# Numerically integrate a function whose lower limit depends on a parameter?

Posted 4 months ago
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 I have a function f(r) that I want to integrate numerically. The code is NIntegrate[f[r], {r, lowerlimit[r], Infinity}] . The lower limit is given by lowerlimit=c if r<=100 and lowerlimit=2*r otherwise, note that r can not be negative and c is some positive number. I wonder how to implement this integral.
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Posted 4 months ago
 This doesn't make any sense. You appear to be using the name r to refer to the variable of integration as well as an independent parameter. Please be clearer.
Posted 4 months ago
Posted 4 months ago
 Thank you John. I attached a picture of the problem to make it clearer
 On your paper, you have x as the variable of integration, which makes sense. Do that in your code. Define your function lowerlimit using Piecewise, Condition, or even If. Then your expression will, given a value for r, yield a result.