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Almost maximal topologies on groups

100%

Zelenyuk Y.

Fundamenta Mathematicae

2016

tom 234

nr 1

91-100

Let G be a countably infinite group. We show that for every finite absolute coretract S, there is a regular left invariant topology on G whose ultrafilter semigroup is isomorphic to S. As consequences we prove that (1) there is a right maximal idempotent in βG∖G which is not strongly right maximal, and (2) for each combination of the properties of being extremally disconnected, irresolvable, and nodec, except for the combination (-,-,+), there is a corresponding regular almost maximal left invariant topology on G.

Longer chains of idempotents in βG

51%

Hindman N., Strauss D., Zelenyuk Y.

Fundamenta Mathematicae

2013

tom 220

nr 3

243-261

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.

Ograniczanie wyników

2 Fundamenta Mathematicae

2 Zelenyuk Y.

1 Hindman N.

1 Strauss D.

1 2016

1 2013

Wyniki wyszukiwania

Almost maximal topologies on groups

Longer chains of idempotents in βG