| Résume | The classical von Neumann inequality provides a fundamental link between complex analysis and operator theory. It shows that for any contraction T on a Hilbert space and any polynomial p, the operator norm of $p(T)$ satisfies
\[ \|p(T)\| \le \sup_{|z| \le 1} |p(z)|. \]
Whereas Ando extended this inequality to pairs of commuting contractions, the corresponding statement for triples of commuting contractions is false. However, it is still not known whether von Neumann's inequality for triples of commuting contractions holds up to a constant. I will talk about this question and about new upper and lower bounds for $\|p(T)\|$.
This partly joint work with Dexie Lin and Yi Wang. |
