Applications of the Fuglede-Kadison determinant: Szegö's theorem and outers for noncommutative $H^p$

David P. Blecher, Louis E. Labuschagne

Published 2006-09-25, updated 2007-02-23Version 2

We first use properties of the Fuglede-Kadison determinant on $L^p(M)$, for a finite von Neumann algebra $M$, to give several useful variants of the noncommutative Szeg\"{o} theorem for $L^p(M)$, including the one usually attributed to Kolmogorov and Krein. As an application, we solve the longstanding open problem concerning the noncommutative generalization, to Arveson's noncommutative $H^p$ spaces, of the famous `outer factorization' of functions $f$ with $\log |f|$ integrable. Using the Fuglede-Kadison determinant, we also generalize many other classical results concerning outer functions.