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If $f$ is a complex-valued function on a compact Lie group $G$, we have a decomposition $f = \sum_\mu f_\mu$ corresponding to the Peter-Weyl decomposition $L^2(G) = \oplus_\mu (\dim \mu) V_\mu$.

For $f \in C^\infty(G)$, it can be shown that for any $n > 0$, there are $k, C > 0$ so that: $$\|f_\mu\| \leq C\frac{\|f\|_{C^k}}{\|\mu\|^n}$$ where $\|\mu\|$ is defined to be the infimum of the Lipschitz constants of the orbit maps $g \mapsto \mu(g) \cdot v$, taken over the vectors $v \in V_\mu^1$ of norm $1$. This is proven using the Weyl dimension formula and some representation theory (see lemma 4.4.2.2/3 in Garth Warner's Harmonic Analysis on Semisimple Lie Groups).

My question: is something along these lines true for Hölder functions? A bound involving $\|f\|_{C^\alpha}$ and $\|\mu\|$ as above would be ideal, but I think most things vaguely resembling the inequality above would be helpful for what I'm doing. I suspect something like this is probably well-known, but I haven't had much luck searching for this or asking around.

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  • $\begingroup$ I guess that in the first claim $k,C$ don't depend on $f$ (which is not clear from the wording). $\endgroup$
    – YCor
    Jun 22, 2018 at 10:03
  • $\begingroup$ Yes; $k$ and $C$ don't depend on $f$ or $\mu$. $\endgroup$
    – Salman
    Jun 22, 2018 at 13:19

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