# Functional calculus of unitary matrices and commutator norms: reference request

Suppose we have a normal matrix $A$ and a general matrix $B$. For a continuous function $f$ on the disk we can find upper-bounds on $\Vert[f(A),B]\Vert$ in terms of $\Vert[A,B]\Vert$. The more we know about $B$ and $f$ the more hope we have of a tight estimate. I work here with the operator norm, and add the restrictions $\left\Vert A\right\Vert \leq1$ and $\left\Vert B\right\Vert \leq1$ for simplicity.

If the spectrum of $A$ is in $[-1,1]$ then we are assuming $A$ is a Hermitian contraction. One method of attack uses the Fourier transform, where for nice $f$ we write $$f^{\prime}(x)=\int_{-\infty}^{\infty}g(t)e^{itx}\, dx$$ and $$\left[f(A),B\right]=\int_{-\infty}^{\infty}\frac{g(t)}{it}\left[e^{itA},B\right]\, dx.$$ From here we get quickly the estimate $$\left\Vert \left[f(A),B\right]\right\Vert \leq\left\Vert \widehat{f^{\prime}}\right\Vert _{1}\left\Vert \left[A,B\right]\right\Vert .$$ Matt Hastings familiarized me with this trick. I suppose this is folklore. Uffe Haggerup thinks such a result and generalizations are in some old papers on derivations.

My current interest is in another special case, assuming $A$ has spectrum in the unit circle. Let's replace $A$ by $U$, now a unitary matrix. This is easier, as we can watch what happens for a trig-polynomial. Assume $f$ on the circle is given by $$f(z)=\sum_{-N}^{N}a_{n}z^{n}$$ and use the estimate $$\left\Vert \left[U^{n},B\right]\right\Vert \leq|n|\left\Vert \left[U,B\right]\right\Vert$$ to find $$\left\Vert \left[f(U),B\right]\right\Vert \leq\sum_{-N}^{N}|n||a_{n}|\left\Vert \left[U,B\right]\right\Vert .$$ If we let $q(x)=f(e^{ix})$ then we can use classical Fourier series notation and can use the replacement $$\left\Vert \widehat{q^{\prime}}\right\Vert _{1}=\sum_{-N}^{N}|n||a_{n}|.$$

This leads to a useful result.

For nice periodic functions $q$ we define $f$ so that $q(x)=f(e^{ix})$. The formula $$\left\Vert \left[f(U),B\right]\right\Vert \leq\left\Vert \widehat{q^{\prime}}\right\Vert_{1}\left\Vert \left[U,B\right]\right\Vert$$ should hold true for at least all unitary matrices and all matrices $B$ of norm at most one. Where is this written down? I am flexible as to the meaning of nice here. Thrice differentiable handles most cases I find interesting.

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Since I remember noticing this kind of estimate when I was reading about the Friis-Rordam proof of Lin's theorem, and doing some back of the envelope jottings, I rather suspect it's folklore. – Yemon Choi Sep 29 '12 at 21:20
I haven't seen the unitary case before either, but I would cite Theorem 3.2.32 of Bratteli and Robinson vol. 1 for the hermitian case. – Nik Weaver Oct 1 '12 at 17:04
Thanks, Nik. I was browsing Bratteli and Robinson when for related reasons, but failed to notice that result. – Terry Loring Oct 2 '12 at 17:21