The greatest mathematicians, as Archimedes, Newton, and Gauss, always united theory and applications in equal measure.

Erster Band; Die gruppentheoretischen Grundlagen (in German) Leipzig: B. Annalen Bd. In the 1890s Klein turned to mathematical physics a subject from which he had never strayed far writing on the gyroscope with Arnold Sommerfeld.

Christian Felix Klein (25 April 1849 – 22 June 1925) was a German mathematician known for his work in group theory complex analysis non-Euclidean geometry and on the connections between geometry and group theory.