Thursday, March 28, 2019

calculus - Closed form for int10efrac1ln(x)dx?



I want to evaluate and find a closed form for this definite integral:10e1ln(x)dx.



I don't know where to start. I've tried taking the natural logarithm of the integral, substitution and expressing the integrand in another way but they haven't led anywhere. The approximation should be between .2 and .3, and is probably a transcendental number.



Thanks.



Answer



Let's verify Robert Israel's find. Observe that



I:=10exp1lnxdx=01y2exp(y+y1)dy=120(1+1/y2)exp(y+y1)dy,

where we have substituted y=1/lnx and then averaged with y1/y.



In view of the integral Kα(x)=0exp[xcosht]cosh(αt)dt and the substitution y=expt, 2K1(2)=exp[2cosht]coshtdt=120exp[y1/y](1+1/y2)dy.


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