ABSTRACT
Abº Sahl Wayjan (Bijæn) ibn Rustam al-Qºhî (al-Kºhî) was one of the principal astronomers and mathematicians of the school of Baghdad, and in particular of the Buyid court. We can measure the importance of his works by the references made to them by his contemporaries, like al-Sijzî or Ibn Sahl, and by his successors, like Ibn al-Haytham and al-Bîrºnî. In his time, according to the report transmitted by the man of letters Abº Îayyæn alTawÌîdî, al-Qºhî was presented as an eminent scholar who was concerned neither with theology nor with metaphysics.1 This mathematician developed to their farthest point the epistemic characteristics that had distinguished this tradition since its foundation, a century earlier, by the Banº Mºsæ, as well as throughout its successive transformations since Thæbit ibn Qurra and his grandson Ibræhîm ibn Sinæn. Al-Qºhî was interested in the application of mathematics to astronomy and to statics, and in the study of mathematical instruments such as the perfect compass.2 In addition, he took an active part in the broadening of research on geometrical transformations: in this regard, it suffices to mention his Treatise on the Art of the Astrolabe by Demonstration.3 Al-Qºhî and the mathematicians of his tendency, among whom was Ibn Sahl, combined the two traditions of Greek geometry – that of Archimedes and that of Apollonius – in order to advance onto a terrain that was not truly Hellenistic: that of transformations. Al-Qºhî also had the
advantage of his chronological situation, and he gathered the fruits of the already considerable accumulation of work carried out since the Banº Mºsæ and Thæbit ibn Qurra.
