⇒ The MacTutor History of Mathematics Archive
Huygens, Christian, Horologium oscillatorium sive de motu pendulorum ad horologia aptato demonstrationes geometricæ, apud F. Muguet, Parisiis 1673.
⇒ download from Kinematic Models for Design Digital Library
Huygens, Christian, Christiani Hugenii, dynastae in Zülechem, solutio ejusdem problematis, “Acta Eruditorum”, 1691, pp. 281-282.
⇒ download from Gallica