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Proposition Let $\operatorname{Aut}_{ \mathrm{Dy} }([0,1])$ be the Thompson group of Definition and let $X = B_{\bullet } \operatorname{Aut}_{ \mathrm{Dy} }([0,1])$ denote its classifying simplicial set (Construction Then the homomorphism $\alpha $ of Construction induces a homotopy idempotent endomorphism $e: X \rightarrow X$ in the $\infty $-category $\operatorname{\mathcal{S}}$.

Proof. We wish to show that the diagram of Kan complexes

\[ \xymatrix@R =50pt@C=50pt{ & X \ar [dr]^{e} & \\ X \ar [ur]^{e} \ar [rr]^{e } & & X } \]

commutes up to homotopy. By virtue of Proposition, this is equivalent to the assertion that the homomorphisms $\alpha , \alpha ^2: \operatorname{Aut}_{ \mathrm{Dy} }([0,1]) \rightarrow \operatorname{Aut}_{ \mathrm{Dy} }([0,1])$ are conjugate: that is, there exists an element $g \in \operatorname{Aut}_{ \mathrm{Dy} }([0,1])$ satisfying the identity $\alpha (f) \circ g = g \circ \alpha ^2(f)$ for every element $f \in \operatorname{Aut}_{ \mathrm{Dy}}( [0,1] )$. Concretely, we can take $g$ to be any dyadic homeomorphism satisfying the identity $g(x) = 2x$ for $0 \leq x \leq 1/4$. $\square$