# The Math Tea argument: must there be numbers we can neither describe nor define? Barcelona March 2023

This will be a talk 15 March 2023 for the Mathematics Department of the University of Barcelona, organized jointly with the Set Theory Seminar.

Abstract. According to the math tea argument, perhaps heard at a good afternoon tea,
there must be some real numbers that we can neither describe nor define, since there
are uncountably many real numbers, but only countably many definitions. Is it correct?
In this talk, I shall discuss the phenomenon of pointwise definable structures in
mathematics, structures in which every object has a property that only it exhibits. A
mathematical structure is Leibnizian, in contrast, if any pair of distinct objects in it
exhibit different properties. Is there a Leibnizian structure with no definable elements?
We shall discuss many interesting elementary examples, eventually working up to the
proof that every countable model of set theory has a pointwise definable extension, in
which every mathematical object is definable, including every real number, every
function, every set. We shall discuss the relevance for the math tea argument.

# Pointwise definable and Leibnizian extensions of models of arithmetic and set theory, MOPA seminar CUNY, November 2022

This will be an online talk for the MOPA Seminar at CUNY on 22 November 2022 1pm. Contact organizers for Zoom access.

Abstract. I shall introduce a flexible new method showing that every countable model of PA admits a pointwise definable end-extension, one in which every individual is definable without parameters. And similarly for models of set theory, in which one may also achieve the Barwise extension result—every countable model of ZF admits a pointwise definable end-extension to a model of ZFC+V=L, or indeed any theory arising in a suitable inner model. A generalization of the method shows that every model of arithmetic of size at most continuum admits a Leibnizian extension, and similarly in set theory.

# Pointwise definable and Leibnizian models of arithmetic and set theory, realized in end extensions of a given model, Notre Dame Logic Seminar, October 2022

This will be a talk for the Notre Dame logic seminar, 11 October 2022, 2pm in Hales-Healey Hall.

Abstract.  I shall present very new results on pointwise definable and Leibnizian end-extensions of models of arithmetic and set theory. Using the universal algorithm, I shall present a new flexible method showing that every countable model of PA admits a pointwise definable $\Sigma_n$-elementary end-extension. Also, any model of PA of size at most continuum admits an extension that is Leibnizian, meaning that any two distinct points are separated by some expressible property. Similar results hold in set theory, where one can also achieve V=L in the extension, or indeed any suitable theory holding in an inner model of the original model.