∫ 0 ∞ e x − 1 x ln x d x
The integral above can be expressed as b π a ( ln ( c π ) − d ln ( A ) + f ) ,
where a , b , c , d , f are positive integers.
Find a + b + c + d + f .
Notations :
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Yes, the format in which the answer has to be submitted is quite persplexing at first glance.
But anyways, awesome problem @Rajdeep Dhingra
Keep posting more!
Log in to reply
I had not followed rajdeep. This question reminded to follow him :P. Nice one @Rajdeep Dhingra
Problem Loading...
Note Loading...
Set Loading...
If we define F ( α ) = = = ∫ 0 ∞ e x − 1 x α d x = ∫ 0 ∞ x α e − x ( 1 − e − x ) − 1 d x n = 0 ∑ ∞ ∫ 0 ∞ x α e − ( n + 1 ) x d x = n = 0 ∑ ∞ ( n + 1 ) α + 1 1 ∫ 0 ∞ y α e − y d y ζ ( α + 1 ) Γ ( α + 1 ) for α > − 1 , noting that the term-by-term integration is justified by the Monotone Convergence Theorem, then it is clear that I = ∫ 0 ∞ e x − 1 x ln x d x = = = F ′ ( 1 ) = ζ ′ ( 2 ) Γ ( 2 ) + ζ ( 2 ) Γ ′ ( 2 ) ζ ′ ( 2 ) + 6 1 π 2 Γ ′ ( 2 ) = 6 1 π 2 [ γ + ln ( 2 π ) − 1 2 ln A ] + 6 1 π 2 ( 1 − γ ) 6 1 π 2 [ γ + ln ( 2 π ) − 1 2 ln A − ( γ − 1 ) ] so that the answer is 2 + 6 + 2 + 1 2 + 1 = 2 3 . It is a bit strange that the answer does not cancel the γ terms.