The Lax-Onsager regression 'theorem' revisited

It is stated by Ford and O'Connell in this festschrift issue and elsewhere that "there is no quantum regression theorem'' although Lax "obtained a for mula for correlation in a driven quantum system that has come to be called the quantum regression theorem". This produces a puzzle: "How can it be tha t a non-existent theorem gives correct results?" Clarification will be prov ided in this paper by a description of the Lax procedure, with a quantitati ve estimate of the error for a damped harmonic oscillator based on expressi ons published in the 1960's. (C) 2000 Elsevier Science B.V. All rights rese rved.