http://faculty.missouristate.edu/l/lesreid/potw.html
Missouri State University’s Advanced Problem of February 2010
Vincent Pantaloni (Orléans, France)
Problem : Evaluate the following integral : Z 1
0
1 x
dx where ((x))denotes the fractional part of a numberx.
Solution :. . . .
∀x∈R, ((x)) =x−[x]. Where[x] is the integer part of x. The above integral is undefined in 0 so we have to evaluate :
n→+∞lim Z 1
n1
1 x
dx Letnbe an integer greater than 2 and consider :In=R1
1 n
1 x
dx.Then :
In= Z 1
n1
1 x−
1 x
dx
= Z 1
n1
1 x dx−
Z 1
n1
1 x
dx
= ln(n)− Z 1
1n
1 x
dx
= ln(n)−
n
X
k=2
Z k−11
1 k
1 x
dx
= ln(n)−
n
X
k=2
Z k−11
1 k
k−1 dx
= ln(n)−
n
X
k=2
1 k−1 −1
k
×(k−1)
= ln(n)−
n
X
k=2
1−k−1 k
= ln(n)−
n
X
k=2
1 k
= 1 + ln(n)−
n
X
k=1
1 k
We know that lim
n→+∞
Pn
k=1 1 k
−ln(n)
=γ. Whereγ≈0.5772156649is theEulerconstant. Hence :
Z 1
0
1 x
dx= 1−γ
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