∫ 0 1 x 2 / 3 tan − 1 x d x
Given the integral above equals to
π ( b a − d c ) + f e ln ( g )
where a , b , c , d , e and f are positive integers with prime g and g cd ( a , b ) = g cd ( c , d ) = g cd ( e , f ) = 1 .
Find the value of the 7-digit integer, a b c d e f g .
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LOL, why you applied such an obscure thing? You can evaluate n = 0 ∑ ∞ n + 3 2 ( − 1 ) n by digamma functions or by Weierstrass factorization theorem.
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@Pi Han Goh I'm not familiar with Weierstrass factorization theorem and Lerch was that sprouted up.! Btw can you post how can the two methods you mentioned be used??
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Hint
:
ϕ
(
a
)
−
ϕ
(
b
)
=
n
=
0
∑
∞
n
+
b
1
−
n
+
a
1
where
a
,
b
are not positive integers. And apply reflection formula:
ϕ
(
1
−
x
)
−
ϕ
(
x
)
=
π
cot
(
π
x
)
.
@Pi Han Goh . How do we get the (-1 )^n part in the numerator when we use digamma?. I used taylor series to convert it into a summation and then made some rearrangements so that the new summation that is formed can be again converted back to an integral but this time it would be an integral of a rational function. I have tried this trick before many times and it always works.
@Kartik Sharma would you like to add something??
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No, probably not. It is exactly the same as I did. Nicely explained! +1!
But yeah, I believe you should look out for Pi Han Goh's advice.
All right. At least x^(1/ 3)[1 - x^2 + x^4 - x^6 + ...] can still be integrated. Is there any other alternatives since we have 3 solvers other than me?
Since P(-1, 1, 2/ 3) is not found on the page of Hurwitz Lerch Transcendent, could you please explain why or how it is equaled to (1/ 3) Pi Sqrt(3) - Ln 2 ?
Using the taylor series of tan − 1 ( x ) we have :-
tan − 1 ( x ) = r = 0 ∑ ∞ 2 r + 1 ( − 1 ) r x 2 r + 1
So ∫ 0 1 x 3 2 tan − 1 ( x ) d x = r = 0 ∑ ∞ ( − 1 ) r ( 2 r + 1 ) ( 2 r + 3 4 ) 1 = r = 0 ∑ ∞ ( − 1 ) r ( 2 r + 1 ) ( 6 r + 4 ) 3 = r = 0 ∑ ∞ ( − 1 ) r 2 ⋅ ( 2 r + 1 ) ( 3 r + 2 ) 3
= 2 3 r = 0 ∑ ∞ r + 1 ( − 1 ) r ( 2 r + 1 1 − 3 r + 2 1 )
= 2 3 r = 0 ∑ ∞ ( ( 2 r + 1 ) ( r + 1 ) ( − 1 ) r − ( r + 1 ) ( 3 r + 2 ) ( − 1 ) r )
= 3 r = 0 ∑ ∞ ( 2 r + 2 ) ( 2 r + 1 ) ( − 1 ) r − 2 9 r = 0 ∑ ∞ ( 3 r + 3 ) ( 3 r + 2 ) ( − 1 ) r
= 3 r = 0 ∑ ∞ ( 2 r + 1 ( − 1 ) r − 2 r + 2 ( − 1 ) r ) − 2 9 r = 0 ∑ ∞ ( 3 r + 2 ( − 1 ) r − 3 r + 3 ( − 1 ) r )
Now we know the taylor series 1 + x 1 = r = 0 ∑ ∞ ( − 1 ) r x r . So in this series....replacing x by x 2 or x 3 as required and multiplying by x or x 2 in the numerator as required and then integrating from 0 to 1 we would get the appropriate summations.
For example :-
∫ 0 1 1 + x 2 1 d x = r = 0 ∑ ∞ 2 r + 1 ( − 1 ) r and ∫ 0 1 1 + x 2 x d x = r = 0 ∑ ∞ 2 r + 2 ( − 1 ) r and so on.....
So we have :-
3 ∫ 0 1 ( 1 + x 2 1 − 1 + x 2 x ) d x − 2 9 ∫ 0 1 ( 1 + x 3 x − 1 + x 3 x 2 ) d x
Essentially what we did was we converted the integral into a series form and then rearranged some terms and then again reconverted it to an integral but this time it is an integral of rational algebraic functions.
Now each of these integrals posses an antiderivative and are quite easy to figure out. Let me provide the hints one by one:-
1 + x 2 1 :- Standard arctan form.
1 + x 2 x :- substitute x 2 = t
1 + x 3 x = 1 + x 3 x + 1 − 1 + x 3 1 = 1 − x + x 2 1 − 1 + x 3 1 :- The first one can be made into the arctan format by some rearrangements and the second one can be solved using partial fractions. Here is a video showing this .
1 + x 3 x 2 :- Substitute x 3 = u .
So after solving all of these integrals we get our answer as:-
π ( 4 3 − 2 3 ) + 2 3 ln ( 2 ) .
I did not show any working regarding the integral. I used desmos to calculate my integral and the integral in question and it gave the same value proving that my method is correct. Here is a picture:-
Complete solution is not here.
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I = ∫ 0 1 x 2 / 3 tan − 1 x d x Integrating by parts we get,(with x 2 / 3 1 as the first function ) 4 3 π − 3 ∫ 0 1 1 + x 2 x 1 / 3 d x Now, write: 1 + x 2 1 = n = 0 ∑ ∞ ( − 1 ) n x 2 n So that: I = 4 3 π − 2 3 n = 0 ∑ ∞ n + 3 2 ( − 1 ) n As, n = 0 ∑ ∞ n + a / b ( − 1 ) n = Ψ ( − 1 , 1 , b a ) where Ψ ( z , s , a ) is the Hurwitz Lerch Transcendent So, I = π [ 4 3 − 2 3 ] + 2 3 ln 2