76,677 research outputs found
The finiteness dimension of local cohomology modules and its dual notion
Let \fa be an ideal of a commutative Noetherian ring R and M a finitely
generated R-module. We explore the behavior of the two notions f_{\fa}(M), the
finiteness dimension of M with respect to \fa, and, its dual notion q_{\fa}(M),
the Artinianess dimension of M with respect to \fa. When (R,\fm) is local and
r:=f_{\fa}(M) is less than f_{\fa}^{\fm}(M), the \fm-finiteness dimension of M
relative to \fa, we prove that H^r_{\fa}(M) is not Artinian, and so the filter
depth of \fa on M doesn't exceeds f_{\fa}(M). Also, we show that if M has
finite dimension and H^i_{\fa}(M) is Artinian for all i>t, where t is a given
positive integer, then H^t_{\fa}(M)/\fa H^t_{\fa}(M) is Artinian. It
immediately implies that if q:=q_{\fa}(M)>0, then H^q_{\fa}(M) is not finitely
generated, and so f_{\fa}(M)\leq q_{\fa}(M).Comment: 14 pages, to appear in Journal of Pure and Applied Algebr
Cohomological Finiteness Conditions in Bredon Cohomology
We show that any soluble group of type Bredon-\FP_{\infty} with respect
to the family of all virtually cyclic subgroups such that centralizers of
infinite order elements are of type \FP_{\infty} must be virtually cyclic. To
prove this, we first reduce the problem to the case of polycyclic groups and
then we show that a polycyclic-by-finite group with finitely many conjugacy
classes of maximal virtually cyclic subgroups is virtually cyclic. Finally we
discuss refinements of this result: we only impose the property Bredon-\FP_n
for some and restrict to abelian-by-nilpotent, abelian-by-polycyclic
or (nilpotent of class 2)-by-abelian groups.Comment: Corrected a mistake in Lemma 2.4 of the previous version, which had
an effect on the results in Section 5 (the condition that all centralisers of
infinite order elements are of type was added
Invariant expectations and vanishing of bounded cohomology for exact groups
We study exactness of groups and establish a characterization of exact groups
in terms of the existence of a continuous linear operator, called an invariant
expectation, whose properties make it a weak counterpart of an invariant mean
on a group. We apply this operator to show that exactness of a finitely
generated group implies the vanishing of the bounded cohomology of with
coefficients in a new class of modules, which are defined using the Hopf
algebra structure of .Comment: Final version, to appear in the Journal of Topology and Analysi
Few smooth d-polytopes with n lattice points
We prove that, for fixed n there exist only finitely many embeddings of
Q-factorial toric varieties X into P^n that are induced by a complete linear
system. The proof is based on a combinatorial result that for fixed nonnegative
integers d and n, there are only finitely many smooth d-polytopes with n
lattice points. We also enumerate all smooth 3-polytopes with at most 12
lattice points. In fact, it is sufficient to bound the singularities and the
number of lattice points on edges to prove finiteness.Comment: 20+2 pages; major revision: new author, new structure, new result
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