ACI-matrices of constant rank over arbitrary fields

The columns of a $m\times n$ ACI-matrix over a field $\mathbb{F}$ are independent affine subspaces of ${\mathbb{F}}^m$. An ACI-matrix has constant rank $\rho$ if all its completions have rank $\rho$. Huang and Zhan (2011) characterized the $m\times n$ ACI-matrices of constant rank when $\mathrm{\mid }\mathbb{F}\mathrm{\mid }\ge \min \{m,n+1\}$. We complete their result characterizing the $m\times n$ ACI-matrices of constant rank over arbitrary fields. Quinlan and McTigue (2014) proved that every partial matrix of constant rank $\rho$ has a $\rho \times \rho$ submatrix of constant rank $\rho$ if and only $\mathrm{\mid }\mathbb{F}\mathrm{\mid }\ge \rho$. We obtain an analogous result for ACI-matrices over arbitrary fields by introducing the concept of complete irreducibility.