Une fonction de WEIERSTRASS à l'oral Mines-Ponts ? (sup/L1)

In this Mines-Ponts MP 2024 oral exam, we're looking at a Weierstrass function. The goal here is to prove that it is not differentiable at zero... even though it is actually continuous on R, but nowhere differentiable. 00:00 Introduction 00:28 Statement + Cultural Aspect 02:47 The "Tricky" Method: Hint 04:02 Clever Solution 07:13 The Natural Method: The Idea 12:05 Natural Method: A Cut 15:11 End of Story