finite subsets proof

Question

If $A$ is an infinite set then $\mathcal{F}(A),$ the set of finite subsets of $A$ has the same cardinality as $A$. Is the following proof of the above fact correct?

Note that $|A_k|\leq |A^k|$ for any $k\geq 0,$ where $A_k$ is the set of subsets of $A$ of cardinality $k$ since the map $f : A_k\to A^k, f(\{a_1,\cdots, a_k\}) = (a_1,\cdots, a_k)$ is injective (indeed for two different subsets, at least one element differs, so that element will be mapped to a different tuple by $f$). Then $\mathcal{F}(A) = \cup_{k\geq 0} A_k$, which is a disjoint union as $A_i \cap A_j = \emptyset$ for $i\neq j$ (the cardinality of a finite set is unique). So $|\mathcal{F}(A)| = \sum_{k\geq 0} |A_k| \leq \sum_{k\geq 0}|A^k| = \sum_{k\geq 1}|A^k| = \sum_{k\geq 1} |A| = \aleph_0 |A|$. Also, $g : A\to A_1, g(a) = \{a\}$ is clearly a bijection, so $|A| = |A_1| \leq |\mathcal{F}(A)|$ as $A_1\subseteq \mathcal{F}(A)$ and if $B\subseteq A$ then there is clearly an injective (identity) function from $B$ to $A.$ So $|\mathcal{F}(A)| = |A|$ by transitivity (for infinite sets this also applies).

Answer 1

Small issue: the function $f : A_k \to A^k$ you wrote down is not well-defined! For example, should $f(\{2,3\}) = f(\{3,2\})$ be $(2,3)$ or $(3,2)$? You gave no way to decide. You could try to fix this in a few ways:

1. Put a total order on $A$ (if you assume the axiom of choice, this is always possible), and then define $f$ so that it lists the elements in ascending order.
2. Put a total order on every element of $A_k$ (if you assume the axiom of choice, this is always possible), and then define $f$ so that it lists the elements in ascending order.
3. Redesign the argument: instead build a surjection $A^k \to A_k$.

To me, approach #3 feels most natural -- there is an "obvious map" $(a_1, \dots, a_k) \mapsto \{a_1, \dots, a_k\}$ that feels like it should be a surjection. The only issue is that $\lvert \{a_1, \dots, a_k\} \rvert$ need not equal $k$! So we have the wrong codomain: instead, this is a surjection $A^k \to A_{\leq k}$. This shows $\lvert A_{\leq k} \rvert \leq \lvert A^k \rvert$, and we also have $\lvert A_k \rvert \leq \lvert A_{\leq k} \rvert$, so transitively $\lvert A_k \rvert \leq \lvert A^k \rvert$.

Another small issue: you definitely showed $\lvert \mathcal{F}(A) \rvert \leq \aleph_0 \lvert A \rvert$, and that $\lvert A \rvert \leq \lvert \mathcal{F}(A) \rvert$. For your proof to be complete, you should say somewhere that $\aleph_0 \lvert A \rvert = \lvert A \rvert$.

Edit: as a side note, I should also mention that it's necessary to assume the axiom of choice here: choice is equivalent to the claim that $a^2 = a$ for all infinite cardinals $a$, and if we had $\lvert A \rvert^2 > \lvert A \rvert$ here the claim would be false.