## Turing computable embeddings.(English)Zbl 1123.03026

Summary: In [W. Calvert, D. Cummins, J. F. Knight and S. Miller, “Comparing classes of finite structures”, Algebra Logic 43, No. 6, 374–392 (2004; Zbl 1097.03026)], two different effective versions of Borel embedding are defined. The first, called computable embedding, is based on uniform enumeration reducibility, while the second, called Turing computable embedding, is based on uniform Turing reducibility. While [loc. cit.] focused mainly on computable embeddings, the present paper considers Turing computable embeddings. Although the two notions are not equivalent, we can show that they behave alike on the mathematically interesting classes chosen for investigation in [loc. cit.]. We give a “Pull-back Theorem”, saying that if $$\Phi$$ is a Turing computable embedding of $$K$$ into $$K'$$, then for any computable infinitary sentence $$\varphi$$ in the language of $$K'$$, we can find a computable infinitary sentence $$\varphi^*$$ in the language of $$K$$ such that for all $${\mathcal A}\in K$$, $${\mathcal A} \models\varphi^*$$ iff $$\Phi({\mathcal A})\models\varphi$$, and $$\varphi^*$$ has the same “complexity” as $$\varphi$$ (i.e., if $$\varphi$$ is computable $$\Sigma_\alpha$$, or computable $$\Pi_\alpha$$, for $$\alpha\geq 1$$, then so is $$\varphi^*)$$. The Pull-back Theorem is useful in proving non-embeddability, and it has other applications as well.

### MSC:

 03C57 Computable structure theory, computable model theory

Zbl 1097.03026
Full Text:

### References:

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