839 research outputs found
Universality of group embeddability
Working in the framework of Borel reducibility, we study various notions of
embeddability between groups. We prove that the embeddability between countable
groups, the topological embeddability between (discrete) Polish groups, and the
isometric embeddability between separable groups with a bounded bi-invariant
complete metric are all invariantly universal analytic quasi-orders. This
strengthens some results from [Wil14] and [FLR09].Comment: Minor corrections. 15 pages, submitte
Computational Processes and Incompleteness
We introduce a formal definition of Wolfram's notion of computational process
based on cellular automata, a physics-like model of computation. There is a
natural classification of these processes into decidable, intermediate and
complete. It is shown that in the context of standard finite injury priority
arguments one cannot establish the existence of an intermediate computational
process
The complexity of classifying separable Banach spaces up to isomorphism
It is proved that the relation of isomorphism between separable Banach spaces
is a complete analytic equivalence relation, i.e., that any analytic
equivalence relation Borel reduces to it. Thus, separable Banach spaces up to
isomorphism provide complete invariants for a great number of mathematical
structures up to their corresponding notion of isomorphism. The same is shown
to hold for (1) complete separable metric spaces up to uniform homeomorphism,
(2) separable Banach spaces up to Lipschitz isomorphism, and (3) up to
(complemented) biembeddability, (4) Polish groups up to topological
isomorphism, and (5) Schauder bases up to permutative equivalence. Some of the
constructions rely on methods recently developed by S. Argyros and P. Dodos
Real Computational Universality: The Word Problem for a class of groups with infinite presentation
The word problem for discrete groups is well-known to be undecidable by a
Turing Machine; more precisely, it is reducible both to and from and thus
equivalent to the discrete Halting Problem.
The present work introduces and studies a real extension of the word problem
for a certain class of groups which are presented as quotient groups of a free
group and a normal subgroup. Most important, the free group will be generated
by an uncountable set of generators with index running over certain sets of
real numbers. This allows to include many mathematically important groups which
are not captured in the framework of the classical word problem.
Our contribution extends computational group theory from the discrete to the
Blum-Shub-Smale (BSS) model of real number computation. We believe this to be
an interesting step towards applying BSS theory, in addition to semi-algebraic
geometry, also to further areas of mathematics.
The main result establishes the word problem for such groups to be not only
semi-decidable (and thus reducible FROM) but also reducible TO the Halting
Problem for such machines. It thus provides the first non-trivial example of a
problem COMPLETE, that is, computationally universal for this model.Comment: corrected Section 4.
Invariantly universal analytic quasi-orders
We introduce the notion of an invariantly universal pair (S,E) where S is an
analytic quasi-order and E \subseteq S is an analytic equivalence relation.
This means that for any analytic quasi-order R there is a Borel set B invariant
under E such that R is Borel bireducible with the restriction of S to B. We
prove a general result giving a sufficient condition for invariant
universality, and we demonstrate several applications of this theorem by
showing that the phenomenon of invariant universality is widespread. In fact it
occurs for a great number of complete analytic quasi-orders, arising in
different areas of mathematics, when they are paired with natural equivalence
relations.Comment: 31 pages, 1 figure, to appear in Transactions of the American
Mathematical Societ
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