TY - JOUR T1 - Projection singularities of extremals for planar systems Y1 - 1999 A1 - Ugo Boscain A1 - Benedetto Piccoli PB - SISSA Library UR - http://hdl.handle.net/1963/1304 U1 - 3151 U2 - Mathematics U3 - Functional Analysis and Applications ER -