Gilbert Levitt
Translated by R. Matveev in 2025 from the French original published at Images des mathématiques
Discrete groups appear in every area of mathematics — and even in Escher’s art. Even if the y are defined algebraically, we often understand them better by their action on geometric objects. More and more often, they are viewed as geometric entities in their own right. Their properties are especially striking when the curvature is negative.
