A Generalization of the {\L}o\'s-Tarski Preservation Theorem - Dissertation Summary

Abhisekh Sankaran

This article gives a summary of the author's Ph.D. dissertation (arXiv:1609.06297). In addition to an overview of notions and results, it also provides sketches of various proofs and simplified presentations of certain abstract results of the dissertation, that concern tree representations of structures. Further, some extensions of the dissertation results are presented. These include the connections of the model-theoretic notions introduced in the thesis with fixed parameter tractability and notions in the structure theory of sparse graph classes. The constructive aspects of the proofs of the model-theoretic results of the dissertation are used to obtain (algorithmic) meta-kernels for various dense graphs such as graphs of bounded clique-width and subclasses of these like $m$-partite cographs and graph classes of bounded shrub-depth. Finally, the article presents updated definitions and results concerning the notion of logical fractals which is a generalization of the Equivalent Bounded Substructure Property from the dissertation. In particular, our results show that (natural finitary adaptations of) both the upward and downward versions of the L\"owenheim-Skolem theorem from classical model theory can be recovered in a variety of algorithmically interesting settings, and further in most cases, in effective form and even for logics beyond first order logic.

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