The stable maximum nullity of digraphs and $1$-DAGs

Abstract

Given a digraph $D=(V,A)$ with vertex-set $V=\{1,\dots ,n\}$ and arc-set $A$, we denote by $Q(D)$ the set of all real $n\times n$ matrices $B=[b_{u,w}]$ with $b_{u,u}\ne 0$ for all $u\in V$, $b_{u,w}\ne 0$ if $u\ne w$ and there is an arc from $u$ to $w$, and $b_{u,w}=0$ if $u\ne w$ and there is no arc from $u$ to $w$. We say that a matrix $B\in Q(D)$ has the Asymmetric Strong Arnold Property (ASAP) if $X\circ B=0$, $X^{T}B=0$, and $B{X}^T=0$ implies $X=0$. We define the stable maximum nullity, $\overrightarrow{\hat{I}}½(D)$, of a digraph $D$ as the largest nullity of any matrix $A\in Q(D)$ that has the ASAP\@. We show that a digraph $D$ has $\overrightarrow{\hat{I}}½(D)\le 1$ if and only $D$ and $\overleftarrow{D}$ a partial $1$-DAGs.