r = 1 ∑ 1 0 ( r 1 0 ) ( − 1 ) r − 1 J r = b a
Given that J r = k = 1 ∑ r k 2 1 .
If a and b are coprime positive integers, evaluate a + b .
You may use a calculator for the final step of your calculation.
Notation:
(
N
M
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=
N
!
(
M
−
N
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!
M
!
denotes the
binomial coefficient
.
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Splendid solution. I proceeded with the approach that Kartik Sharma applied in his original problem from which this is inspired from. I can't seem to figure out which one is better. As of now for me, it's both of them. (+1)
How did transit from the first to second line of K N ?
Changing integral and summation seems tricky, could you link some readings ?
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The first to the second line in K N is just the Binomial Theorem. The last summation/integration interchange is justified by the Monotone Convergence Theorem. All the others are finite sums, so there is no problem.
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Since ∫ 0 1 x k ln x d x = − ( k + 1 ) 2 1 k ≥ 0 we see that J r = − k = 0 ∑ r − 1 ∫ 0 1 x k ln x d x = − ∫ 0 1 1 − x 1 − x r ln x d x r ∈ N and hence, for N ∈ N , K N = r = 1 ∑ N ( r N ) ( − 1 ) r − 1 J r = ∫ 0 1 r = 1 ∑ N ( r N ) ( − 1 ) r ( 1 − x r ) 1 − x ln x d x = ∫ 0 1 { ( 1 − 1 ) N − 1 − ( 1 − x ) N + 1 } 1 − x ln x d x = − ∫ 0 1 ( 1 − x ) N − 1 ln x d x = − ∫ 0 1 x N − 1 ln ( 1 − x ) d x = ∫ 0 1 x N − 1 ( j = 1 ∑ ∞ j 1 x j ) d x = j = 1 ∑ ∞ j ( j + N ) 1 = N 1 j = 1 ∑ ∞ ( j 1 − j + N 1 ) = N 1 H N where H N is the N th Harmonic number. We want K 1 0 = 1 0 1 H 1 0 = 2 5 2 0 0 7 3 8 1 , making the answer 3 2 5 8 1 .