In mathematics, Euler's identity is the equality where Euler's identity is named after the Swiss mathematician Leonhard Euler. It is considered to be an exemplar of mathematical beauty as it shows a profound connection between the most fundamental numbers in mathematics.
Furthermore, the equation is given in the form of an expression set equal to zero, which is common practice in several areas of mathematics. Stanford University mathematics professor Keith Devlin has said, "like a Shakespearean sonnet that captures the very essence of love, or a painting that brings out the beauty of the human form that is far more than just skin deep, Euler's equation reaches down into the very depths of existence". And Paul Nahin, a professor emeritus at the University of New Hampshire, who has written a book dedicated to Euler's formula and its applications in Fourier analysis, describes Euler's identity as being "of exquisite beauty". Mathematics writer Constance Reid has opined that Euler's identity is "the most famous formula in all mathematics". And Benjamin Peirce, a 19th-century American philosopher, mathematician, and professor at Harvard University, after proving Euler's identity during a lecture, stated that the identity "is absolutely paradoxical; we cannot understand it, and we don't know what it means, but we have proved it, and therefore we know it must be the truth". A poll of readers conducted by The Mathematical Intelligencer in 1990 named Euler's identity as the "most beautiful theorem in mathematics". In another poll of readers that was conducted by Physics World in 2004, Euler's identity tied with Maxwell's equations as the "greatest equation ever". A study of the brains of sixteen mathematicians found that the "emotional brain" lit up more consistently for Euler's identity than for any other formula. At least three books in popular mathematics have been published about Euler's identity:
Dr. Euler's Fabulous Formula: Cures Many Mathematical Ills, by Paul Nahin
A Most Elegant Equation: Euler's formula and the beauty of mathematics, by David Stipp
Euler's Pioneering Equation: The most beautiful theorem in mathematics, by Robin Wilson.
Explanations
Imaginary exponents
Fundamentally, Euler's identity asserts that is equal to −1. The expression is a special case of the expression, where is any complex number. In general, is defined for complex by extending one of the definitions of the exponential function from real exponents to complex exponents. For example, one common definition is: Euler's identity therefore states that the limit, as approaches infinity, of is equal to −1. This limit is illustrated in the animation to the right. Euler's identity is a special case of Euler's formula, which states that for any real number, where the inputs of the trigonometric functions sine and cosine are given in radians. In particular, when, Since and it follows that which yields Euler's identity:
Geometric interpretation
Any complex number can be represented by the point on the complex plane. This point can also be represented in polar coordinates as, where r is the absolute value of z, and is the argument of z. By the definitions of sine and cosine, this point has cartesian coordinates of, implying that. According to Euler's formula, this is equivalent to saying. Euler's identity says that. Since is for r = 1 and, this can be interpreted as a fact about the number −1 on the complex plane: its distance from the origin is 1, and its angle from the positive x-axis is radians. Additionally, when any complex number z is multiplied by, it has the effect of rotating z counterclockwise by an angle of on the complex plane. Since multiplication by −1 reflects a point across the origin, Euler's identity can be interpreted as saying that rotating any point radians around the origin has the same effect as reflecting the point across the origin.
Generalizations
Euler's identity is also a special case of the more general identity that the th roots of unity, for, add up to 0: Euler's identity is the case where. In another field of mathematics, by using quaternion exponentiation, one can show that a similar identity also applies to quaternions. Let be the basis elements; then, In general, given real,, and such that, then, For octonions, with real such that, and with the octonion basis elements,
History
It has been claimed that Euler's identity appears in his monumental work of mathematical analysis published in 1748, Introductio in analysin infinitorum. However, it is questionable whether this particular concept can be attributed to Euler himself, as he may never have expressed it. Moreover, while Euler did write in the Introductio about what we today call Euler's formula, which relates with cosine and sine terms in the field of complex numbers, the English mathematician Roger Cotes also knew of this formula and Euler may have acquired the knowledge through his Swiss compatriot Johann Bernoulli. Robin Wilson states the following.
