## Saturday, February 25, 2012

### EULER !

Richard Feynman called Euler's formula "our jewel"[2] and "one of the most remarkable, almost astounding, formulas in all of mathematics."[3]
 You can buy the ring above for a mere \$245 here. One equation to bring them all and in the darkness bind them. In the land of Euler (that would be Switzerland, although Euler did most of his work and lived in St. Petersberg and Prussia).

Dr. Euler's Fabulous Formula: Cures Many Mathematical Ills by Paul J. Nahin, Professor Emeritus, University of New Hampshire, PhDEE

From + Plus magazine ... Living Mathematics Review by Lewis Dartnell

The hero of this book is Euler's formula:
eiπ + 1 = 0
This simple equation has been widely considered through the last two centuries to be one of the most beautiful formulae of mathematics, and Nahin tells us why.
The constant e is the base of the natural logarithm (and you might have used it in calculations on radioactive decay in physics lessons, for example), whereas &pi, as we all know, is the ratio of a circle's circumference to its diameter. Bothe and &pi are irrational numbers, that is, you could never write down all of their decimal places as they can be proven to continue for ever. The symbol i denotes the square root of -1, a value that does not even exist on the standard number line. Each of these three quantities is therefore curious in its own right, and they were originally developed within very different areas of maths. So how on Earth does ei&pi equal exactly -1? It seems like the biggest fluke in the Universe — and this is part the formula's exquisite beauty. It is also fairly easy to derive the formula yourself, and the proof can be found in any textbook on complex numbers. And as Nahin's book shows, it is also one of the most influential formulae in the history of mathematics.
The book starts off gently enough, with an Introduction leading the reader through a few examples of the most fundamental skill in mathematics; constructing a proof. We see, for example, why √2 must be irrational through a simple proof by contradiction. The process of proving things is mostly ignored at GCSE and A-level, but really does give an eye-opening insight into how real mathematics is often like solving a intriguing puzzle, rather than slogging through homework exercises to practice the basic skills.
Through the following six chapters of the book, Nahin shows us many of the applications of imaginary numbers, Euler's formula, and other mathematical formulae and techniques that have been built on this eighteenth century foundation, both in pure and applied maths. Regular Plusreaders might already know a little bit about some of these, and the sections in the book include: drawing regular prime number polygons, such as the 17-gon, using only a ruler and a compass; how to deconstruct any continuous function, such as a sound wave, into a set of sine waves — a technique that is crucial to modern gadgets like mp3 players. Nahin also shows us the maths of complex numbers lying behind the uncertainty principle of quantum mechanics, listening to the radio, and even how to build a telephone voice scrambler from simple electronics.
The book is well-written and illustrated with numerous diagrams and graphs. For those wanting a little more detail, or to follow up on the bibliography, the book is usefully cross-referenced and has hordes of end notes. But I do have one major warning for Plus readers. Although this book is written to be more easy-reading and popularist than a text book, it is still very heavy-going. More importantly it is pitched at the level of university undergraduates. However, if you're enjoying maths A-level, then this book has a lot to offer, even if you don't understand everything. Every chapter begins with an interesting introduction on the history of a particular problem and the lives of the mathematicians involved, before heading into streams of equations and derivations. The final section of the book gives a detailed biography of the genius behind all of this mathematics through the ages, Leonhard Euler. If you have a class project to write an essay on an influential mathematician, then Euler would certainly be an inspirational choice, and this final chapter a good place to start your research.

We will be discussing two equations. The ring illustrated above is Euler's Identity, which we will discuss second. Feynman's quote refers to Euler's formula, which we will discuss first. From Wikipedia:

Euler's formula, named after Leonhard Euler, is a mathematical formula in complex analysis that establishes the deep relationship between the trigonometric functions and the complex exponential function. Euler's formula states that, for any real number x,
$e^{ix} = \cos x + i\sin 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, with the argument x given in radians. This complex exponential function is sometimes called cis(x). The formula is still valid if x is a complex number, and so some authors refer to the more general complex version as Euler's formula.[1]

## History

It was Bernoulli [1702] who noted that
$\frac{1}{1+x^2}=\frac{1}{2} \left(\frac{1}{1-ix}+\frac{1}{1+ix} \right) \ .$
And since
$\int \frac{dx}{1+ax}=\frac{1}{a}\ln(1+ax)+C \ ,$
the above equation tells us something about complex logarithms. Bernoulli, however, did not evaluate the integral. His correspondence with Euler (who also knew the above equation) shows that he didn't fully understand logarithms. Euler also suggested that the complex logarithms can have infinitely many values.

Meanwhile, Roger Cotes, in 1714, discovered
$\ln(\cos x + i\sin x)=ix \$
(where "ln" means natural logarithm, i.e. log with base e).[4] We now know that the above equation is only true modulo integer multiples of i, but Cotes missed the fact that a complex logarithm can have infinitely many values which owes to the periodicity of the trigonometric functions.

It was Euler (presumably around 1740) who turned his attention to the exponential function instead of logarithms, and obtained the correct formula now coined after his name. It was published in 1748, and his proof was based on the infinite series of both sides being equal. Neither of these men saw the geometrical interpretation of the formula: the view of complex numbers as points in the complex plane arose only some 50 years later (see Caspar Wessel).

## Applications in complex number theory

This formula can be interpreted as saying that the function eix traces out the unit circle in the complex number plane as x ranges through the real numbers. Here, x is the angle that a line connecting the origin with a point on the unit circle makes with the positive real axis, measured counter clockwise and in radians.

The original proof is based on the Taylor series expansions of the exponential function ez (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 z.

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 reduces the number of terms from two to one, which simplifies the mathematics when used in multiplication or powers of complex numbers. Any complex number z = x + iy can be written as
$z = x + iy = |z| (\cos \phi + i\sin \phi ) = r e^{i \phi} \$
$\bar{z} = x - iy = |z| (\cos \phi - i\sin \phi ) = r e^{-i \phi} \$
where
$x = \mathrm{Re}\{z\} \,$ the real part
$y = \mathrm{Im}\{z\} \,$ the imaginary part
$r = |z| = \sqrt{x^2+y^2}$ the magnitude of z
$\phi = \arg z = \,$ atan2(y, x) .
$\phi \,$ is the argument of z—i.e., the angle between the x axis and the vector z measured counterclockwise and in radians—which is defined up to addition of 2π. Many texts write tan-1(y/x) instead of atan2(y,x) but this needs adjustment when x ≤ 0.

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) that
$a = e^{\ln (a)} \$
and that
$e^a e^b = e^{a + b} \$
both valid for any complex numbers a and b.

Therefore, one can write:
$z = |z| e^{i \phi} = e^{\ln |z|} e^{i \phi} = e^{\ln |z| + i \phi} \$
for any z ≠ 0. Taking the logarithm of both sides shows that:
$\ln z= \ln |z| + i \phi \ .$
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 φ is multi-valued.

Finally, the other exponential law
$(e^a)^k = e^{a k} \ ,$
which can be seen to hold for all integers k, together with Euler's formula, implies several trigonometric identities as well as de Moivre's formula.

## Relationship to trigonometry

Euler's formula 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:
$\cos x = \mathrm{Re}\{e^{ix}\} ={e^{ix} + e^{-ix} \over 2}$
$\sin x = \mathrm{Im}\{e^{ix}\} ={e^{ix} - e^{-ix} \over 2i} \ .$
The two equations above can be derived by adding or subtracting Euler's formulas:
$e^{ix} = \cos x + i \sin x \;$
$e^{-ix} = \cos(- x) + i \sin(- x) = \cos x - i \sin x \;$
and solving for either cosine or sine.

These formulas can even serve as the definition of the trigonometric functions for complex arguments x. For example, letting x = iy, we have:
$\cos(iy) = {e^{-y} + e^{y} \over 2} = \cosh(y)$
$\sin(iy) = {e^{-y} - e^{y} \over 2i} = -{1 \over i} {e^{y} - e^{-y} \over 2} = i\sinh(y) \ .$
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:
\begin{align} \cos x\cdot \cos y & = \frac{(e^{ix}+e^{-ix})}{2} \cdot \frac{(e^{iy}+e^{-iy})}{2} \\ & = \frac{1}{2}\cdot \frac{e^{i(x+y)}+e^{i(x-y)}+e^{i(-x+y)}+e^{i(-x-y)}}{2} \\ & = \frac{1}{2} \left[ \underbrace{ \frac{e^{i(x+y)} + e^{-i(x+y)}}{2} }_{\cos(x+y)} + \underbrace{ \frac{e^{i(x-y)} + e^{-i(x-y)}}{2} }_{\cos(x-y)} \right] \ . \end{align}
Another technique is to represent the sinusoids in terms of the real part of a more complex expression, and perform the manipulations on the complex expression. For example:
\begin{align} \cos(nx) & = \mathrm{Re} \{\ e^{inx}\ \} = \mathrm{Re} \{\ e^{i(n-1)x}\cdot e^{ix}\ \} \\ & = \mathrm{Re} \{\ e^{i(n-1)x}\cdot (e^{ix} + e^{-ix} - e^{-ix})\ \} \\ & = \mathrm{Re} \{\ e^{i(n-1)x}\cdot \underbrace{(e^{ix} + e^{-ix})}_{2\cos(x)} - e^{i(n-2)x}\ \} \\ & = \cos[(n-1)x]\cdot 2 \cos(x) - \cos[(n-2)x] \ . \end{align}
This formula is used for recursive generation of cos(nx) for integer values of n and arbitrary x (in radians).

## Other applications

In differential equations, the function eix is often used to simplify derivations, even if the final answer is a real function involving sine and cosine. The reason for this is that the complex exponential is the eigenfunction of differentiation. Euler's identity is an easy consequence of Euler's formula.

In electrical engineering and other fields, signals that vary periodically over time are often described as a combination of sine and cosine functions (see Fourier analysis), and these are more conveniently expressed as the real part 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.

## Definitions of complex exponentiation

The exponential function ex 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 ez for complex values of z simply by substituting z in place of x and using the complex algebraic operations. In particular we may use either of the two following definitions which are equivalent. From a more advanced perspective, each of these definitions may be interpreted as giving the unique analytic continuation of ex to the complex plane.

### Power series definition

For complex z
$e^z = 1 + \frac{z}{1!} + \frac{z^2}{2!} + \frac{z^3}{3!} + \cdots = \sum_{n=0}^{\infty} \frac{z^n}{n!} ~.$
Using the ratio test it is possible to show that this power series has an infinite radius of convergence, and so defines ez for all complex z.

### Limit definition

For complex z
$e^z = \lim_{n \rightarrow \infty} \left(1+\frac{z}{n}\right)^n ~.$

## Proofs

Various proofs of the formula are possible.

### Using power series

Here is a proof of Euler's formula using power series expansions as well as basic facts about the powers of i:
\begin{align} i^0 &{}= 1, \quad & i^1 &{}= i, \quad & i^2 &{}= -1, \quad & i^3 &{}= -i, \\ i^4 &={} 1, \quad & i^5 &={} i, \quad & i^6 &{}= -1, \quad & i^7 &{}= -i, \end{align}
and so on. Using now the power series definition from above we see that for real values of x
\begin{align} e^{ix} &{}= 1 + ix + \frac{(ix)^2}{2!} + \frac{(ix)^3}{3!} + \frac{(ix)^4}{4!} + \frac{(ix)^5}{5!} + \frac{(ix)^6}{6!} + \frac{(ix)^7}{7!} + \frac{(ix)^8}{8!} + \cdots \\[8pt] &{}= 1 + ix - \frac{x^2}{2!} - \frac{ix^3}{3!} + \frac{x^4}{4!} + \frac{ix^5}{5!} - \frac{x^6}{6!} - \frac{ix^7}{7!} + \frac{x^8}{8!} + \cdots \\[8pt] &{}= \left( 1 - \frac{x^2}{2!} + \frac{x^4}{4!} - \frac{x^6}{6!} + \frac{x^8}{8!} - \cdots \right) + i\left( x - \frac{x^3}{3!} + \frac{x^5}{5!} - \frac{x^7}{7!} + \cdots \right) \\[8pt] &{}= \cos x + i\sin x \ . \end{align}
In the last step we have simply recognized the Taylor series for sin(x) and cos(x). The rearrangement of terms is justified because each series is absolutely convergent.

### Using calculus

Treating i as a constant, albeit an imaginary constant, note that
$\frac{d}{dx} e^{ix} = i e^{ix} \ .$
Then define the function
$f(x) = (\cos x - i \sin x) \cdot e^{ix} \ .$
Because the product rule holds for complex valued functions of a real variable for the same reason as in the real case, the derivative of ƒ(x) according to the product rule is:
\begin{align} \frac{d}{dx}f(x) &= (\cos x - i\sin x)\cdot\frac{d}{dx}e^{ix} + \frac{d}{dx}(\cos x - i\sin x)\cdot e^{ix} \\ &= (\cos x - i\sin x)(i e^{ix}) + (-\sin x - i\cos x)\cdot e^{ix} \\ &= (i\cos x + \sin x - \sin x - i\cos x)\cdot e^{ix} \\ &= 0 \ . \end{align}
Therefore, ƒ(x) must be a constant function in x. Because ƒ(0) = 1 by inspection, ƒ(x) = 1, giving
$1 = (\cos x - i \sin x) \cdot e^{ix} \ .$
Multiplying both sides by cos x + i sin x, we obtain
\begin{align} \cos x + i \sin x &= (\cos x + i \sin x)(\cos x - i \sin x) \cdot e^{ix} \\ &= (\cos^2 x -(i \sin x)^2) \cdot e^{ix} = (\cos^2 x + \sin^2 x) \cdot e^{ix} = e^{ix} \ . \end{align}

### Using differential equations

Here is another proof that follows from the differential identity above. Define a new function ƒ(x) of the real variable x as
$f(x) = \cos x + i \sin x \ .$
Then we may check that
\begin{align} \frac{d}{dx}f(x) &= -\sin x + i \cos x \\ &= i f(x) \ . \end{align}
Thus ƒ(x) and eix satisfy the same first-order ordinary differential equation (here the complex values are considered as points in the plane ℝ2). Note also that both functions are equal to 1 at x = 0, then by the uniqueness of solutions to ordinary differential equations they must be equal everywhere (see Picard–Lindelöf theorem and note the comments concerning global uniqueness in the proof section there).

## References

1. ^ Moskowitz, Martin A. (2002). A Course in Complex Analysis in One Variable. World Scientific Publishing Co.. pp. 7. ISBN 981-02-4780-X.
2. ^ Feynman, Richard P. (1977). The Feynman Lectures on Physics, vol. I. Addison-Wesley. pp. 22–10. ISBN 0-201-02010-6.
3. ^ Feynman, Richard P. (1977). The Feynman Lectures on Physics, vol. I. Addison-Wesley. pp. 22–1. ISBN 0-201-02010-6.
4. ^ John Stillwell (2002). Mathematics and Its History. Springer.

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