At the time, it seems, some people disguised themselves as École polytechnique students to make others believe they were on the right side! So how could these costumed impostors be exposed? "Those people, the authors tell us, whenever we met them, we asked them for the derivative of sin(x) or of log(x), and if they failed to answer, they were immediately locked up." The anecdote does not say whether those who knew the right answer had learned it by attending Cauchy's lectures.
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Fighting outside the Hôtel de Ville on 28 July 1830.

Reference: L’argot de l’X, Albert Lévy & Gaston Pinet, Testard, 1894.