Movable algebraic singularities of second-order ordinary differential equations

Any nonlinear equation of the form y′′ = ∑N n=0 an(z)y n has a (generally branched) solution with leading order behaviour proportional to (z − z0) about a point z0, where the coefficients an are analytic at z0 and aN (z0) 6= 0. We consider the subclass of equations for which each possible leading order term of this form corresponds to a one-parameter family of solutions represented near z0 by a Laurent series in fractional powers of z−z0. For this class of equations we show that the only movable singularities that can be reached by analytic continuation along finite-length curves are of the algebraic type just described. This work generalizes previous results of S. Shimomura. The only other possible kind of movable singularity that might occur is an accumulation point of algebraic singularities that can be reached by analytic continuation along infinitely long paths ending at a finite point in the complex plane. This behaviour cannot occur for constant coefficient equations in the class considered. However, an example of R. A. Smith shows that such singularities do occur in solutions of a simple autonomous second-order differential equation outside the class we consider here.