Willem Klein (4 December 1912 – 1 August 1986), also known as Wim Klein^[1] or under his stage names Pascal and Willy Wortel, was a Dutch mental calculator, famous for ability to perform very complicated calculations in his head very fast. On 27 August 1976, he calculated the 73rd root of a 500-digit number in 2 minutes and 43 seconds. This feat was recorded by the Guinness Book of Records.^[2]

Wim Klein was born in Amsterdam on 4 December 1912 to Jewish parents,^[3] Henry Klein, GP, and Emma Cohen. Klein had a rough childhood because his father wanted him to become a Doctor (as his father was), even though he was quite opposed to the idea. In addition to this pressure, his mother also committed suicide in 1929. In 1932, when Klein finished high school, despite his strong desire to pursue his love of mathematics, he gave in to his father's demands and enrolled at the University of Amsterdam for Medicine. He graduated with a bachelor's degree in 1935. His father died in 1937. Although Klein passed the first part of his doctoral exam, he eventually gave up. It was around this time that he discovered his homosexuality. Both Klein and his older brother Leo were regularly examined by a neurologist in Amsterdam for their incredible computing capabilities. Stokvis labeled Wim as an "auditory calculator," and his brother Leo as a "visual calculator."

When the Germans invaded in May 1940, Klein began working in a Jewish hospital and continued with his doctoral studies in 1941. In 1942, though, he had to hide; his brother was captured and taken to the Sobibór extermination camp, where he died. After the war, Klein returned to his doctoral studies. Additionally, he also worked in circuses in France, Belgium, and the Netherlands, performing fast calculations as an act, often under the stage name 'Pascal'. He lived a fairly nomadic lifestyle and performed in such shows until 1952.

In 1952,^[4] Klein was hired as a scientific calculator at the Mathematisch Centrum (English: Mathematical Center) in Amsterdam. In 1954, he attended the International Congress of Mathematicians in Amsterdam, which inspired him to return to performing and embrace his nomadic lifestyle. He performed internationally again for a few years until he was hired by CERN in 1958. However, as computers became more powerful in the 1960s and physicists began programming more, Klein found himself often used as a mascot for CERN, to perform calculations for visitors, which he found disheartening.

In 1975, the commemoration of the 700th anniversary of Amsterdam made him feel homesick, leading to his early retirement from CERN in 1976. He continued to perform calculations for shows, this time with the stage name "Willy Wortel". However, he became increasingly interested in breaking records, trying hard to improve his time and beat new records. This continued until 1 August 1986, when Klein's housekeeper found him dead in his home in Amsterdam, brutally murdered with a knife. Although a young man was arrested, there was no evidence linking him to the case, and he was soon released. The murder remains unsolved.^[5]

• List of unsolved murders

• Wim Klein's retirement show, CERN 1976
• Smith, Steven B. (1983), "Chapter 34 – Wim Klein", The Great Mental Calculators: The Psychology, Methods, and Lives of Calculating Prodigies, Past and Present, Columbia University Press, ISBN 0-231-05640-0, archived from the original on 17 July 2007, retrieved 10 November 2007.
• van den Brandhof, Alex (July 2005), "Willem Klein (1912-1986)", Biografisch Woordenboek van Nederlandse Wiskundingen [Biographical Dictionary of Dutch mathematicians] (in Dutch), archived from the original on 24 July 2011, retrieved 3 April 2010.
• Haag, Den (12 November 2013). "Klein, Willem". Huygens ING. Retrieved 20 January 2015.
• "The Birth of the Web". CERN. 2015. Retrieved 20 January 2015.