Between the von Neumann inequality and the Crouzeix conjecture

A new concept of a deformed numerical range $W^{\rho }(T)$ is introduced. Here $T$ is a bounded linear operator or a matrix and $\rho \in [1,+\mathrm{\infty })$ is a parameter. Each $W^{\rho }(T)$ is a closed convex set that contains the spectrum of $T$. Furthermore, $W^{\rho }(T)$ is decreasing with respect to $\rho$ and $W^2(T)$ coincides with the numerical range. It is also shown that $W^{\rho }(T)$ is contained in the closed unit disc if and only if $T$ has a $\rho$ unitary dilation in the sense of N'agy-Foia\c s. The spectral constants of $W^{\rho }(T)$ are investigated, it is shown that it is monotone and continuous with respect to the parameter $\rho$.