I'm trying to show all the non-projectively equivalent configurations of linear subspaces.
Two subspaces $\Lambda_1,\Lambda_2$ are projectively equivalent if
$\exists\omega:\mathbb{P}^n\longrightarrow \mathbb{P}^n$ homography such that $\omega(\Lambda_1)=\Lambda_2$.
For example, in $\mathbb{P}^4$ let $l_1,l_2$ be two distinct lines and $P$ a point such that such that $P \notin l_1\cup l_2$.
What's the number of non-projectively equivalent configurations of the two lines and the point $P$?
My idea is to study the dimension of the intersection space of the two lines.
Infact, for Grassmann's formula we have:
$$\mathrm{dim}(l_1 \cup l_2)=\mathrm{dim}(l_1)+\mathrm{dim}(l_2)-\mathrm{dim}(l_1 \cap l_2) \le4$$
We know that $\mathrm{dim}(l_1 \cap l_2)= \begin{cases} 0 \\ -1 \end{cases}$
If $\mathrm{dim}(l_1 \cap l_2)=0$ it means that $l_1 \cap l_2=\{Q\}$ (case $1$), while if $\mathrm{dim}(l_1 \cap l_2)=-1$ it means that $l_1 \cap l_2 = \emptyset$ (case $2$). We can now study all these cases.
$1$a) $<l_1,l_2>=\pi$, where $\pi$ is a plane, $P \in \pi$ and $\mathrm{dim}(<l_1,l_2,P>)=2$
$1$b) $<l_1,l_2>=\pi$, where $\pi$ is a plane, $P \notin \pi$ and $\mathrm{dim}(<l_1,l_2,P>)=3$
$2$a) $<l_1,l_2>=\Theta$, where $\Theta$ is a hyperplane, $P \in \Theta$ and $\mathrm{dim}(<l_1,l_2,P>)=3$
$2$b) $<l_1,l_2>=\Theta$, where $\Theta$ is a hyperplane, $P \notin \Theta$ and $\mathrm{dim}(<l_1,l_2,P>)=4$
All these are non-projectively equivalent because a homography $\omega$ doesn't change the dimension of linear subspaces in $\mathbb{P}^n$. Is that correct? Thank you in advance.