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Euler's formula : ウィキペディア英語版
Euler's formula

Euler's formula, named after Leonhard Euler, is a mathematical formula in complex analysis that establishes the fundamental relationship between the trigonometric functions and the complex exponential function. Euler's formula states that, for any real number :
:e^=\cos x+i\sin x
where is the base of the natural logarithm, is the imaginary unit, and and are the trigonometric functions cosine and sine respectively, with the argument ''x'' given in radians. This complex exponential function is sometimes denoted ("cosine plus i sine"). The formula is still valid if is a complex number, and so some authors refer to the more general complex version as Euler's formula.
Euler's formula is ubiquitous in mathematics, physics, and engineering. The physicist Richard Feynman called the equation "our jewel" and "the most remarkable formula in mathematics."
==History==
Johann Bernoulli noted that〔Johann Bernoulli, Solution d'un problème concernant le calcul intégral, avec quelques abrégés par rapport à ce calcul, ''Mémoires de l'Académie Royale des Sciences de Paris'', 197-289 (1702).〕
\dfrac \left( \dfrac +\dfrac \right)
And since
\int \dfrac =\dfrac \ln \left( 1+ax\right) +C
the above equation tells us something about complex logarithms. Bernoulli, however, did not evaluate the integral.
Bernoulli's correspondence with Euler (who also knew the above equation) shows that Bernoulli did not fully understand complex logarithms. Euler also suggested that the complex logarithms can have infinitely many values.
Meanwhile, Roger Cotes, in 1714, discovered that
ix=\ln \left( \cos x+i\sin x\right)
(\ln is the natural logarithm).
Cotes missed the fact that a complex logarithm can have infinitely many values, differing by multiples of , due to the periodicity of the trigonometric functions.
Around 1740 Euler turned his attention to the exponential function instead of logarithms, and obtained the formula used today that is named after him. It was published in 1748, obtained by comparing the series expansions of the exponential and trigonometric expressions.〔
None of these mathematicians saw the geometrical interpretation of the formula; the view of complex numbers as points in the complex plane was described some 50 years later by Caspar Wessel.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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