Structure functions of 2d integrable asymptotically free models

We investigate structure functions in the 2–dimensional (asymptotically free) non–linear O$(n)\sigma$–models using the non–perturbative S–matrix bootstrap program. In particular the {\it exact small (Bjorken) $x$ behavior is exhibited}; the structure is rather universal and is probably the same in a wide class of (integrable) asymptotically free models. Structurally similar universal formulae may also hold for the small $x$ behavior of QCD in 4–dimensions. Structure functions in the special case of the $n=3$ model are accurately computed over the whole $x$ range for $-q^2\mathrm{/}{M}^2<{10}^5$, and some moments are compared with results from renormalized perturbation theory. Some remarks concerning the structure functions in the $1\mathrm{/}n$ approximation are also made.