Periodic solutions of second-order wave equations. III (Q1108485)

[For Part II see ibid. 38, 617-623 (1986); translation from Ukr. Mat. Zh. 38, No.6, 733-739 (1986; Zbl 0637.35054)]. The authors investigate the existence and the structure of generalized periodic solutions of boundary value problems of the form \[ u_{tt}- u_{xx}=f(x,t,u,u_ t,u_ x),\quad 0<x<\pi,\quad t\in {\mathbb{R}}, \] \[ u(0,t)=u(\pi,t)=0,\quad u(x,t+2\pi)=u(x,t). \]