Felix Christian Klein (April 25, 1849, Düsseldorf, (then) Prussian Empire – June 22, 1925, Göttingen, Germany) was a German mathematician. He became known for his work in group theory, function theory, non-Euclidean geometry, and on the connections between geometry and group theory. His 1872 Erlangen Program, which ate geometries by their underlying symmetry groups, was a hugely influential synthesis of much of the mathematics of the day.