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- From: alopez-o@neumann.uwaterloo.ca (Alex Lopez-Ortiz)
- Subject: sci.math FAQ: e^(i Pi) = -1 Euler's formula
- Summary: Part 14 of many, New version,
- Originator: alopez-o@neumann.uwaterloo.ca
- Message-ID: <DI76Ks.FF5@undergrad.math.uwaterloo.ca>
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- Approved: news-answers-request@MIT.Edu
- Date: Fri, 17 Nov 1995 17:14:52 GMT
- Expires: Fri, 8 Dec 1995 09:55:55 GMT
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-
- Archive-Name: sci-math-faq/specialnumbers/eulerFormula
- Last-modified: December 8, 1994
- Version: 6.2
-
-
- Euler's formula: e^(i pi) = -1
-
-
-
- The definition and domain of exponentiation has been changed several
- times. The original operation x^y was only defined when y was a
- positive integer. The domain of the operation of exponentation has
- been extended, not so much because the original definition made sense
- in the extended domain, but because there were (almost) unique ways to
- extend exponentation which preserved many of what seemed to be the
- ``important" properties of the original operation. So in part, these
- definitions are only convention, motivated by reasons of aesthetics
- and utility.
-
- The original definition of exponentiation is, of course, that x^y = x
- *x * ... * x, where x is multiplied by itself y times. This is only a
- reasonable definition for y = 1, 2, 3, ... (It could be argued that it
- is reasonable when y = 0 , but that issue is taken up in a different
- part of the FAQ). This operation has a number of properties, including
-
-
- 1. x^1 = x
- 2. For any x , n , m , x^n x^m = x^(n + m) .
- 3. If x is positive, then x^n is positive.
-
- Now, we can try to see how far we can extend the domain of
- exponentiation so that the above properties (and others) still
- hold. This naturally leads to defining the operation x^y on the
- domain x positive real; y rational, by setting x^(p/q) = the
- q^(th) root of x^p . This operation agrees with the original
- definition of exponentiation on their common domain, and also
- satisfies (1), (2) and (3). In fact, it is the unique operation on
- this domain that does so. This operation also has some other
- properties:
-
- 4. If x > 1 , then x^y is an increasing function of y .
- 5. If 0 < x < 1 , then x^y is a decreasing function of y .
-
- Again, we can again see how far we can extend the domain of
- exponentiation while still preserving properties (1)-(5). This
- leads naturally to the following definition of x^y on the domain x
- positive real; y real:
-
- If x > 1 , x^y is defined to be sup_q { x^q } , where q runs over
- all rationals less than or equal to y .
-
- If x < 1 , x^y is defined to be inf_q { x^q } , where q runs over
- all rationals less than or equal to y .
-
- If x = 1 , x^y is defined to be 1 .
-
- Again, this operation satisfies (1)-(5), and is in fact the only
- operation on this domain to do so.
-
- The next extension is somewhat more complicated. As can be proved
- using the methods of calculus or combinatorics, if we define e to
- be the number
-
- e = 1 + 1/1! + 1/2! + 1/3! + ... = 2.71828...
-
- it turns out that for every real number x ,
-
- 6. e^x = 1 + x/1! + x^2/2! + x^3/3! + ...
-
- e^x is also denoted exp(x) . (This series always converges
- regardless of the value of x ).
-
- One can also define an operation ln(x) on the positive reals,
- which is the inverse of the operation of exponentiation by e. In
- other words, exp(ln(x)) = x for all positive x . Moreover,
-
- 7. If x is positive, then x^y = exp(y ln(x)) . Because of this, the
- natural extension of exponentiation to complex exponents, seems to
- be to define
-
- exp(z) = 1 + z/1! + z^2/2! + z^3/3! + ...
-
- for all complex z (not just the reals, as before), and to define
-
- x^z = exp(z ln(x))
-
- when x is a positive real and z is complex.
-
- This is the only operation x^y on the domain x positive real, y
- complex which satisfies all of (1)-(7). Because of this and other
- reasons, it is accepted as the modern definition of
- exponentiation.
-
- From the identities
-
- sin x = x - x^3/3! + x^5/5! - x^7/7! + ...
-
- cos x = 1 - x^2/2! + x^4/4! - x^6/6! + ...
-
- which are the Taylor series expansion of the trigonometric sine
- and cosine functions respectively. From this, one sees that, for
- any real x,
-
- 8. exp(ix) = cos x + i sin x.
-
- Thus, we get Euler's famous formula
-
- e^(pi i) = -1
-
- and
-
- e^(2 pi i) = e^0 = 1.
-
- One can also obtain the classical addition formulae for sine and
- cosine from (8) and (1).
-
-
-
- All of the above extensions have been restricted to a positive real
- for the base. When the base x is not a positive real, it is not as
- clear-cut how to extend the definition of exponentiation. For example,
- (-1)^(1/2) could well be i or -i, (-1)^(1/3) could be -1 , 1/2 +
- sqrt(3)i/2 , or 1/2 - sqrt(3)i/2 , and so on. Some values of x and y
- give infinitely many candidates for x^y , all equally plausible. And
- of course x = 0 has its own special problems. These problems can all
- be traced to the fact that the exp function is not injective on the
- complex plane, so that ln is not well defined outside the real line.
- There are ways around these difficulties (defining branches of the
- logarithm, for example), but we shall not go into this here.
-
- The operation of exponentiation has also been extended to other
- systems like matrices and operators. The key is to define an
- exponential function by (6) and work from there. [Some reference on
- operator calculus and/or advanced linear algebra?]
-
-
-
- References
-
- Complex Analysis. Ahlfors, Lars V. McGraw-Hill, 1953.
-
-
-
-
- _________________________________________________________________
-
-
-
- alopez-o@barrow.uwaterloo.ca
- Tue Apr 04 17:26:57 EDT 1995
-
-
-