@Article{ DeL9, author = { De Lellis, Camillo }, title = { Ill-posedness for bounded admissible solutions of the $2$-dimensional $p$-system }, journal = { Hyperbolic problems: theory, numerics and applications. }, year = { 2009 }, volume = { 67 of Proc. Sympos. Appl. Math. }, pages = { 269-278 }, URL = { http://cvgmt.sns.it/paper/2192/ }, note = { cvgmt preprint} }