Sorry, I see now. 2. "there's no rank/co-rank relation (like rk+co-rk=dim) for PDE", even PDE is linear? Indeed, I haven't seen anything about dimension of solution (usually submanifold) and number of pdes in any literature. Would you mind referring me to textbooks discussing Cartan's "generality"? I think I need something about like fundamental solution theorem of pde or what. Thank you very much.