Coba jelasin 1890. "Nicht-Euklidische Geometrie " dari Felix Klein dong

David Mumford, Caroline Series, and David Wright Indra's Pearls: The Vision of Felix Klein. Cambridge Univ. Press. 2002.

Tobies, Renate (with Fritz König) Felix Klein. Teubner Verlag, Leipzig 1981.

Rowe, David "Felix Klein, David Hilbert, and the Göttingen Mathematical Tradition", in Science in Germany: The Intersection of Institutional and Intellectual Issues, Kathryn Olesko, ed., Osiris, 5 (1989), 186-213.