An important aspect of Iamblichus' citation of Aristotle's Protrepticus in his Protr. VI (but one misunderstood in the scholarly literature) is the fact his citation there overlaps (at exactly four stretches, it turns out) with a citation (evidently from the same work) in his De communi mathematici scientia XXVI. Because of the overlap, scholars going back to Rose 1889 have attributed various stretches of DCM XXVI to Aristotle's Protrepticus. Our study of Iamblichus' techniques of chapter construction in his Pythagorean Sequence (of which the Protr. and the DCM are the second and third works) indicates that Iamblichus would have utilized Aristotle's Protrepticus as a source throughout that chapter.
We have recently posted to www.protrepticus.info draft translations of Iamblichus, Protr. VI and DCM XXVI. For Protr. VI, we have also posted a text with apparatus criticus and commentary. The text in Bold we attribute verbatum to Aristotle; italics for Iamblichus; plain text is used for zones of uncertainty. (As always, we would greatly appreciate your feedback.)
Once it has been established that Iamblichus used Aristotle's Protrepticus as a source in DCM XXVI, the next question that needs to be asked is: where exactly did Iamblichus start and stop using Aristotle's Protrepticus as a source in the DCM?
Our answer, that we plan to defend in a series of upcoming talks in the UK and Italy (details forthcoming), is that Iamblichus used Aristotle's Protrepticus as a source throughout DCM XXI-XXVII. Realizing that has allowed us to attribute and authenticate around 600 lines of new material to Aristotle's Protrepticus.