on our labelling, so we have gotten from $\psi$ to $\psi'$ in a choice-free way.


Some notes making a start on this problem:

Now, by acting as $\bar{\sigma_A}$ and $\bar{\sigma_B}$, $\sigma_A$ and $\sigma_B$ have actions on $iso(P(A), P(B))$ that commute with each other, so we have an action of $\Gamma = Sym(A) \times Sym(B)$ on $iso(P(A), P(B))$ We can thus partition $iso(P(A), P(B))$ into orbits, and if a valid $\phi$ exists, we can restrict its domain to $O$ for each orbit $O$, to make a mini $\phi$ with the same property.

Conversely, if for each orbit $O$ we can make a mini $\phi$, we can stitch these together to make a $\phi$ for the whole of $iso(P(A), P(B))$. Stitching together the orbits can't cause there to be a $g, \sigma_A, \sigma_B$ such that $\phi(\bar{\sigma_B} g \bar{\sigma_A}) \ne \sigma_B f \sigma_A$, because $\bar{\sigma_B} g \bar{\sigma_A}$ is in the same orbit as $g$, so it would have broken the mini $\phi$ for that orbit already, if such a counterexample existed. This means we have a $\phi$ iff we have a mini $\phi$ for each orbit.

On a given orbit $O$, if we fix $\phi(g) = f$, that already fixes the value of every $\phi$ on the whole of $O$, because everything in $O$ is of the form $\bar{\sigma_B} g \bar{\sigma_A}$ and $\phi(\bar{\sigma_B} g \bar{\sigma_A}) = \sigma_B f \sigma_A$. It might fix the value of $g' \in O$ multiple ways if $g'$ can be written in multiple ways $g' = \bar{\sigma_B} g \bar{\sigma_A} = \bar{\tau_B} g \bar{\tau_A}$ so we get a well-defined function mini $\phi$ iff whenever $\bar{\sigma_B} g \bar{\sigma_A} = \bar{\tau_B} g \bar{\tau_A}$ we have the induced values of $\phi(\bar{\sigma_B} g \bar{\sigma_A}), \phi(\bar{\tau_B} g \bar{\tau_A})$ equal, i.e. $\sigma_B f \sigma_A = \tau_B f \tau_A$.

But we can simplify this condition, saying whenever $\bar{\tau_B}^{-1} \bar{\sigma_B} g \bar{\sigma_A} \bar{\tau_A}^{-1} = g$ we have $\tau_B^{-1} \sigma_B f \sigma_A \tau_A^{-1}$, and $\tau_B^{-1} \sigma_B$ and $\sigma_A \tau_A^{-1}$ can both take any value, in $Sym(A)$, $Sym(B)$ respectively, so our condition becomes:

There is some $f \in iso(A, B)$ such that whenever $\bar{\sigma_B} g \bar{\sigma_A} = g$ we have $\sigma_B f \sigma_A = f$.

It is my suspicion that such an $f$ will exist for any orbit $O$, and thus that a $\phi$ does exist, regardless of what we take $n = |A| = |B|$ to be.

Zoe Allen
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