Discuss, on physical grounds, the behavior of $f_i(x)$ in the limit as $x \rightarrow 1$ when parton $i$ carries all the momentum of the proton. Counting rules have been proposed which argue that
$$
f_i(x) \underset{x \rightarrow 1}{\longrightarrow}(1-x)^{2 n_s-1},
$$
where $n_s$ is the number of spectator valence quarks which share between them the residual, vanishingly small momentum of the proton. Contrast the $x \rightarrow 1$ behavior of $u^p(x)$ with that of $u^{\nabla}(x)$, the u-quark structure function of a $\pi^{+}$-meson.