Constraint structure of O(3) nonlinear sigma model revisited
  We study the constraint structure of the O(3) nonlinear sigma model in the framework of the Lagrangian, symplectic, Hamilton-Jacobi as well as the Batalin-Fradkin-Tyutin embedding procedure. 
Soon-Tae Hong, Yong-Wan Kim, Young-Jai Park and Klaus D. Rothe