On oscillatory solutions of the fourth order differential equations with the middle term

In the paper the fourth order nonlinear differential equation y((4)) + (q(t)y')' + r(t)f(y) = 0, where q, r is an element of C-0 ([0, infinity)), f is an element of C-0(R), q less than or equal to 0, r greater than or equal to 0 and f(x)x > 0 for x not equal 0 is considered. The structure of proper solutions, the existence of oscillatory solutions and asymptotic properties of solutions of this equation are investigated.