Bourbaki

Exercise 14, § 3

General Topology, X

Let Γ\Gamma be the group of all homeomorphisms of the real plane R2\mathbf{R}^2, endowed with the topology of pointwise convergence. Show that the mapping (u,v)→v∘u(u, v) \to v \circ u of Γ×Γ\Gamma \times \Gamma into Γ\Gamma is not continuous. [Consider a sequence of homeomorphisms (vn)(v_n) such that vnv_n leaves fixed every point (x,y)(x, y) for which y∈[1/(n+1),1/n]y \in [1/(n+1), 1/n], and such that the restriction of vnv_n to the line y=2/(2n+1)y = 2/(2n + 1) is the translation (x,y)→(x+1,y)(x, y) \to (x + 1, y); on the other hand, consider the sequence (un)(u_n) of translations

(x,y)→(x,y+2/(2n+1)). (x, y) \to (x, y + 2/(2n + 1)).
Solution

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