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Birkhoff's variety theorem for relative algebraic theories
An algebraic theory, sometimes called an equational theory, is a theory
defined by finitary operations and equations, such as the theories of groups
and of rings. It is well known that algebraic theories are equivalent to
finitary monads on . In this paper, we generalize this phenomenon
to locally finitely presentable categories using partial Horn logic. For each
locally finitely presentable category , we define an "algebraic
concept" relative to , which will be called an
-relative algebraic theory, and show that -relative
algebraic theories are equivalent to finitary monads on . Finally,
we generalize Birkhoff's variety theorem for classical algebraic theories to
our relative algebraic theories.Comment: 34 page
The number of clones determined by disjunctions of unary relations
We consider finitary relations (also known as crosses) that are definable via
finite disjunctions of unary relations, i.e. subsets, taken from a fixed finite
parameter set . We prove that whenever contains at least one
non-empty relation distinct from the full carrier set, there is a countably
infinite number of polymorphism clones determined by relations that are
disjunctively definable from . Finally, we extend our result to
finitely related polymorphism clones and countably infinite sets .Comment: manuscript to be published in Theory of Computing System
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