Monday, June 12, 2017

calculus - Show that int10|frac1xsinfrac1x|dx diverges



I read that the improper Riemann integral 10|1xsin1x| dx diverges.



I have tried comparison criteria for 10|1xsin1x|dx, but I cannot find a function f with a divergent integral such that 0f(x)|1xsin1x|. I also notice, by using a change of variable, that 10|1xsin1x|dx=1|1xsinx|dx, but I have not found a use of this equality to prove the divergence of the integral. I have also tried to think about some use of complex analysis, but found none useful to prove the desired result. Coud anybody give a proof of this divergence? thanks!


Answer





I also notice, by using a change of variable, that 10|1xsin1x|dx=1|1xsinx|dx, but I have not found a use of this equality to prove the divergence of the integral.




Subdivide (1,) into an infinite number of intervals of the form (kπ, (k+1)π), and notice that for each of them, their graphic is tangent to a hyperbola, and that the area of each slice is greater than that of a triangle of height 1π (k+12), due to the convexity/concavity of the sine function. This shows that the value of our integral is greater than a constant multiple of the harmonic series, which is known to be divergent.


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