April 30th, 2010, 11:10 PM  #1 
Newbie Joined: Apr 2010 Posts: 2 Thanks: 0  Riemann/Darboux Integral
Hello to everyone. I have a question. Is the Riemann Integral really equivalent to Darboux Integral?. For example, consider the function f(x)=sin(x)/x. I want to know if it is integrable between 1 and 1. The problem arises at x=0. Now, this function is clearly Darboux integrable, since you solve the problem of the nonexistance of f(0) with the supremum operator. But I don't think it is Riemann integrable, since you can not evaluate f(0), and the Riemann definition clearly states that the sum must exist for ANY election of evaluation points. If you defined f(0) as 1, you would not be solving the problem, because you would be integrating a different function,not f(x), since you are changing the domain of the function. So, is it possible to say that the Riemann and Darboux integrals are equivalent iff the function is well defined for all points in the interval?. Please correct me if I am wrong. I will deeply appreciate any replys. 
May 1st, 2010, 12:46 PM  #2 
Global Moderator Joined: May 2007 Posts: 6,612 Thanks: 617  Re: Riemann/Darboux Integral
I would call your question a quibble, since you want to leave sinx/x undefined at x=0, rather than accepting any value at all. Whatever you define it to be, the resultant function will be Riemann integrable, since the x interval containing x=0 can be made as small as you want, so its contribution > 0, no matter what the (finite) value assigned at that point.


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