Derivatives and AntiderivativesDate: 10/31/2001 at 19:41:31 From: Robert Subject: integral sqrt(cos(x^2)) dx Hi, When I run the formula int(sqrt(cos(x^2)),x=0..1) in Maple it just returns the formula. I know it's because there is no antiderivative, but why isn't there one? Date: 11/02/2001 at 14:43:17 From: Doctor Rob Subject: Re: integral sqrt(cos(x^2)) dx Thanks for writing to Ask Dr. Math, Robert. There are many, many functions that behave similarly. For any of them, its antiderivative exists, but there is no expression for it in closed form in terms of familiar functions and constants. Examples are x^x, e^(x^2), e^(e^x)), e^(-x)/x, 1/ln(x), ln(1-x)/x, sin(x)/x, sin(x^2), sin(sin(x)), sqrt(sin(x)), e^sin(x), ln(sin(x)), sqrt(x^3+a*x^2+b*x+c), and so on. The nonexistence of such an expression is a consequence of the fact that we only have a fairly small restricted set of functions to use to try to express antiderivatives: constant, rational, irrational algebraic, exponential, logarithmic, trigonometric, inverse trigonometric, hyperbolic, and inverse hyperbolic functions. This may seem like a big list to you, but it is not big enough for those purposes. One way to think of this is that the antiderivative is the solution of a certain differential equation: dy/dx = sqrt(cos(x^2)). You probably are aware that not all such solutions are expressible that way. Bessel functions and many other so-called "special" functions that crop up in engineering and physics arise as such solutions. A similar situation exists with trying to solve the general quintic equation. The roots cannot be expressed in terms of the equation's coefficients using the above functions. The roots exist, and are continuous functions of the coefficients, but those functions just don't have simple expressions. Sorry! - Doctor Rob, The Math Forum http://mathforum.org/dr.math/ |
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