目：Set Theoretic Tools in Abstract Density Problem

间：2018.05.21（星期一）, 14:00--16:00

点：数学院南楼N820

: Maximal almost disjoint family and almost containment sequence for sets of natural numbers modulo a non-principal ideal are common set theoretic objects in many researches. We will show how these tools can be used to prove the following: (1) There exists a “rich” abstract upper density on sets of natural numbers such that its zero-sets form precisely the ideal of all finite sets. (2) There does not exist any such abstract upper density which is “too rich”. These results answer a question of Georges Grekos. The results mentioned above are from a joint paper by Mauro Di Nasso and the speaker.