Below is the ascii version of the abstract for 07-139.
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The absolutely continuous spectrum of Jacobi matrices
ABSTRACT. I explore some consequences of a groundbreaking result of Breimesser and Pearson on the absolutely continuous spectrum of one-dimensional Schr"odinger operators. These include an Oracle Theorem that predicts the potential and rather general results on the approach to certain limit potentials. In particular, we prove a Denisov-Rakhmanov type theorem for the general finite gap case.
The main theme is the following: It is extremely difficult to produce
absolutely continuous spectrum in one space dimension and thus its existence has strong implications.