1. [1827] C. F. Gauss, Disquisitiones generales circa superficies curvas, Commentationes Gottingensis 6 (Werke 4, 218-258; German summary, 341-347).
2. [1829-30] N. I. Lobachevsky, On the foundations of geometry, Kazan Messenger 25-28; German transl., ’Zwei geometrische Abhandlungen’, Leipzig, 1898.
3. [1832] T. Clausen, Ueber die Function sin φ + (1/2., J. Reine Angew. Math. 8, 298-300.
4. [1832] J. Bolyai, The absolute science of space, independent of the truth or falsity of Euclid’s Axiom 11 (which can never be proved a priori), Maros-Vásárhelyini.
5. [1836] N. I. Lobachevsky, Imaginary geometry and its application to integration, Kazan (German transl. Abh. Gesch. Math. 19, 1904).