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Euler's identity

In mathematics, Euler's identity, a special case of Euler's formula, is the following:

<math> e^{i \pi} + 1 = 0\; <math>

The equation appears in Leonhard Euler's Introduction, published in Lausanne in 1748. In this equation, e is the base of the natural logarithm, <math>i<math> is the imaginary unit (an imaginary number with the property i² = -1), and <math> \pi <math> is Archimedes' constant pi (π, the ratio of the circumference of a circle to its diameter).

The formula is a special case of Euler's formula from complex analysis, which states that

<math>e^{ix} = \cos x + i \sin x \,\!<math>

for any real number <math>x<math>. If we set <math>x = \pi<math>, then

<math>e^{i \pi} = \cos \pi + i \sin \pi \,\!<math>

and since cos(π) = −1 and sin(π) = 0, we get

<math>e^{i \pi} = -1 \,\!<math>

and therefore

<math>e^{i \pi} + 1 = 0 \,\!<math>

Perceptions of the identity

It was called "the most remarkable formula in mathematics" by Richard Feynman. Feynman found this formula remarkable because it links some very fundamental mathematical constants:

• The number 0, the identity element for addition (for all a, a+0=0+a=a). See Group (mathematics).
• The number 1, the identity element for multiplication (for all a, a×1=1×a=a).
• The number <math>\pi <math> is fundamental in trigonometry, <math>\pi <math> is a constant in a world which is Euclidean, on small scales at least (otherwise, the ratio of the length of the circumference of circle to its diameter would not be a universal constant, i.e. the same for all circumferences).
• The number <math>e<math> is a fundamental in connections to the study of logarithms and in calculus (such as in describing growth behaviors, as the solution to the simplest growth equation <math>dy / dx = y<math> with initial condition <math>y(0) = 1<math> is <math>y = e^x<math>).
• The imaginary unit <math>i<math> (where i² = −1) is a unit in the complex numbers. Introducing this unit yields all non-constant polynomial equations soluble in the field of complex numbers (see fundamental theorem of algebra).

Furthermore, all the most fundamental operators of arithmetic are also present: equality, addition, multiplication and exponentiation. All the fundamental assumptions of complex analysis are present, and the integers 0 and 1 are related to the field of complex numbers.

In addition, the result is remarkable to most students learning it for the first time because it is so highly counterintuitive. Consider that

<math>e^{\pi} = 23.1406... \,\!<math> while <math>e^{i \pi} = -1 \,\!<math>

The simple addition of i changes the result dramatically.

References

• Feynman R.P. - The Feynman Lectures on Physics, vol. I - part 1. Inter European Editions, Amsterdam (1975)