On the convergence of exotic formal series solutions of an ODE. A proof by the implicit mapping theorem

Abstract

We propose a sufficient condition of the convergence of a complex power type formal series of the form $\varphi=\sum_{k=1}^{\infty}\alpha_k(x^{{\rm i}\gamma})\,x^k$, where $\alpha_k$ are functions meromorphic at the origin and $\gamma\in{\mathbb R}\setminus\{0\}$, that satisfies an analytic ordinary differential equation (ODE) of a general type. An example of a such type formal solution of the third Painlev\'e equation is presented and the proposed sufficient condition is applied to check its convergence.