The Euler formula, sometimes also called the Euler identity (e.g., Trott 2004, p. 174), states
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(1)
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where i is the imaginary unit. Note that Euler's polyhedral formula is sometimes also called the Euler formula, as is the Euler curvature formula. The equivalent expression
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(2)
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had previously been published by Cotes (1714).
The special case of the formula with 
gives the beautiful identity
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(3)
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an equation connecting the fundamental numbers i, pi, e, 1, and 0 (zero),
the fundamental operations 
, 
,
and exponentiation, the most important relation 
, and nothing else. Gauss is reported to have commented that
if this formula was not immediately obvious, the reader would never be a first-class
mathematician (Derbyshire 2004, p. 202).
The Euler formula can be demonstrated using a series expansion
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(4)
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(5)
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(6)
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It can also be demonstrated using a complex integral. Let
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(7)
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(8)
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(9)
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(10)
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(11)
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(12)
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so
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(13)
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(14)
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A mathematical joke asks, "How many mathematicians does it take to change a light bulb?" and answers "
" (which, of course, equals 1).
