Past exam of the mathematics course of the University of Cambridge / 2003 / iii / Paper 19 / 4 / Solution

An inner model of a universe of ZF is a transitive class $M$ containing every ordinal and satisfying the required set-theoretic axioms with the inherited membership relation. Transitivity means that every member of a member of $M$ belongs to $M$. Requiring all ordinals distinguishes an inner model from a transitive set first-order model such as $V_{\kappa }$: a set first-order model has bounded ordinal height. One may specify an inner model of ZF, ZFC or a stronger theory; choice is not built into the term unless specified.

The fundamental example is the constructible universe. Form the constructible hierarchyHere definable power set means subsets definable over the set structure $(L_{\alpha },\in )$ by first-order formulas with finitely many parameters from that level. Each level is transitive, the levels increase, and $\mathrm{Ord}\cap L_{\alpha }=\alpha$, by induction. Thus $L$ is transitive and contains all ordinals.

The construction supplies a definable well-order: order objects first by their earliest construction stage, then by a least first-order formula code and a finite tuple of parameters in the already constructed level's well-order. Recursively extend the earlier well-order at each successor stage and unite compatible orders at limits. First-order formula codes and finite tuples of well-ordered parameters are well-orderable. This yields a canonical global ordering definable within $L$; its restrictions to sets will give choice once the ZF axioms are checked.

For that check, a useful reflection argument is explicit. Given finitely many first-order formulas and a starting level, pass to a higher level containing witnesses for their existential subformulas on all parameter tuples from the current level, choosing least witnesses in the canonical order. There are set-many tuples, so ambient replacement bounds their witness stages. Repeat for $\omega$ steps and take the union level $L_{\theta }$. Every relevant witness with parameters in that level appears at a later step. Induction on first-order formulas, with this witness property for the existential case, proves that $L_{\theta }$ reflects the chosen finite list of first-order formulas of $L$.

Separation on a constructible set is then definability over a sufficiently large reflecting level, so its subset appears at the next level. For replacement, reflect the first-order formula specifying a functional image and its required existence statements. All inputs belong to a level containing the domain as a set; all their unique outputs then lie in one reflecting level. The image is definable there and hence is constructible. For power set, the ambient set of constructible subsets of $x$ has bounded construction stages by replacement in the ambient universe. At a level containing them all, this internal collection is definable by the bounded condition $y\subseteq x$, and therefore is itself in $L$. Pairing, union and axiom of infinity follow directly from definability over sufficiently large levels, while extensionality and foundation follow from transitivity. Thus $L\models \mathsf{ZF}$, and its canonical ordering gives $L\models \mathsf{AC}$.

This proves the important relative-consistency applicationOne does not conclude that choice is true in the original universe. Instead the inner model supplies a possibly smaller universe in which choice holds.

Constructibility also supplies GCH. Here is the cardinal-counting argument, making clear the role of the condensation lemma for the constructible universe. Work inside $L$. If $x\subseteq \kappa$ is constructible, take a large limit $L_{\theta }$ containing $x$ and an elementary hull of $\kappa \cup \{\kappa ,x\}$ of size $\kappa$. Its transitive collapse is $L_{\beta }$ by condensation. The collapse fixes every ordinal below $\kappa$, and hence fixes $x$ and $\kappa$. Since the hull has size $\kappa$, $\beta <{\kappa }^{+}$. Therefore every constructible subset of $\kappa$ appears below $L_{{\kappa }^{+}}$.

The condensation fact expresses the rigidity of this definability construction: first-order formula codes, finite parameter tuples and satisfaction in set-sized lower levels are preserved by the collapse. Inducting over the hull's ordinal indices identifies each collapsed lower level with the corresponding $L$-level; elementarity supplies every definable subset coded by parameters in that collapsed lower level. Taking unions identifies the entire collapsed hull with a level $L_{\beta }$.

Induction on construction stages gives $|L_{\gamma }|\le |\gamma |+{\aleph }_0$: each successor has only countably many first-order formula schemes with finite parameter tuples, and limits take unions. Hence $|L_{{\kappa }^{+}}|={\kappa }^{+}$ for infinite $\kappa$. The preceding containment gives $2^{\kappa }\le {\kappa }^{+}$ in $L$, and Cantor's theorem gives the reverse inequality. ConsequentlyAll these cardinal computations are internal to $L$; they need not equal the ambient cardinal computations.

Inner models are also useful for analyzing stronger axioms. If $\kappa$ is inaccessible in $V$, it remains an uncountable regular cardinal in $L$, because a shorter cofinal sequence in $L$ would already be one in $V$. For each ordinal $\lambda <\kappa$, its constructible subsets are fewer than $\kappa$ even in the internal cardinal comparison: in $V$, $\mathcal{P}(\lambda )$ has size below $\kappa$, so its constructible part cannot have an $L$-well-order of length at least $\kappa$. Thus it remains strong limit. Accordingly $L$ supplies relative-consistency information compatible with inaccessibles and GCH. Stronger embedding or measure properties require extra analysis, since the witnessing objects may disappear upon passing to an inner model.

There is a useful limitation: no uniform inner-model construction can refute choice from every starting ZF universe. Every inner model $M$ of ZF contains the ambient $L$. Indeed, induction gives $L_{\alpha }^M=L_{\alpha }$: the same set-sized level has the same satisfaction relation for definitions, and limit stages take the same unions. If the starting universe already satisfies $V=L$, any inner model containing all ordinals must therefore be the whole universe, where choice holds. To obtain a choice-failing first-order model uniformly one needs a different method, such as the symmetry restriction in the preceding solution. Inner-model arguments establish consistency and preservation through carefully controlled smaller universes; they do not freely manufacture arbitrary desired axiom failures.