Page "Leonhard Euler" Paragraph 28
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Notably, Euler directly proved the power series expansions for and the inverse tangent function.
( Indirect proof via the inverse power series technique was given by Newton and Leibniz between 1670 and 1680.
) His daring use of power series enabled him to solve the famous Basel problem in 1735 ( he provided a more elaborate argument in 1741 ):
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