2021/05/20 by Cory Christopherson, Christopherson, Cory, John H. Johnson +1
Mathematics · #08A02 (Secondary) #54D35 #54D80 (Primary) 22A15 #FOS: Mathematics #General Topology (math.GN) #Rings and Algebras (math.RA)
paper · doi:10.48550/arxiv.2105.09723
We obtain algebraic characterizations of relative notions of size in a discrete semigroup that generalize the usual combinatorial notions of syndetic, thick, and piecewise syndetic sets. "Filtered" syndetic and piecewise syndetic sets were defined and applied earlier by Shuungula, Zelenyuk, and Zelenyuk [24]. Other instances of these relative notions of size have appeared explicitly (and more often implicitly) in the literature related to the algebraic structure of the Stone-Čech compactification. Building on this prior work, we observe a natural duality and demonstrate how these notions of size may be composed to characterize previous notions of size (like piecewise syndetic sets) and serve as a convenient description for new notions of size.