Finite set theory and the Axiom of the Power Set

Since you don't include Foundation, we can get a very strange model indeed. Namely, consider an $\omega$-model $M$ of ZF-Foundation+"there is an infinite set $A$ of Quine atoms" (a Quine atom is a set satisfying $x=\{x\}$, and the existence of many Quine atoms is consistent with ZF-Foundation). Now look at the structure $W$ which is the closure of $$V_\omega^M\cup A\cup\{[A]^n: n\in\mathbb{N}\}$$ under Pairing, Union, and Separation. Here "$[X]^k$" denotes the set of $k$-element subsets of $X$.

$W$ satisfies finite set theory, but not Powerset ($\mathcal{P}(A)$ doesn't exist) or Choice (there is no choice function for the set of two-element subsets of $A$).

I am not sure whether $W$ satisfies Replacement, but I suspect it does, and I don't immediately see how to add a failure of Replacement. Meanwhile, incorporating Foundation seems difficult.


If we define $W$ as the Pairing/Union/Separation-closure of $$V_\omega^M\cup A\cup\mathcal{P}(A)\cup\{[\mathcal{P}(A)]^n: n\in\mathbb{N}\}$$ instead, where $M$ satisfies "$A$ is an amorphous set of Quine atoms", then I believe we get a model of finite set theory together with the negation of Replacement and Powerset; powerset fails since we don't have $\mathcal{P}^2(A)$, and Replacement fails since the image of the map sending each subset of $A$ to its cardinality if it is finite, or the cardinality of its complement otherwise, does not exist.


Here is a model of your "finite set theory" (including Foundation) in which there is an infinite set and Power Set and Replacement fail. Let $A=\{\emptyset,\{\emptyset\},\{\{\emptyset\}\},\{\{\{\emptyset\}\}\},\dots\}$ and let $M$ be the closure of $V_\omega\cup\{A\}$ under Pairing, Union, and taking subsets (so if $X\in M$ and $Y\subseteq X$ then $Y\in M$). It is clear that $M$ satisfies all of your axioms except possibly the negation of Infinity. To prove the negation of Infinity, note that if $X\in M$, then the transitive closure of $X$ contains only finitely many elements of cardinality $>1$ (since this is true of $A$ and for every element of $V_\omega$, and is preserved by taking pairs, unions, and subsets). So $M$ cannot contain any inductive set.

However, $M$ does contain an infinite set, namely $A$. It is also clear that $M$ fails to satisfy Power Set, since $M$ contains every subset of $A$ but $\mathcal{P}(A)\not\in M$ (either by the criterion mentioned above, or by noting that every element of $M$ is countable). Replacement also fails, since the usual recursive definition of the obvious bijection $A\to\omega$ can be implemented in $M$, so Replacement would imply $\omega$ is a set.

This model does satisfy Choice in the form "if $X$ is a set of disjoint nonempty sets then there is a set that contains one element from each of them" (since $M$ contains all subsets of $\bigcup X$). It does not satisfy Choice in the form "if $X$ is a set of nonempty sets then there exists a choice function $X\to\bigcup X$", basically because it is very hard to construct functions as sets in $M$ (for instance, if $X=A\setminus\{\emptyset\}$, the unique choice function for $X$ would have infinitely many 2-element sets in its transitive closure). Probably it is possible to build a model where any reasonable form of Choice fails, but I don't know how exactly to do that at the moment.