Todhunter, Isaac (1873). "Note on the history of certain formulæ in spherical trigonometry". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 45 (298): 98–100. doi:10.1080/14786447308640820.
Chamberlain, Robert G.; Duquette, William H. (17 April 2007). Some algorithms for polygons on a sphere. Association of American Geographers Annual Meeting. NASA JPL. Retrieved 7 August 2020.
O'Connor, John J.; Robertson, Edmund F., "Nasir al-Din al-Tusi", MacTutor History of Mathematics Archive, University of St Andrews "One of al-Tusi's most important mathematical contributions was the creation of trigonometry as a mathematical discipline in its own right rather than as just a tool for astronomical applications. In Treatise on the quadrilateral al-Tusi gave the first extant exposition of the whole system of plane and spherical trigonometry. This work is really the first in history on trigonometry as an independent branch of pure mathematics and the first in which all six cases for a right-angled spherical triangle are set forth"
