Complete Pick Positivity and Unitary Invariance

Abstract

The characteristic function for a contraction is a classical complete unitary invariant devised by Sz.-Nagy and Foias. Just as a contraction is related to the Szego kernel $k_S(z,w) = (1 - z\ow)^{-1}$ for $|z|,|w|<1$, by means of $(1/k_S)(T,T^{\ast })\ge 0$, we consider an arbitrary open connected domain $\Omega$ in $\BC^n$, a complete Nevanilinna-Pick kernel $k$ on $\Omega$ and a tuple $T=(T_1,...,T_n)$ of commuting bounded operators on a complex separable Hilbert space $\clh$ such that $(1/k)(T,T^{\ast })\ge 0$. For a complete Pick kernel the $1/k$ functional calculus makes sense in a beautiful way. It turns out that the model theory works very well and a characteristic function can be associated with $T$. Moreover, the characteristic function then is a complete unitary invariant for a suitable class of tuples $T$.

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