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Advice: Errors in symbolic integration

As most calculus students know, integration can be a difficult process. Even in Maple, the difficulties have not all been overcome, and symbolic integration should not be considered as completely reliable in all cases. In general it is prudent to check the correctness of integrals, either by comparing definite integrals to their floating-point approximations or by comparing the derivative of an indefinite integral to the integrand.

One common source of trouble is multivalued functions. In particular, a definite integral int(f(x), x = a .. b) may give a wrong answer because the antiderivative is a multivalued function with a branch cut crossing the path of integration. Maple may not recognize the branch cut, and return as the value of the definite integral. Note that this can happen even when the integrand is single-valued.

In other cases, when is multivalued Maple may find an antiderivative for the wrong choice of branch of .

Examples:

The following example is in Maple 6 as of January 2000. It may be corrected in later releases.

We begin with the function and its antiderivative . Note that is continuous on the whole real line.

> f:= t/(2-cos(t^2)^2);

> F:= simplify(int(f,t));

This is correct in the sense that the difference between and the derivative of simplifies (with a little help) to 0.

> simplify(diff(F,t)-f);

> combine(%,trig);

Here is a definite integral with this integrand.

> J1:=int(f, t= 0 .. 2);

> evalf(J1);

This result is equivalent to that obtained by evaluating the antiderivative at the endpoints of the interval.

> simplify(eval(F,t=2)-eval(F,t=0) - J1);

However, the answer is wrong (rather obviously, because the integrand is positive on this interval). Here is the correct value, obtained by numerical integration.

> evalf(Int(f, t=0 .. 2));