Definability, in particular existential definability, is a central concept in Julia Robinson’s work.
I will discuss its dual role in relation to (algebraic) differential equations. This leads naturally to the idea of transseries. My work on this is joint with Matthias Aschenbrenner and Joris van der Hoeven, and has resulted in a complete theory of solving differential equations with initial conditions in the differential field of transseries. I will also briefly talk about the connection to Hardy fields that we have established recently.