Abstract
The present paper contains a fairly detailed verification of a reasonably general form of the Interpretation Existence Lemma. In first approximation, this lemma tells us that if a theory U proves the consistency of V then U interprets V. What are theories here? What is proving the consistency of V in this context? The paper will explain how the lemma works, providing rather general answers to these questions. We apply the Interpretation Existence Lemma to verify well-known characterization theorems for interpretability: the Friedman Characterization and the Orey–Hájek Characterization. Finally, we provide three randomly chosen examples of application of the Interpretation Existence Lemma.
I am grateful to Ali Enayat, Paula Henk and Volodya Shavrukov for helpful comments, references and scholarly support. I thank the anonymous referee for his/her suggestions.
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Notes
- 1.
My most extensive earlier attempt to give a presentation of the proof was in [37]. However, the present one gives more detail and isolates the special features of the construction in a better way.
- 2.
One-dimensional means that the formula representing the object domain of the interpretation just has one free variable, apart from variables serving as parameters. See Sect. 2.2.
- 3.
Alternatively, we could start from the theory \(\mathsf{PA}^-\).
- 4.
The verification of the equivalence between axioms-interpretability and theorems-interpretability demands \(\Sigma _1\)-collection. See [37] for more details.
- 5.
Here we restrict ourselves to the case without parameters. We can add parameters to the definition in different ways, either by employing a parameter domain or by stipulating parameters locally for each model. These ways are more closely connected than one would think thanks to the Omitting Types Theorem. One can find examples of theories U and V, where V model-interprets U with parameters but does not do so without parameters.
- 6.
In my forthcoming paper Restricted Theories, I will treat the various properties of the complexity measure in more detail.
- 7.
We note that in the case of many sorted theories, we need to use a standard translation into a one-sorted theory to apply the definition.
- 8.
Our Sect. 4.2 contains a bit more detail on the construction of satisfaction predicates.
- 9.
The induction axioms are only for the original language of arithmetic.
- 10.
For the notion of synonymity of theories, see e.g. [40].
- 11.
I choose for this representation of \(P_\eta \) since it contains no ambiguities. One would like to say that \(P_\tau (x_0,\cdots ,x_{n-1})\) is \(P (c_0,\cdots ,c_{n-1}) \in \Delta ^\star \), were the \(c_i\) are the \(x_i\).
- 12.
We are a bit sloppy, since the finite set need not have a unique code. The insight works for every code of the displayed set.
- 13.
We allow the domain D to be empty. We consider the the empty relation to be an equivalence relation on the empty set.
- 14.
In \(\mathsf{S}^1_2\), the supremum of the elements of a sequence always exists. However, we do not need that insight for the present definition to be meaningful.
- 15.
We remind the reader that we allow axiomatizations of any complexity.
- 16.
We treat the A in the subscript of \( \Diamond _{\alpha ,A,n}\) as the number bounding the axiom of the theory A, which is par abus de langage, again A.
- 17.
It would be more natural to work with the theory \(\mathsf{FC}_\infty \) with class-variables for any arity. However, this would not fit with our policy to only consider finite signatures.
- 18.
Regrettably, I do not have a reference for the argument in full generality. It is proven for the case of \(\mathsf{ACA}_0\) in [32, pp. 311–312]. However, the argument there does not easily lift to the general case.
- 19.
We do have, by a direct application of the Interpretation Existence Lemma, that there is a large class of models \(\mathcal M\) that have a parametric internal strict end-extension \(\mathcal N\) that is elementarily equivalent to \(\mathcal M\). In fact, the recursively saturated models are in this class. So, the difference between elementary extension and extension that is elementarily equivalent is essential in the extended Feferman-Scott result. An open end in our story is that it is unclear whether there is a model \(\mathcal M\) with a parameter-free internal model \(\mathcal N\) that is a strict end-extension and elementarily equivalent to \(\mathcal M\).
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Visser, A. (2017). The Interpretation Existence Lemma. In: Jäger, G., Sieg, W. (eds) Feferman on Foundations. Outstanding Contributions to Logic, vol 13. Springer, Cham. https://doi.org/10.1007/978-3-319-63334-3_5
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