340 research outputs found
Infinite Lexicographic Products
We generalize the lexicographic product of first-order structures by
presenting a framework for constructions which, in a sense, mimic iterating the
lexicographic product infinitely and not necessarily countably many times. We
then define dense substructures in infinite products and show that any
countable product of countable transitive homogeneous structures has a unique
countable dense substructure, up to isomorphism. Furthermore, this dense
substructure is transitive, homogeneous and elementarily embeds into the
product. This result is then utilized to construct a rigid elementarily
indivisible structure.Comment: 20 pages, 3 figure
Invariant measures concentrated on countable structures
Let L be a countable language. We say that a countable infinite L-structure M
admits an invariant measure when there is a probability measure on the space of
L-structures with the same underlying set as M that is invariant under
permutations of that set, and that assigns measure one to the isomorphism class
of M. We show that M admits an invariant measure if and only if it has trivial
definable closure, i.e., the pointwise stabilizer in Aut(M) of an arbitrary
finite tuple of M fixes no additional points. When M is a Fraisse limit in a
relational language, this amounts to requiring that the age of M have strong
amalgamation. Our results give rise to new instances of structures that admit
invariant measures and structures that do not.Comment: 46 pages, 2 figures. Small changes following referee suggestion
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