Iterated convolutions and endless Riemann surfaces

We discuss a version of \'Ecalle's definition of resurgence, based on the notion of endless continuability in the Borel plane. We relate this with the notion of \Omega-continuability, where \Omega\ is a discrete filtered set, and show how to construct a universal Riemann surface X_\Omega\ whose holomorphic functions are in one-to-one correspondence with \Omega-continuable functions. We then discuss the \Omega-continuability of convolution products and give estimates for iterated convolutions of the form \hat\phi_1*\cdots *\hat\phi_n. This allows us to handle nonlinear operations with resurgent series, e.g. substitution into a convergent power series.