Bourbaki

Remark 2

Tag 03AO, permanent. Cite it as [Bourbaki, Tag 03AO].

Théories spectrales, chapter V, § 2. Représentations des groupes localement compacts

Let κ′:G→R+∗\kappa ': G\rightarrow \mathbf{R}_+^* be such that

κ′(xh)κ′(x)=ΔH(h)ΔG(h)=κ(xh)κ(x) \frac{\kappa'(xh)}{\kappa'(x)}=\frac{\Delta_H(h)}{\Delta_G(h)}=\frac{\kappa(xh)}{\kappa(x)}

for every (x,h)∈G×H(x, h)\in G\times H. Let ν′\nu ' be the quasi-invariant measure (κ′⋅μ)/β(\kappa '\cdot \mu)/\beta on G/HG/H. The function κ′κ−1\kappa '\kappa^{-1} defines, by passing to quotients, a continuous function ξ:G/H→R+∗\xi : G/H\rightarrow \mathbf{R}_+^* such that ν′=ξ⋅ν\nu '=\xi \cdot \nu. The endomorphism α\alpha of Kπ(G)\mathscr{K}_{\pi}(G) defined by f↦(κ′κ−1)1/pff\mapsto (\kappa '\kappa^{-1})^{1/p}f satisfies

∫G/H∗∥f∥pdν′=∫G/H∗∥α(f)∥pdν \int_{G/H}^*\|f\|^pd\nu '=\int_{G/H}^*\|\alpha (f)\|^pd\nu

and makes it possible to identify the spaces Lπp(G,ν′)\mathscr{L}_{\pi}^p(G, \nu ') and Lπp(G,ν)\mathscr{L}_{\pi}^p(G, \nu ), as well as the spaces Lπp(G,ν′)L^p_{\pi}(G, \nu ') and Lπp(G,ν)L^p_{\pi}(G, \nu ). Moreover, α\alpha induces an isometric isomorphism of the representations IndHG(π,κ′)^G_H(\pi , \kappa ') and IndHG(π,κ)^G_H(\pi , \kappa ).

Bourbaki sets the proof directly after the statement, and the corpus does not separate them, so what is above is the statement and its proof together.

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Machine translation of the English, by gpt-5.4, gpt-5-6-mini, against glossary version 34. Not checked by a person. That model is a cut down version of the one the rest of this language was written by.