Longer chains of idempotents in βG

• Autorzy: Neil Hindman , Dona Strauss , Yevhen Zelenyuk
• Języki publikacji: EN

Abstrakty

EN

Given idempotents e and f in a semigroup, e ≤ f if and only if e = fe = ef. We show that if G is a countable discrete group, p is a right cancelable element of G* = βG∖G, and λ is a countable ordinal, then there is a strictly decreasing chain $⟨q_{σ}⟩_{σ<λ}$ of idempotents in $C_p$, the smallest compact subsemigroup of G* with p as a member. We also show that if S is any infinite subsemigroup of a countable group, then any nonminimal idempotent in S* is the largest element of such a strictly decreasing chain of idempotents. (It had been an open question whether there was a strictly decreasing chain $⟨q_{σ}⟩_{σ<ω+1}$ in ℕ*.) As other corollaries we show that if S is an infinite right cancellative and weakly left cancellative discrete semigroup, then βS contains a decreasing chain of idempotents of reverse order type λ for every countable ordinal λ and that if S is an infinite cancellative semigroup then the set U(S) of uniform ultrafilters contains such decreasing chains.

Kategorie tematyczne

• 22A15: Structure of topological semigroups
• 54H13: Topological fields, rings, etc.
• 54D80: Special constructions of spaces (spaces of ultrafilters, etc.)

• Wydawca: Institute of Mathematics Polish Academy of Sciences
• Czasopismo: Fundamenta Mathematicae
• Rocznik: 2013
• Tom: 220
• Numer: 3
• Strony: 243-261

Daty

wydano

2013

Twórcy

autor

Neil Hindman

• Department of Mathematics, Howard University, Washington, DC 20059, U.S.A.

autor

Dona Strauss

• Department of Pure Mathematics, University of Leeds, Leeds LS2 9J2, UK

autor

Yevhen Zelenyuk

• School of Mathematics, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa

Identyfikatory

DOI

10.4064/fm220-3-5