TITLE: Is containment of images of linear maps Zariski closed?
QUESTION [5 upvotes]: Consider the set
$$\\\{ (A,B) \in \mathbb{P}^{n\times n-1} \times \mathbb{P}^{n\times n -1} : \text{im}(A) \subseteq \text{im}(B)\}.$$
That is, this is the set of pairs of square matrices $(A,B)$ so that the image of $A$ is contained in the image of $B$. Is this Zariski closed? I would be happy if this were at least true over an algebraically closed field.
I tried to write down explicit equations for this, but my first attempt would involved using determinants of submatrices of $A$, and would fail if $A$ was singular. I had thought that this would follow from the projection theorem for for projective varieties, but am unsure.

REPLY [4 votes]: Another way to see your set is not closed: it is evidently not the whole space of pairs, but yet it contains the open dense set $U\times U$ where $U \subset \mathbb{P}^{n^2-1}$ consists of (classes of) invertible matrices. So your set is different from its closure.