Speaker
Thibaut Kouptchinsky
TUWien
Talks at this conference:
Wednesday, 14:25, J431 
The limits of determinacy in thirdorder arithmetic 
Authors: Thibaut Kouptchinsky and Juan Aguilera This talk is about the foundations of mathematics, studying determinacy axioms derived from game theory, with a reverse mathematics point of view. Reverse Mathematics is an endeavour to compare theorems according to the “strength” of the axioms a mathematician needs to prove them (see [5]). We study a refined case of the proof from Martin of Borel determinacy [2], which showed how to use the existence of highorder objects when one wants to show that infinite games of increasing difficulty in the Borel hierarchy have winning strategies (and thus are called determined). The use of such principles had already been shown to be necessary by Friedmann [1]. In the terms that will be ours, Martin provided the final proof that \((2+\gamma)\)thorder arithmetic (\(\mathsf{Z}_{2+\gamma}\)) is the first to witness the determinacy of \(\Pi^0_{1+\gamma+2}\) GaleStewart games (\(\gamma < \omega_1^{\mathsf{CK}}\)). For our part, we will look in more detail at the situation in secondorder and thirdorder arithmetic, examining a paper by Montalbán and Shore [3]. We present a generalisation of their results, in some natural interpretation of thirdorder arithmetic about infinite games which complexity is described by the difference hierarchy of \(\Pi^0_4\) sets. Along the way, we underline a shift compared to the analogous situation in the countable case (about differences of \(\Pi^0_3\) sets), which is the object of the paper of Montalbán and Shore. Namely, we need less of the separation scheme than in the countable case. Finally, we use these generalisations following the results of Pacheco and Yokoyama to [4] show that, while \[\forall m \in \omega \mathsf{Z}_{2+\gamma} \vdash (\Pi^0_{1+\gamma+2})_m\text{}\mathsf{Det} \qquad \text{but} \qquad \mathsf{Z}_{2+\gamma} \not\vdash \forall m (\Pi^0_{1+\gamma+2})_m\text{}\mathsf{Det},\]the last theorem is equivalent to a form of reflection principle. Bibliography
