16,346 research outputs found
Generating infinite symmetric groups
Let S=Sym(\Omega) be the group of all permutations of an infinite set \Omega.
Extending an argument of Macpherson and Neumann, it is shown that if U is a
generating set for S as a group, respectively as a monoid, then there exists a
positive integer n such that every element of S may be written as a group word,
respectively a monoid word, of length \leq n in the elements of U.
Several related questions are noted, and a brief proof is given of a result
of Ore's on commutators that is used in the proof of the above result.Comment: 9 pages. See also http://math.berkeley.edu/~gbergman/papers To
appear, J.London Math. Soc.. Main results as in original version. Starting on
p.4 there are references to new results of others including an answer to
original Question 8; "sketch of proof" of Lemma 11 is replaced by a full
proof; 6 new reference
Critical point for the strong field magnetoresistance of a normal conductor/perfect insulator/perfect conductor composite with a random columnar microstructure
A recently developed self-consistent effective medium approximation, for
composites with a columnar microstructure, is applied to such a
three-constituent mixture of isotropic normal conductor, perfect insulator, and
perfect conductor, where a strong magnetic field {\bf B} is present in the
plane perpendicular to the columnar axis. When the insulating and perfectly
conducting constituents do not percolate in that plane, the
microstructure-induced in-plane magnetoresistance is found to saturate for
large {\bf B}, if the volume fraction of the perfect conductor is greater
than that of the perfect insulator . By contrast, if , that
magnetoresistance keeps increasing as without ever saturating. This
abrupt change in the macroscopic response, which occurs when , is a
critical point, with the associated critical exponents and scaling behavior
that are characteristic of such points. The physical reasons for the singular
behavior of the macroscopic response are discussed. A new type of percolation
process is apparently involved in this phenomenon.Comment: 4 pages, 1 figur
Direct limits and fixed point sets
For which groups G is it true that whenever we form a direct limit of G-sets,
dirlim_{i\in I} X_i, the set of its fixed points, (dirlim_I X_i)^G, can be
obtained as the direct limit dirlim_I(X_i^G) of the fixed point sets of the
given G-sets? An easy argument shows that this holds if and only if G is
finitely generated.
If we replace ``group G'' by ``monoid M'', the answer is the less familiar
condition that the improper left congruence on M be finitely generated.
Replacing our group or monoid with a small category E, the concept of set on
which G or M acts with that of a functor E --> Set, and the concept of fixed
point set with that of the limit of a functor, a criterion of a similar nature
is obtained. The case where E is a partially ordered set leads to a condition
on partially ordered sets which I have not seen before (pp.23-24, Def. 12 and
Lemma 13).
If one allows the {\em codomain} category Set to be replaced with other
categories, and/or allows direct limits to be replaced with other kinds of
colimits, one gets a vast area for further investigation.Comment: 28 pages. Notes on 1 Aug.'05 revision: Introduction added; Cor.s 9
and 10 strengthened and Cor.10 added; section 9 removed and section 8
rewritten; source file re-formatted for Elsevier macros. To appear, J.Al
On group topologies determined by families of sets
Let be an abelian group, and a downward directed family of subsets of
. The finest topology on under which converges to
has been described by I.Protasov and E.Zelenyuk. In particular, their
description yields a criterion for to be Hausdorff. They then
show that if is the filter of cofinite subsets of a countable subset
, there is a simpler criterion: is Hausdorff if and
only if for every and positive integer , there is an
such that does not lie in the n-fold sum .
In this note, their proof is adapted to a larger class of families . In
particular, if is any infinite subset of , any regular infinite
cardinal , and the set of complements in of
subsets of cardinality , then the above criterion holds.
We then give some negative examples, including a countable downward directed
set of subsets of not of the above sort which satisfies the
"" condition, but does not induce a Hausdorff
topology.
We end with a version of our main result for noncommutative .Comment: 10 pages. Copy at http://math.berkeley.edu/~gbergman/papers may be
updated more frequently than arXiv cop
More Abelian groups with free duals
In answer to a question of A. Blass, J. Irwin and G. Schlitt, a subgroup G of
the additive group Z^{\omega} is constructed whose dual, Hom(G,Z), is free
abelian of rank 2^{\aleph_0}. The question of whether Z^{\omega} has subgroups
whose duals are free of still larger rank is discussed, and some further
classes of subgroups of Z^{\omega} are noted.Comment: 9 pages. Copy at http://math.berkeley.edu/~gbergman/papers may be
updated more frequently than arXiv cop
Can one factor the classical adjoint of a generic matrix?
Let k be a field, n a positive integer, X a generic nxn matrix over k (i.e.,
a matrix (x_{ij}) of n^2 independent indeterminates over the polynomial ring
k[x_{ij}]), and adj(X) its classical adjoint. It is shown that if char k=0 and
n is odd, then adj(X) is not the product of two noninvertible nxn matrices over
k[x_{ij}]. If n is even and >2, a restricted class of nontrivial factorizations
occur. The nonzero-characteristic case remains open.
The operation adj on matrices arises from the (n-1)st exterior power functor
on modules; the same question can be posed for matrix operations arising from
other functors.Comment: Revised version contains answer to "even n" question left open in
original version. (Answer due to Buchweitz & Leuschke; simple proof in this
note.) Copy at http://math.berkeley.edu/~gbergman/papers will always have
latest version; revisions sent to arXiv only for major change
Strong inner inverses in endomorphism rings of vector spaces
For a vector space over a field, or more generally, over a division ring,
it is well-known that every has an inner inverse,
i.e., an element satisfying We show here that a
large class of such have inner inverses that satisfy with an
infinite family of additional monoid relations, making the monoid generated by
and what is known as an inverse monoid (definition recalled). We
obtain consequences of these relations, and related results.
P. Nielsen and J. \v{S}ter, in a paper to appear, show that a much larger
class of elements of rings including all elements of von Neumann
regular rings, have inner inverses satisfying arbitrarily large finite
subsets of the abovementioned set of relations. But we show by example that the
endomorphism ring of any infinite-dimensional vector space contains elements
having no inner inverse that simultaneously satisfies all those relations.
A tangential result proved is a condition on an endomap of a set that
is necessary and sufficient for to belong to an inverse submonoid of the
monoid of all endomaps of Comment: 18pp. The main change from the preceding version is the discussion of
three questions posed by the referee, two on p.10, starting on line 6, and
one starting at the top of p.16. There are also many small revisions of
wording et
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