Tuesday, November 13, 2018

integration - Evaluating a nested log integral


Question:10dxloglog1x(1+x)2=12logπ2γ2




I’ve had some practice with similar integrals, but this one eludes me for some reason. I first made the transformation xlogx to get rid of the nested log. ThereforeI=0dxexlogx(1+ex)2

The inside integrand can be rewritten as an infinite series to getI=n0(n+1)(1)n0dxex(n+1)logxThe inside integral, I thought, could be evaluated by differentiating the gamma function to get0dtet(n+1)logt=γn+1log(n+1)n+1
However, when I simplify everything and split the sum, neither sum converges. If we consider it as a Cesaro sum, then I know for sure thatn0(1)n=12Which eventually does give the right answer. But I’m not sure if we’re quite allowed to do that especially because in a general sense, neither sum converges.

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