Distributive Semilattices as Retracts of Ultraboolean Ones; Functorial Inverses without Adjunction
نویسندگان
چکیده
A 〈∨, 0〉-semilattice is ultraboolean, if it is a directed union of finite Boolean 〈∨, 0〉-semilattices. We prove that every distributive 〈∨, 0〉-semilattice is a retract of some ultraboolean 〈∨, 0〉-semilattice. This is established by proving that every finite distributive 〈∨, 0〉-semilattice is a retract of some finite Boolean 〈∨, 0〉-semilattice, and this in a functorial way. This result is, in turn, obtained as a particular case of a category-theoretical result that gives sufficient conditions, for a functor Π, to admit a right inverse. The particular functor Π used for the abovementioned result about ultraboolean semilattices has neither a right nor a left adjoint.
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