• J. D. Hamkins and K. J. Williams, “The $\Sigma_1$-definable universal finite sequence,” Journal of Symbolic Logic, 2021.
  [Bibtex]

  @ARTICLE{HamkinsWilliams2021:The-universal-finite-sequence,
  author = {Joel David Hamkins and Kameryn J. Williams},
  title = {The $\Sigma_1$-definable universal finite sequence},
  journal = {Journal of Symbolic Logic},
  year = {2021},
  volume = {},
  number = {},
  pages = {},
  month = {},
  note = {},
  abstract = {},
  keywords = {},
  eprint = {1909.09100},
  archivePrefix = {arXiv},
  primaryClass = {math.LO},
  source = {},
  doi = {10.1017/jsl.2020.59},
  }

Abstract. We introduce the $\Sigma_1$-definable universal finite sequence and prove that it exhibits the universal extension property amongst the countable models of set theory under end-extension. That is, (i) the sequence is $\Sigma_1$-definable and provably finite; (ii) the sequence is empty in transitive models; and (iii) if $M$ is a countable model of set theory in which the sequence is $s$ and $t$ is any finite extension of $s$ in this model, then there is an end extension of $M$ to a model in which the sequence is $t$. Our proof method grows out of a new infinitary-logic-free proof of the Barwise extension theorem, by which any countable model of set theory is end-extended to a model of $V=L$ or indeed any theory true in a suitable submodel of the original model. The main theorem settles the modal logic of end-extensional potentialism, showing that the potentialist validities of the models of set theory under end-extensions are exactly the assertions of S4. Finally, we introduce the end-extensional maximality principle, which asserts that every possibly necessary sentence is already true, and show that every countable model extends to a model satisfying it.

• The universal algorithm,
  • J. D. Hamkins and H. W. Woodin, “The universal finite set,” Mathematics ArXiv, p. 1–16, 2017.
    [Bibtex]

    @ARTICLE{HamkinsWoodin:The-universal-finite-set,
    author = {Joel David Hamkins and W. Hugh Woodin},
    title = {The universal finite set},
    journal = {Mathematics ArXiv},
    year = {2017},
    volume = {},
    number = {},
    pages = {1--16},
    month = {},
    note = {Manuscript under review},
    abstract = {},
    keywords = {under-review},
    source = {},
    doi = {},
    eprint = {1711.07952},
    archivePrefix = {arXiv},
    primaryClass = {math.LO},
    url = {http://jdh.hamkins.org/the-universal-finite-set},
    }

• The modal logic of arithmetic potentialism,
  • J. D. Hamkins, “The modal logic of arithmetic potentialism and the universal algorithm,” Mathematics ArXiv, p. 1–35, 2018.
    [Bibtex]

    @ARTICLE{Hamkins:The-modal-logic-of-arithmetic-potentialism,
    author = {Joel David Hamkins},
    title = {The modal logic of arithmetic potentialism and the universal algorithm},
    journal = {Mathematics ArXiv},
    year = {2018},
    volume = {},
    number = {},
    pages = {1--35},
    month = {},
    eprint = {1801.04599},
    archivePrefix = {arXiv},
    primaryClass = {math.LO},
    note = {Under review},
    url = {http://wp.me/p5M0LV-1Dh},
    abstract = {},
    keywords = {under-review},
    source = {},
    doi = {},
    }

• A new proof of the Barwise extension theorem
• Kameryn’s blog post about the paper