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

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.

Locations

• Journal of the American Mathematical Society - View - PDF
• arXiv (Cornell University) - View - PDF