Here is the program for Logic seminar on Monday 20.4.2026 (16:40, C116). Please see the page of the seminar for more details https://logika.ff.cuni.cz/en/research/seminar/.

Speaker: Alberto Soria Rojas (Institute of Mathematics, Prague)

Title: The Rudin-Keisler order, Blass-Frolik sums and u-limits.

Abstract:

The Rudin-Keisler order on ultrafilters is a fundamental tool that shows
us that not all ultrafilters are created equal. Among the free
ultrafilters on omega, selective ultrafilters are minimal with respect
to said order. In this talk we will present this fact, we will introduce
the concept of Blass-Frolik sums, u-limits and their relationship with
the Rudin-Keisler order.