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**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 x:

where e is the base of the natural logarithm, i is the imaginary unit, and cos and sin are the trigonometric functions cosine and sine respectively. This complex exponential function is sometimes denoted cis

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".^{[2]}

When *x* = *π*, Euler's formula may be rewritten as *e ^{iπ}* + 1 = 0, which is known as Euler's identity.

In 1714, the English mathematician Roger Cotes presented a geometrical argument that can be interpreted (after correcting a misplaced factor of ) as:^{[3]}^{[4]}^{[5]}

Around 1740 Leonhard Euler turned his attention to the exponential function and derived the equation named after him by comparing the series expansions of the exponential and trigonometric expressions.^{[6]}^{[4]} The formula was first published in 1748 in his foundational work *Introductio in analysin infinitorum*.^{[7]}

Johann Bernoulli had found that^{[8]}

And since

Bernoulli's correspondence with Euler (who also knew the above equation) shows that Bernoulli did not fully understand complex logarithms. Euler also suggested that complex logarithms can have infinitely many values.

The view of complex numbers as points in the complex plane was described about 50 years later by Caspar Wessel.

The exponential function *e ^{x}* for real values of x may be defined in a few different equivalent ways (see Characterizations of the exponential function). Several of these methods may be directly extended to give definitions of

The exponential function is the unique differentiable function of a complex variable for which the derivative equals the function

For complex z

Using the ratio test, it is possible to show that this power series has an infinite radius of convergence and so defines *e ^{z}* for all complex z.

For complex z

Here, n is restricted to positive integers, so there is no question about what the power with exponent n means.

Various proofs of the formula are possible.

This proof shows that the quotient of the trigonometric and exponential expressions is the constant function one, so they must be equal (the exponential function is never zero,^{[9]} so this is permitted).^{[10]}

Consider the function *f*(*θ*)

Here is a proof of Euler's formula using power-series expansions, as well as basic facts about the powers of i:^{[11]}

Using now the power-series definition from above, we see that for real values of x

Another proof^{[12]} is based on the fact that all complex numbers can be expressed in polar coordinates. Therefore, for some r and θ depending on x,

This formula can be interpreted as saying that the function *e*^{iφ} is a unit complex number, i.e., it traces out the unit circle in the complex plane as φ ranges through the real numbers. Here φ is the angle that a line connecting the origin with a point on the unit circle makes with the positive real axis, measured counterclockwise and in radians.

The original proof is based on the Taylor series expansions of the exponential function *e*^{z} (where z is a complex number) and of sin *x* and cos *x* for real numbers x (see below). In fact, the same proof shows that Euler's formula is even valid for all *complex* numbers x.

A point in the complex plane can be represented by a complex number written in cartesian coordinates. Euler's formula provides a means of conversion between cartesian coordinates and polar coordinates. The polar form simplifies the mathematics when used in multiplication or powers of complex numbers. Any complex number *z* = *x* + *iy*, and its complex conjugate, *z* = *x* − *iy*, can be written as

*x*= Re*z*is the real part,*y*= Im*z*is the imaginary part,*r*= |*z*| = √*x*^{2}+*y*^{2}is the magnitude of z and*φ*= arg*z*= atan2(*y*,*x*).

φ is the argument of z, i.e., the angle between the *x* axis and the vector *z* measured counterclockwise in radians, which is defined up to addition of 2*π*. Many texts write *φ* = tan^{−1} *y/x* instead of *φ* = atan2(*y*,*x*), but the first equation needs adjustment when *x* ≤ 0. This is because for any real x and y, not both zero, the angles of the vectors (*x*, *y*) and (−*x*, −*y*) differ by π radians, but have the identical value of tan *φ* = *y*/*x*.

Now, taking this derived formula, we can use Euler's formula to define the logarithm of a complex number. To do this, we also use the definition of the logarithm (as the inverse operator of exponentiation):

Finally, the other exponential law

Euler's formula, the definitions of the trigonometric functions and the standard identities for exponentials are sufficient to easily derive most trigonometric identities. It provides a powerful connection between analysis and trigonometry, and provides an interpretation of the sine and cosine functions as weighted sums of the exponential function:

The two equations above can be derived by adding or subtracting Euler's formulas:

These formulas can even serve as the definition of the trigonometric functions for complex arguments x. For example, letting *x* = *iy*, we have:

Complex exponentials can simplify trigonometry, because they are easier to manipulate than their sinusoidal components. One technique is simply to convert sinusoids into equivalent expressions in terms of exponentials. After the manipulations, the simplified result is still real-valued. For example:

Another technique is to represent the sinusoids in terms of the real part of a complex expression and perform the manipulations on the complex expression. For example:

This formula is used for recursive generation of cos *nx* for integer values of n and arbitrary x (in radians).

In the language of topology, Euler's formula states that the imaginary exponential function is a (surjective) morphism of topological groups from the real line to the unit circle . In fact, this exhibits as a covering space of . Similarly, Euler's identity says that the kernel of this map is , where . These observations may be combined and summarized in the commutative diagram below:

In differential equations, the function *e ^{ix}* is often used to simplify solutions, even if the final answer is a real function involving sine and cosine. The reason for this is that the exponential function is the eigenfunction of the operation of differentiation.

In electrical engineering, signal processing, and similar fields, signals that vary periodically over time are often described as a combination of sinusoidal functions (see Fourier analysis), and these are more conveniently expressed as the sum of exponential functions with imaginary exponents, using Euler's formula. Also, phasor analysis of circuits can include Euler's formula to represent the impedance of a capacitor or an inductor.

In the four-dimensional space of quaternions, there is a sphere of imaginary units. For any point r on this sphere, and x a real number, Euler's formula applies:

**^**Moskowitz, Martin A. (2002).*A Course in Complex Analysis in One Variable*. World Scientific Publishing Co. p. 7. ISBN 981-02-4780-X.**^**Feynman, Richard P. (1977).*The Feynman Lectures on Physics, vol. I*. Addison-Wesley. p. 22-10. ISBN 0-201-02010-6.**^**Cotes wrote:*"Nam si quadrantis circuli quilibet arcus, radio*CE*descriptus, sinun habeat*CX*sinumque complementi ad quadrantem*XE*; sumendo radium*CE*pro Modulo, arcus erit rationis inter &*CE*mensura ducta in ."*(Thus if any arc of a quadrant of a circle, described by the radius*CE*, has sinus*CX*and sinus of the complement to the quadrant*XE*; taking the radius*CE*as modulus, the arc will be the measure of the ratio between &*CE*multiplied by .) That is, consider a circle having center*E*(at the origin of the (x,y) plane) and radius*CE*. Consider an angle*θ*with its vertex at*E*having the positive x-axis as one side and a radius*CE*as the other side. The perpendicular from the point*C*on the circle to the x-axis is the "sinus"*CX*; the line between the circle's center*E*and the point*X*at the foot of the perpendicular is*XE*, which is the "sinus of the complement to the quadrant" or "cosinus". The ratio between and*CE*is thus . In Cotes' terminology, the "measure" of a quantity is its natural logarithm, and the "modulus" is a conversion factor that transforms a measure of angle into circular arc length (here, the modulus is the radius (*CE*) of the circle). According to Cotes, the product of the modulus and the measure (logarithm) of the ratio, when multiplied by , equals the length of the circular arc subtended by*θ*, which for any angle measured in radians is*CE*•*θ*. Thus, . This equation has the wrong sign: the factor of should be on the right side of the equation, not the left side. If this change is made, then, after dividing both sides by*CE*and exponentiating both sides, the result is: , which is Euler's formula.

See:- Roger Cotes (1714) "Logometria,"
*Philosophical Transactions of the Royal Society of London*,**29**(338) : 5-45 ; see especially page 32. Available on-line at: Hathi Trust - Roger Cotes with Robert Smith, ed.,
*Harmonia mensurarum*… (Cambridge, England: 1722), chapter: "Logometria", p. 28.

- Roger Cotes (1714) "Logometria,"
- ^
^{a}^{b}John Stillwell (2002).*Mathematics and Its History*. Springer. ISBN 9781441960528. **^**Sandifer, C. Edward (2007),*Euler's Greatest Hits*, Mathematical Association of America ISBN 978-0-88385-563-8**^**Leonard Euler (1748) Chapter 8: On transcending quantities arising from the circle of Introduction to the Analysis of the Infinite, page 214, section 138 (translation by Ian Bruce, pdf link from 17 century maths).**^**Conway & Guy, pp. 254–255**^**Bernoulli, Johann (1702). "Solution d'un problème concernant le calcul intégral, avec quelques abrégés par rapport à ce calcul" [Solution of a problem in integral calculus with some notes relating to this calculation].*Mémoires de l'Académie Royale des Sciences de Paris*.**1702**: 289–297.**^**Apostol, Tom (1974).*Mathematical Analysis*. Pearson. p. 20. ISBN 978-0201002881. Theorem 1.42**^**user02138 (https://math.stackexchange.com/users/2720/user02138), How to prove Euler's formula: $e^{i\varphi}=\cos(\varphi) +i\sin(\varphi)$?, URL (version: 2018-06-25): https://math.stackexchange.com/q/8612**^**Ricardo, Henry J. (23 March 2016).*A Modern Introduction to Differential Equations*. p. 428. ISBN 9780123859136.**^**Strang, Gilbert (1991).*Calculus*. Wellesley-Cambridge. p. 389. ISBN 0-9614088-2-0. Second proof on page.

- Elements of Algebra