Title: Carlsson's rank conjecture and a conjecture on square-zero upper triangular matrices,
Abstract: Let $k$ be an algebraically closed field and $A$ the polynomial algebra in
$r$ variables with coefficients in $k$. In case the characteristic of $k$ is
$2$, Carlsson conjectured that for any $DG$-$A$-module $M$ of dimension $N$ as
a free $A$-module, if the homology of $M$ is nontrivial and finite dimensional
as a $k$-vector space, then $2^r\leq N$. Here we state a stronger conjecture
about varieties of square-zero upper-triangular $N\times N$ matrices with
entries in $A$. Using stratifications of these varieties via Borel orbits, we
show that the stronger conjecture holds when $N < 8$ or $r < 3$ without any
restriction on the characteristic of $k$. As a consequence, we attain a new
proof for many of the known cases of Carlsson's conjecture and give new results
when $N > 4$ and $r = 2$.