Segre invariant and a stratification of the moduli space of coherent systems
The aim of this paper is to generalize the $m-$Segre invariant for vector bundles to coherent systems. Let $X$ be a non-singular irreducible complex projective curve of genus $g$ over $\mathbb{C}$ and $(E,V)$ be a coherent system on $X$ of type $(n,d,k)$. For any pair of integers $m, t$, $0 < m < n$, $0 \leq t \leq k$ we define the $(m,t)-$Segre invariant, denoted by $S^α_{m,t}$ and show that $S^α_{m,t}$ induces a semicontinuous function on the families of coherent systems. Thus, $S^α_{m,t}$ gives a stratification of the moduli space $G(α;n,d,k)$ of $α-$stable coherent systems of type $(n,d,k)$ on $X$ into locally closed subvarieties $G(α;n,d,k;m,t;s)$ according to the value $s$ of $S^α_{m,t}$. We study the stratification, determine conditions under which the different strata are non-empty and compute their dimension.