Linear Algebra for Computing Gröbner Bases of Linear Recursive Multidimensional Sequences
Résumé
The so-called Berlekamp~-- Massey~-- Sakata algorithm
computes a Gröbner basis of a $0$-dimensional ideal of relations satisfied by an input
table. It extends the Berlekamp~-- Massey algorithm
to $n$-dimensional tables, for $n>1$.
We investigate this problem and design several algorithms for
computing such a Gröbner basis of an ideal of relations using linear
algebra techniques.
The first one performs a lot of table queries and
is analogous to a change of variables on the ideal of relations.
As each query to the table can be expensive,
we design a second algorithm
requiring fewer queries, in general.
This \textsc{FGLM}-like algorithm allows us to compute the relations of the
table by extracting a full rank submatrix of a \emph{multi-Hankel}
matrix (a multivariate generalization of Hankel matrices).
Under some
additional assumptions, we make a third, adaptive, algorithm and reduce
further the number of table queries.
Then, we relate the number of queries of
this third algorithm to the
\emph{geometry} of the final staircase and we show that it is
essentially linear in the size of the output when the staircase is convex.
As a direct application to this, we decode $n$-cyclic codes, a
generalization in dimension $n$ of Reed Solomon codes.
We show that the multi-Hankel
matrices are heavily structured when using the \textsc{LEX} ordering
and that we can speed up the computations using fast algorithms for
quasi-Hankel matrices.
Finally, we design
algorithms for computing the generating series of a linear recursive
table.
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