In 1932, when Klein finished High School, despite wanting badly to pursue his love of mathematics, he gave in to his father's demands and enrolled at the University of Amsterdam for Medicine; he succeeded in getting his bachelor's degree in 1935.
Both Klein and his older brother Leo were regularly examined by a neurologist in Amsterdam for their incredible computing capabilities.
When the Germans invaded in May 1940, Klein began working in a Jewish hospital and continued with his doctoral studies in 1941.
After the war, Klein returned to his doctoral studies, but he also worked in circuses in France, Belgium, and the Netherlands, performing fast calculations as an act, often under the stage name 'Pascal'.
This continued until 1 August 1986, when Klein's housekeeper found him dead in his home in Amsterdam, brutally murdered with a knife.