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, one has
where is the base of the natural logarithm, is the imaginary unit, and and are the trigonometric functions cosine and sine respectively. This complex exponential function is sometimes denoted . The formula is still valid if is a complex number, and is also called Euler's formula in this more general case.
Euler's formula is ubiquitous in mathematics, physics, chemistry, and engineering. The physicist Richard Feynman called the equation "our jewel" and "the most remarkable formula in mathematics".
When, Euler's formula may be rewritten as or, which is known as Euler's identity.

History

In 1714, the English mathematician Roger Cotes presented a geometrical argument that can be interpreted as:
Exponentiating this equation yields Euler's formula. Note that the logarithmic statement is not universally correct for complex numbers, since a complex logarithm can have infinitely many values, differing by multiples of.
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. The formula was first published in 1748 in his foundational work Introductio in analysin infinitorum.
Johann Bernoulli had found that
And since
the above equation tells us something about complex logarithms by relating natural logarithms to imaginary numbers. Bernoulli, however, did not evaluate the integral.
Bernoulli's correspondence with Euler 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.

Definitions of complex exponentiation

The exponential function for real values of may be defined in a few different equivalent ways. Several of these methods may be directly extended to give definitions of for complex values of simply by substituting in place of and using the complex algebraic operations. In particular, we may use any of the three following definitions, which are equivalent. From a more advanced perspective, each of these definitions may be interpreted as giving the unique analytic continuation of to the complex plane.

Differential equation definition

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

Power series definition

For complex
Using the ratio test, it is possible to show that this power series has an infinite radius of convergence and so defines for all complex.

Limit definition

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

Proofs

Various proofs of the formula are possible.

Using differentiation

This proof shows that the quotient of the trigonometric and exponential expressions is the constant function one, so they must be equal.
Consider the function
for real. Differentiating gives by the product rule
Thus, is a constant. Since the exponential function is for, by definition, and the complex trig function also evaluates to there,, then for all real, and thus

Using power series

Here is a proof of Euler's formula using power-series expansions, as well as basic facts about the powers of :
Using now the power-series definition from above, we see that for real values of
where in the last step we recognize the two terms are the Maclaurin series for and. The rearrangement of terms is justified because each series is absolutely convergent.

Using polar coordinates

Another proof is based on the fact that all complex numbers can be expressed in polar coordinates and on the assumption that can be likewise represented; this will be the case if we find a solution. Therefore, for some and depending on,
No assumptions are being made about and ; they will be determined in the course of the proof. From any of the definitions of the exponential function it can be shown that the derivative of is. Therefore, differentiating both sides gives
Substituting for and equating real and imaginary parts in this formula gives and. Thus, is a constant, and is for some constant. We now have
knowing that, for, this becomes
giving us the constant and proving the formula

Applications

Applications in complex number theory

[Image:Euler's formula.svg|thumb|right|Euler's formula illustrated in the complex plane.]

Interpretation of the formula

This formula can be interpreted as saying that the function is a unit complex number, i.e., it traces out the unit circle in the complex plane as ranges through the real numbers. Here 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 and of and for real numbers . In fact, the same proof shows that Euler's formula is even valid for all complex numbers .
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, and its complex conjugate,, can be written as
where
is the argument of, i.e., the angle between the x axis and the vector z measured counterclockwise in radians, which is defined up to addition of. Many texts write instead of, but the first equation needs adjustment when. This is because for any real and, not both zero, the angles of the vectors and differ by radians, but have the identical value of.

Use of the formula to define the logarithm of complex numbers

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 :
and that
both valid for any complex numbers and. Therefore, one can write:
for any. Taking the logarithm of both sides shows that
and in fact, this can be used as the definition for the complex logarithm. The logarithm of a complex number is thus a multi-valued function, because is multi-valued.
Finally, the other exponential law
which can be seen to hold for all integers, together with Euler's formula, implies several trigonometric identities, as well as de Moivre's formula.

Relationship to trigonometry

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:
and solving for either cosine or sine.
These formulas can even serve as the definition of the trigonometric functions for complex arguments. For example, letting, we have:
In addition
Complex exponentials can simplify trigonometry, because they are mathematically easier to manipulate than their sine and cosine components. One technique is simply to convert sines and cosines into equivalent expressions in terms of exponentials sometimes called complex sinusoids. After the manipulations, the simplified result is still real-valued. For example:
Another technique is to represent sines and cosines 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 for integer values of and arbitrary .
Considering a parameter in equation above yields recursive formula for Chebyshev polynomials of the first kind.

Topological interpretation

In the language of topology, Euler's formula states that the imaginary exponential function is a 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:

Other applications

In differential equations, the function 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, 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 on this sphere, and a real number, Euler's formula applies:
and the element is called a versor in quaternions. The set of all versors forms a 3-sphere in the 4-space.

Other special cases

The special cases that evaluate to units illustrate rotation around the complex unit circle:

The special case at yields. This is also argued to link five fundamental constants with three basic arithmetic operations, but, unlike Euler's identity, without rearranging the addends from the general case:
An interpretation of the simplified form is that rotating by a full turn is an identity function.