We describe how the Szlenk index has been used in various areas of the geometry of Banach spaces. We cover the following domains of application of this notion: non existence of universal spaces, linear classification of C(K) spaces, descriptive set theory, renorming problems and non linear classification of Banach spaces.

In the context of the theory of infinite matrices and linear operators, two articles by Peano and by Gramegna on systems of linear differential equations have interesting implications for the reconstruction of research on functional analysis between 1887 and 1910. With the aim of evaluating their historical value, linked to logic and vector calculus, this paper provides a detailed analysis of their treatment, demonstrating the modernity of the analytic tools used. In this paper we also reveal the...