A polynomially bounded operator on Hilbert space which is not similar to a contraction

Pisier, Gilles

doi:10.1090/S0894-0347-97-00227-0

A polynomially bounded operator on Hilbert space which is not similar to a contraction
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by Gilles Pisier

J. Amer. Math. Soc. 10 (1997), 351-369

DOI: https://doi.org/10.1090/S0894-0347-97-00227-0

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Abstract:

Let $\varepsilon >0$. We prove that there exists an operator $T_{\varepsilon }:\ell _{2}\to \ell _{2}$ such that for any polynomial $P$ we have $\|{P(T_{\varepsilon })}\| \leq (1+\varepsilon ) \|{P}\|_{\infty }$, but $T_{\varepsilon }$ is not similar to a contraction, i.e. there does not exist an invertible operator $S: \ell _{2}\to \ell _{2}$ such that $\|{S^{-1}T_{\varepsilon }S}\|\leq 1$. This answers negatively a question attributed to Halmos after his well-known 1970 paper (“Ten problems in Hilbert space"). We also give some related finite-dimensional estimates.

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