# Carlsson's rank conjecture and a conjecture on square-zero upper triangular matrices

## TL;DR

This paper explores a stronger conjecture related to Carlsson's rank conjecture, focusing on varieties of square-zero upper-triangular matrices, and proves it in specific cases, providing new insights and proofs for existing conjectures.

## Contribution

It introduces a stronger conjecture involving matrix varieties and proves it for certain parameters, extending known results on Carlsson's conjecture.

## Key findings

- Stronger conjecture holds when N<8 or r<3
- New proofs for many cases of Carlsson's conjecture
- Results for N>4 and r=2

## 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 $r = 2$.