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Generalization of Scott's formula for retractions from generalized Alexandroff's cube
Authors:Jarosław Achinger
Affiliation:1. Institute of Mathematics Polish Academy of Sciences, Warsaw, Poland
Abstract:In the paper [2] the following theorem is shown: Theorem (Th. 3,5, [2]), If α=0 or δ= or α?δ, then a closure space X is an absolute extensor for the category of 〈α, δ〉 -closure spaces iff a contraction of X is the closure space of all 〈α, δ〉-filters in an 〈α, δ〉-semidistributive lattice. In the case when α=ω and δ=∞, this theorem becomes Scott's theorem: Theorem ([7]). A topological space X is an absolute extensor for the category of all topological spaces iff a contraction of X is a topological space of “Scott's open sets” in a continuous lattice. On the other hand, when α=0 and δ=ω, this theorem becomes Jankowski's theorem: Theorem ([4]). A closure space X is an absolute extensor for the category of all closure spaces satisfying the compactness theorem iff a contraction of X is a closure space of all filters in a complete Heyting lattice. But for separate cases of α and δ, the Theorem 3.5 from [2] is proved using essentialy different methods. In this paper it is shown that this theorem can be proved using, for retraction, one uniform formula. Namely it is proved that if α= 0 or δ= ∞ or α ? δ and (F_{alpha ,delta } left( L right) subseteq B_{alpha ,delta }^mathfrak{n} ) and if L is an 〈α, δ〉-semidistributive lattice, then the function $$r:{text{ }}B_{alpha ,delta }^mathfrak{n} to F_{alpha ,delta } left( L right)$$ such that for x ε ? ( (mathfrak{n}) ): (*) $$rleft( x right) = inf_L left{ {l in L|left( {forall A subseteq L} right)x in Cleft( A right) Rightarrow l in Cleft( A right)} right}$$ defines retraction, where C is a proper closure operator for (B_{alpha ,delta }^mathfrak{n} ) . It is also proved that the formula (*) defines retraction for all 〈α, δ〉, whenever L is an 〈α, δ〉 -pseudodistributive lattice. Moreover it is proved that when α=ω and δ=∞, the formula (*) defines identical retraction to the formula given in [7], and when α = 0 and δ=ω, the formula (*) defines identical retraction to the formula given in [4].
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