Felix C. Klein was a mathematician now famous for his study of group theory in conjunction with geometry (Klein geometry) as laid out in his Erlanger program (Klein 72).
John Baez, Klein geometry XII, nCafe
Felix Klein, A comparative review of recent researches in geometry, 1872 (1892 translation: arxiv/0807.3161
on the Erlangen program
Felix Klein, Vorlesungen über das Ikosaeder und die Auflösung der Gleichungen vom fünften Grade, 1884, translated as Lectures on the Icosahedron and the Resolution of Equations of Degree Five by George Morrice 1888, online version
on the classification of finite rotation groups and specifically the icosahedron