Integrating X^x, Closed Form
Date: 10/31/96 at 15:6:46 From: Simon Judes Subject: Integration of x^x and solution of y= xcosx (1) How would you go about integrating x^x? I don't have a clue where to start. Also, how do you express the equation y = xcosx in terms of y? For both these problems, I have asked other people who have told me that there is no simple solution. If this is the case, please send the solution anyway, or at least give me some idea of the theory on which it is based. Thanks!
Date: 11/1/96 at 8:10:56 From: Doctor Jerry Subject: Re: Integration of x^x and solution of y= xcosx Dear Simon Judes, (1) There is an algorithm due to a contemporary mathematician named Risch which can decide whether the anti-derivative of a continuous function f can be expressed as a finite combination of elementary functions. The elementary functions include polynomials, the trig functions, the inverse trig functions, the exponential function and its inverse, etc. There are many functions - called special functions - which fail to have an anti-derivative expressible as a finite combination of elementary functions. The so-called elliptic functions, the error function, and the gamma function are a few examples. The error function, which is extremely useful in both physics and statistics, is defined as: erf(x) = (2/sqrt(pi))integral from 0 to x of e^(-t^2)dt Extensive tables of the error function would not exist if the anti-derivative of e^(-t^2) were expressible as a finite combination of elementary functions. The anti-derivative of x^x is not expressible as a finite combination of elementary functions. I'm not sufficiently familiar with the Risch algorithm to even hint at a proof of it. (2) As to solving the equation y = xcosx for x in terms of y, this can't be done in "closed form" or "by classical methods." Both of these phrases are used to mean that this equation can't be solved by algebraic methods or with pencil and paper methods. Of course, given a value of y, one can determine numerically (using Newton's method, for example) the values of x for which xcosx = y. I'd like to give a parallel question to y = xcos x. Can you solve the equation y = sinx for x in terms of y? If you say yes and write x = arcsin y, then you have only recognized that the function inverse to sine is a well known function and you know its name. You haven't actually found a finite expression for x. The fact is, neither sine nor arcsin are known in the sense that y = x^2 or x = sqrt(y) is known. Except for a few special cases, specific values of sine and arcsin must be looked up in a table or calculated numerically. -Doctor Jerry, The Math Forum Check out our web site! http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994-2015 The Math Forum