23,143 research outputs found
Complexity of matrix problems
In representation theory, the problem of classifying pairs of matrices up to
simultaneous similarity is used as a measure of complexity; classification
problems containing it are called wild problems. We show in an explicit form
that this problem contains all classification matrix problems given by quivers
or posets. Then we prove that it does not contain (but is contained in) the
problem of classifying three-valent tensors. Hence, all wild classification
problems given by quivers or posets have the same complexity; moreover, a
solution of any one of these problems implies a solution of each of the others.
The problem of classifying three-valent tensors is more complicated.Comment: 24 page
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
Representations of quivers and mixed graphs
This is a survey article for "Handbook of Linear Algebra", 2nd ed., Chapman &
Hall/CRC, 2014. An informal introduction to representations of quivers and
finite dimensional algebras from a linear algebraist's point of view is given.
The notion of quiver representations is extended to representations of mixed
graphs, which permits one to study systems of linear mappings and bilinear or
sesquilinear forms. The problem of classifying such systems is reduced to the
problem of classifying systems of linear mappings
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