Simplify the following text for a Grade 11 native English-speaking person.
Original: Christian Felix Klein ( 25 April 1849 â `` 22 June 1925 ) was a German mathematician , known for his work in group theory , function theory , non-Euclidean geometry , and on the connections between geometry and group theory .
Simplified: Felix Christian Klein ( April 25 , 1849 , DÃ 1\/4 sseldorf , ( then ) Prussian Empire -- June 22 , 1925 , GÃ ttingen , 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 .