Bourbaki

Proposition 9

Tag 09IV, permanent. Cite it as [Bourbaki, Tag 09IV].

Integration, VIII, p. 14, § 2. Linear representations of groups

Suppose that each function χ(s,⋅)\chi(s, \cdot) is bounded. Then γχ\gamma_\chi leaves LCp(X,β)L_C^p(X, \beta) stable, and the linear representation γχ\gamma_\chi of GG in LCp(X,β)L_C^p(X, \beta) is continuous.

Let f∈LCp(X,β)f \in \mathscr{L}_C^p(X, \beta). Then

∫∗∣χ(s−1,x)f(s−1x)∣pdβ(x) \int^* |\chi(s^{-1}, x)f(s^{-1}x)|^p d\beta(x)
≤sup⁡x∈Xχ(s−1,x)p−1∫∗∣f(s−1x)∣pχ(s−1,x)dβ(x) \leq \sup_{x \in X} \chi(s^{-1}, x)^{p-1} \int^* |f(s^{-1}x)|^p \chi(s^{-1}, x) d\beta(x)
=sup⁡x∈Xχ(s−1,x)p−1∫∣f(x)∣pdβ(x), = \sup_{x \in X} \chi(s^{-1}, x)^{p-1} \int |f(x)|^p d\beta(x),
therefore γχ(s)f∈LCp(X,β)\gamma_\chi(s)f \in \mathscr{L}_C^p(X, \beta), and
∥γχ(s)∥≤sup⁡x∈Xχ(s−1,x)1/q, \| \gamma_\chi(s) \| \leq \sup_{x \in X} \chi(s^{-1}, x)^{1/q},
(5) where qq denotes the exponent conjugate to pp. If f∈K(X)f \in \mathscr{K}(X), then γχ(s)f\gamma_{\chi}(s)f tends to γχ(s0)f\gamma_{\chi}(s_0)f in K(X)\mathscr{K}(X), hence in LCp(X,β)\mathscr{L}_C^p(X, \beta), as ss tends to s0s_0. Therefore the representation γχ\gamma_{\chi} of GG in LCp(X,β)L_C^p(X, \beta) is continuous (No. 1, Prop. 2).

Properties analogous to those of Nos. 3, 4, 5 hold if GG operates on the right in XX.

In particular, if one regards GG as operating on itself by left or right translations, and if one takes χ=1\chi = 1, one obtains the left and right regular representations of GG in C(G)\mathscr{C}(G), K(G)\mathscr{K}(G), K‾(G)\overline{\mathscr{K}}(G), C′(G)\mathscr{C}'(G), M(G)\mathscr{M}(G), M1(G)\mathscr{M}^1(G). If one takes β\beta to be a left (resp. right) Haar measure on GG, and if one takes χ=1\chi = 1, one obtains the left (resp. right) regular representation of GG in LCp(G,β)L_C^p(G, \beta).

Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.

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Transcribed from the printed volume. Not translated.