@Jakobian yesterday we were talking about projection of matrices and you identified, topologically, the space of matrices with $\mathbb R^{mn}$. I have a hard time accepting this association. There is something called
vectorization. In projecting a matrix onto one of its entries, did you implicitly use this map somehow? If we associate a matrix with a point in $\mathbb R^{mn}$, then I guess we have to vectorize it somehow.