Semi-inclusive \pi^\pm production - tests for independent fragmentation and for polarized quark densities

We show that measurements of semi-inclusive $\pi^\pm$ asymmetries on $p$ and $n$ with polarized and unpolarized target and beams allow, without any knowledge of the polarized parton densities 1) to test independent fragmentation and SU(2) symmetry for the polarized sea, and 2) to determine separately the polarized strange and valence quark densities, and the ratio of the fragmentation functions $D_s^{\pi^+ + \pi^-}/D_u^{\pi^+ +\pi^-}$