Deterministic Truncation of Linear Matroids

Let $M=(E,{\cal I})$ be a matroid. A {\em $k$-truncation} of $M$ is a matroid {$M'=(E,{\cal I}')$} such that for any $A\subseteq E$, $A\in {\cal I}'$ if and only if $|A|\leq k$ and $A\in {\cal I}$. Given a linear representation of $M$ we consider the problem of finding a linear representation of the $k$-truncation of this matroid. This problem can be abstracted out to the following problem on matrices. Let $M$ be a $n\times m$ matrix over a field $\mathbb{F}$. A {\em rank $k$-truncation} of the matrix $M$ is a $k\times m$ matrix $M_k$ (over $\mathbb{F}$ or a related field) such that for every subset $I\subseteq \{1,\ldots,m\}$ of size at most $k$, the set of columns corresponding to $I$ in $M$ has rank $|I|$ if and only of the corresponding set of columns in $M_k$ has rank $|I|$. Finding rank $k$-truncation of matrices is a common way to obtain a linear representation of $k$-truncation of linear matroids, which has many algorithmic applications. A common way to compute a rank $k$-truncation of a $n \times m$ matrix is to multiply the matrix with a random $k\times n$ matrix (with the entries from a field of an appropriate size), yielding a simple randomized algorithm. So a natural question is whether it possible to obtain a rank $k$-truncations of a matrix, {\em deterministically}. In this paper we settle this question for matrices over any finite field or the field of rationals ($\mathbb Q$). We show that given a matrix $M$ over a field $\mathbb{F}$ we can compute a $k$-truncation $M_k$ over the ring $\mathbb{F}[X]$ in deterministic polynomial time.