130 research outputs found
The MSO+U theory of (N, <) is undecidable
We consider the logic MSO+U, which is monadic second-order logic extended
with the unbounding quantifier. The unbounding quantifier is used to say that a
property of finite sets holds for sets of arbitrarily large size. We prove that
the logic is undecidable on infinite words, i.e. the MSO+U theory of (N,<) is
undecidable. This settles an open problem about the logic, and improves a
previous undecidability result, which used infinite trees and additional axioms
from set theory.Comment: 9 pages, with 2 figure
Isometry groups among topological groups
It is shown that a topological group G is topologically isomorphic to the
isometry group of a (complete) metric space iff G coincides with its
G-delta-closure in the Rajkov completion of G (resp. if G is Rajkov-complete).
It is also shown that for every Polish (resp. compact Polish; locally compact
Polish) group G there is a complete (resp. proper) metric d on X inducing the
topology of X such that G is isomorphic to Iso(X,d) where X = l_2 (resp. X = Q;
X = Q\{point} where Q is the Hilbert cube). It is demonstrated that there are a
separable Banach space E and a nonzero vector e in E such that G is isomorphic
to the group of all (linear) isometries of E which leave the point e fixed.
Similar results are proved for an arbitrary complete topological group.Comment: 30 page
A topological characterization of LF-spaces
We present a topological characterization of LF-spaces and detect small
box-products that are (locally) homeomorphic to LF-spaces.Comment: 16 page
Stable graphs of bounded twin-width
We prove that every class of graphs that is monadically stable
and has bounded twin-width can be transduced from some class with bounded
sparse twin-width. This generalizes analogous results for classes of bounded
linear cliquewidth and of bounded cliquewidth. It also implies that monadically
stable classes of bounded twin-widthare linearly -bounded.Comment: 44 pages, 2 figure
Detecting topological groups which are (locally) homeomorphic to LF-spaces
We present a simple-to-apply criterion for recognizing topological groups
that are (locally) homeomorphic to LF-spaces.Comment: 10 page
Indiscernibles and Flatness in Monadically Stable and Monadically NIP Classes
Monadically stable and monadically NIP classes of structures were initially
studied in the context of model theory and defined in logical terms. They have
recently attracted attention in the area of structural graph theory, as they
generalize notions such as nowhere denseness, bounded cliquewidth, and bounded
twinwidth.
Our main result is the - to the best of our knowledge first - purely
combinatorial characterization of monadically stable classes of graphs, in
terms of a property dubbed flip-flatness. A class of graphs is
flip-flat if for every fixed radius , every sufficiently large set of
vertices of a graph contains a large subset of vertices
with mutual distance larger than , where the distance is measured in some
graph that can be obtained from by performing a bounded number of
flips that swap edges and non-edges within a subset of vertices. Flip-flatness
generalizes the notion of uniform quasi-wideness, which characterizes nowhere
dense classes and had a key impact on the combinatorial and algorithmic
treatment of nowhere dense classes. To obtain this result, we develop tools
that also apply to the more general monadically NIP classes, based on the
notion of indiscernible sequences from model theory. We show that in
monadically stable and monadically NIP classes indiscernible sequences impose a
strong combinatorial structure on their definable neighborhoods. All our proofs
are constructive and yield efficient algorithms.Comment: v2: revised presentation; renamed flip-wideness to flip-flatness;
changed the title from "Indiscernibles and Wideness [...]" to "Indiscernibles
and Flatness [...]
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