A Game of Scales

Abstract

These notes cover the material of two courses given at SISSA, Trieste, in the academic years 2023-2024 and 2025-2026, together with some material from courses taught in other recent years. The original idea behind these courses emerged from conversations with Andrea Causin and Margherita Solci about the somewhat elusive notion of `multiscale behaviour', which arises in the study of variational problems involving one or more small parameters. Two main mechanisms govern their asymptotic behaviour: concentration and oscillations. Their description is often associated with the emergence of a characteristic `scale', quantifying either the speed of concentration or the period of an optimal oscillation. These mechanisms may originate from different variational principles and may occur separately, coexist, compete, or interact, thus providing a natural interpretation of the term `multiscale'. This perspective inspired the informal title of the courses, which also became the title of these notes. The natural framework for these analyses is $Γ$-convergence. However, these notes are not intended as a general introduction to this now well-established theory. We provide only a brief account of the techniques relevant to our purposes, referring the reader to the author's book chapter {\em A Handbook of $Γ$-convergence}. For a broader introduction to $Γ$-convergence, and in particular to its topological properties, the standard reference is Dal Maso's monograph {\em An Introduction to $Γ$-convergence}.