266 research outputs found
On the complexity of computing with zero-dimensional triangular sets
We study the complexity of some fundamental operations for triangular sets in
dimension zero. Using Las-Vegas algorithms, we prove that one can perform such
operations as change of order, equiprojectable decomposition, or quasi-inverse
computation with a cost that is essentially that of modular composition. Over
an abstract field, this leads to a subquadratic cost (with respect to the
degree of the underlying algebraic set). Over a finite field, in a boolean RAM
model, we obtain a quasi-linear running time using Kedlaya and Umans' algorithm
for modular composition. Conversely, we also show how to reduce the problem of
modular composition to change of order for triangular sets, so that all these
problems are essentially equivalent. Our algorithms are implemented in Maple;
we present some experimental results
On some local cohomology spectral sequences
We introduce a formalism to produce several families of spectral sequences
involving the derived functors of the limit and colimit functors over a finite
partially ordered set. The first type of spectral sequences involves the left
derived functors of the colimit of the direct system that we obtain applying a
family of functors to a single module. For the second type we follow a
completely different strategy as we start with the inverse system that we
obtain by applying a covariant functor to an inverse system. The spectral
sequences involve the right derived functors of the corresponding limit. We
also have a version for contravariant functors. In all the introduced spectral
sequences we provide sufficient conditions to ensure their degeneration at
their second page. As a consequence we obtain some decomposition theorems that
greatly generalize the well-known decomposition formula for local cohomology
modules given by Hochster.Comment: 63 pages, comments are welcome. To appear in International
Mathematics Research Notice
Well known theorems on triangular systems and the D5 principle
International audienceThe theorems that we present in this paper are very important to prove the correctness of triangular decomposition algorithms. The most important of them are not new but their proofs are. We illustrate how they articulate with the D5 principle
Formal Desingularization of Surfaces - The Jung Method Revisited -
In this paper we propose the concept of formal desingularizations as a
substitute for the resolution of algebraic varieties. Though a usual resolution
of algebraic varieties provides more information on the structure of
singularities there is evidence that the weaker concept is enough for many
computational purposes. We give a detailed study of the Jung method and show
how it facilitates an efficient computation of formal desingularizations for
projective surfaces over a field of characteristic zero, not necessarily
algebraically closed. The paper includes a generalization of Duval's Theorem on
rational Puiseux parametrizations to the multivariate case and a detailed
description of a system for multivariate algebraic power series computations.Comment: 33 pages, 2 figure
Rational, Replacement, and Local Invariants of a Group Action
The paper presents a new algorithmic construction of a finite generating set
of rational invariants for the rational action of an algebraic group on the
affine space. The construction provides an algebraic counterpart of the moving
frame method in differential geometry. The generating set of rational
invariants appears as the coefficients of a Groebner basis, reduction with
respect to which allows to express a rational invariant in terms of the
generators. The replacement invariants, introduced in the paper, are tuples of
algebraic functions of the rational invariants. Any invariant, whether
rational, algebraic or local, can be can be rewritten terms of replacement
invariants by a simple substitution.Comment: 37 page
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