Notes on ultrafilter extensions of almost bounded structures
Fuente:
arXiv
Guardado en:
| Autor principal: | |
|---|---|
| Formato: | Preprint |
| Publicado: |
2025
|
| Materias: | |
| Acceso en línea: | |
| Etiquetas: |
Agregar Etiqueta
Sin Etiquetas, Sea el primero en etiquetar este registro!
|
| _version_ | 1866917933499809792 |
|---|---|
| author | Molnár, Zalán |
| author_facet | Molnár, Zalán |
| contents | We extend some of our earlier results on the interconnection between ultrafilter extensions, and ultrapowers. Throughout we restrict ourselves to relational structures with one binary relation. Recently it was shown that for bounded structures, where a universal finite bound on the maximal in-and out-degree is given, ultrafilter extensions are elementary extensions of the original structures. Comparing the constructions, it seems that the real challenge is when the degree has no global finite bound, or there are elements with infinite degree. This is a first step towards this direction by slightly relaxing the notion of boundedness, called almost bounded structures. Among others, we show that members of this class are still elementary substructures of their extensions, elementary embeddings can be lifted up to the extensions, moreover for the countable case, the extensions are isomorphic to certain ultrapowers. We also comment on the modal logics they generate. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2502_09451 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Notes on ultrafilter extensions of almost bounded structures Molnár, Zalán Logic Primary: 03B45, Secondary: 03C20 We extend some of our earlier results on the interconnection between ultrafilter extensions, and ultrapowers. Throughout we restrict ourselves to relational structures with one binary relation. Recently it was shown that for bounded structures, where a universal finite bound on the maximal in-and out-degree is given, ultrafilter extensions are elementary extensions of the original structures. Comparing the constructions, it seems that the real challenge is when the degree has no global finite bound, or there are elements with infinite degree. This is a first step towards this direction by slightly relaxing the notion of boundedness, called almost bounded structures. Among others, we show that members of this class are still elementary substructures of their extensions, elementary embeddings can be lifted up to the extensions, moreover for the countable case, the extensions are isomorphic to certain ultrapowers. We also comment on the modal logics they generate. |
| title | Notes on ultrafilter extensions of almost bounded structures |
| topic | Logic Primary: 03B45, Secondary: 03C20 |
| url | https://arxiv.org/abs/2502.09451 |