Linear Algebra for Computing Gröbner Bases of Linear Recursive Multidimensional Sequences
Résumé
Sakata generalized the Berlekamp -- Massey algorithm to $n$ dimensions in~1988. The Berlekamp -- Massey -- Sakata (BMS)
algorithm can be used for finding a Gröbner basis of a $0$-dimensional
ideal of relations verified by a table. We investigate this problem using
linear algebra techniques, with motivations such as accelerating change of
basis algorithms (FGLM) or improving their complexity.
We first define and characterize multidimensional linear recursive sequences
for $0$-dimensional ideals.
Under genericity assumptions, we propose a randomized preprocessing of the
table that corresponds to performing a linear change of coordinates on the
polynomials associated with the linear recurrences. This technique then
essentially reduces our problem to using the efficient $1$-dimensional Berlekamp -- Massey (BM)
algorithm.
However, the number of probes to the table in this scheme may be elevated.
We thus consider the table in the \emph{black-box} model: we assume probing the
table is expensive and we minimize the number of probes to the table in our
complexity model.
We produce an FGLM-like algorithm for finding the relations in the
table, which lets us use linear algebra techniques. Under some additional
assumptions, we make this algorithm adaptive and reduce further the number
of table probes.
This number can be estimated by counting the number of distinct elements in a
multi-Hankel matrix (a multivariate generalization of Hankel matrices); we can
relate this quantity with the \emph{geometry} of the final staircase. Hence,
in favorable cases such as convex ones, the complexity is essentially linear in
the size of the output. Finally, when using the \textsc{lex} ordering, we can
make use of fast structured linear algebra similarly to the Hankel
interpretation of Berlekamp -- Massey.
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