Computing syzygies in finite dimension using fast linear algebra
Résumé
We consider the computation of syzygies of multivariate polynomials in a
finite-dimensional setting: for a $\mathbb{K}[X_1,\dots,X_r]$-module
$\mathcal{M}$ of finite dimension $D$ as a $\mathbb{K}$-vector space, and
given elements $f_1,\dots,f_m$ in $\mathcal{M}$, the problem is to compute
syzygies between the $f_i$'s, that is, polynomials $(p_1,\dots,p_m)$ in
$\mathbb{K}[X_1,\dots,X_r]^m$ such that $p_1 f_1 + \dots + p_m f_m = 0$ in
$\mathcal{M}$. Assuming that the multiplication matrices of the $r$
variables with respect to some basis of $\mathcal{M}$ are known, we give an
algorithm which computes the reduced Gr\"obner basis of the module of these
syzygies, for any monomial order, using $O(m D^{\omega-1} + r D^\omega
\log(D))$ operations in the base field $\mathbb{K}$, where $\omega$ is the
exponent of matrix multiplication. Furthermore, assuming that $\mathcal{M}$
is itself given as $\mathcal{M} = \mathbb{K}[X_1,\dots,X_r]^n/\mathcal{N}$,
under some assumptions on $\mathcal{N}$ we show that these multiplication
matrices can be computed from a Gr\"obner basis of $\mathcal{N}$ within the
same complexity bound. In particular, taking $n=1$, $m=1$ and $f_1=1$ in
$\mathcal{M}$, this yields a change of monomial order algorithm along the
lines of the FGLM algorithm with a complexity bound which is sub-cubic in
$D$.
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