n = 0 ∑ ∞ ( 2 n + 1 ) ! ( n + 1 ) 2 2 n + 1 n ! 2 = A π G − C B ζ ( D ) + F π E ln H
The equation above holds true for positive integers A , B , C , D , E , F and H , where B , C are coprime and H is minimized.
Find A + B + C + D + E + F + H .
Notations :
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Let the summation be denoted by S = n = 0 ∑ ∞ ( 2 n + 1 ) ! ( n + 1 ) 2 2 n + 1 n ! 2
We rewrite the summation as S = n = 1 ∑ ∞ n 3 ( n 2 n ) 2 n + 1
By Gregory Series , y t a n − 1 y = m = 1 ∑ ∞ 2 m − 1 ( − 1 ) m − 1 y 2 m
Substitute y = 1 − x 2 x ,
1 − x 2 x sin − 1 x = m = 1 ∑ ∞ ( 2 m − 1 ) ( 1 − x 2 ) m ( − 1 ) m − 1 x 2 m
= m = 1 ∑ ∞ 2 m − 1 ( − 1 ) m − 1 k = 0 ∑ ∞ ( k m + k − 1 ) x 2 ( k + m )
1 − x 2 x sin − 1 x = r = 1 ∑ ∞ x 2 r m = 1 ∑ r Γ ( r ) Γ ( m ) Γ ( r − m + 1 ) ( 2 m − 1 )
where co-efficient of x 2 r is half of j = 1 ∑ ∞ j ( j 2 j ) ( 2 x ) 2 j i.e.
r ( r 2 r ) v = 0 ∑ r − 1 ( − 1 ) v ( v r − 1 ) 2 v + 1 1 = 2 2 r − 1
To verify rewrite as ,
r ( r 2 r ) v = 0 ∑ r − 1 ( − 1 ) v ( v r − 1 ) 2 v + 1 1 = r ( r 2 r ) ∫ 0 1 v = 0 ∑ r − 1 ( − 1 ) v ( v r − 1 ) y 2 v d y = r ( r 2 r ) ∫ 0 1 ( 1 − y 2 ) r − 1 d y
Hence , r ( r 2 r ) v = 0 ∑ r − 1 ( − 1 ) v ( v r − 1 ) 2 v + 1 1 = r ( r 2 r ) ∫ 0 1 ( s i n θ ) 2 r − 1 d θ
Using Walli's integral , r ( r 2 r ) v = 0 ∑ r − 1 ( − 1 ) v ( v r − 1 ) 2 v + 1 1 = 2 2 r − 1
So we are verified with the co-efficient equating task and hence j = 1 ∑ ∞ j ( j 2 j ) ( 2 x ) 2 j = 1 − x 2 2 x sin − 1 x
Dividing by 2 x and integrating from 0 to x twice on the green sum we get,
m = 1 ∑ ∞ m 3 ( m 2 m ) ( 2 x ) 2 m = 4 ∫ 0 x y ( sin − 1 y ) 2 d y
If we put x = 2 1 then the equation becomes,
2 S = 4 ∫ 0 2 1 y ( sin − 1 y ) 2 d y , where S is our required sum
R.H.S is a representation of Log-Sine Integral which we can use as a standard result or even evaluate ,
Using IBP on RHS we get the second term as a Log-Sine integral ∫ 0 2 π t ln ∣ sin 2 t ∣ d t where we could make a substitution t → 2 t to get upper limit π
We are missing a ′ 2 ′ inside the Logarithm but that doesn't create any difference as we are not using any standard formulae for the evaluation , So let us consider a function ,
F ( a , b ) = ∫ 0 π ( sin z ) a cos b z d z = Γ ( 2 a + b + 2 ) Γ ( 2 a − b + 2 ) 2 − a π Γ ( a + 1 ) cos 2 b π
So our integral equals ∂ a ∂ b ∂ 2 F ( a , b ) ] a=b=1 which can now be calculated using Trigamma & Catalan's constant.
Putting all the values successively which is lengthy and an easy exercise we get 8 S = 4 π G − 6 4 3 5 ζ ( 3 ) + 3 2 π 2 ln 2
n = 0 ∑ ∞ ( 2 n + 1 ) ! ( n + 1 ) 2 2 n + 1 n ! 2 = 2 π G − 8 3 5 ζ ( 3 ) + 4 π 2 ln 2
Thus making the answer 2 + 3 5 + 8 + 3 + 2 + 4 + 2 = 5 6