Before revision:  It is well known that I(n,l) = \Theta( (nl)^{2/3 + n + l) 2,3,7 . Let C _{SzTr} denote the  constant of proportionality of the (nl)^{2/3 term.
After revision:  It is well known that I(n,l) = \Theta( (nl)^{2/3 + n + l) 2,3,7 . Let C _{SzTr} denote the lower bound on the constant of proportionality of the (nl)^{2/3 term.