Some other non-trivial sequence in disguise

Calculus Level 5

r = 1 10 ( 10 r ) ( 1 ) r 1 J r = a b \large \sum_{r=1}^{10} \dbinom{10}{r} {(-1)}^{r-1} J_r = \dfrac{a}{b}

Given that J r = k = 1 r 1 k 2 \displaystyle J_r =\sum_{k=1}^{r} \dfrac{1}{k^2} .

If a a and b b are coprime positive integers, evaluate a + b a+b .

You may use a calculator for the final step of your calculation.


Notation: ( M N ) = M ! N ! ( M N ) ! \dbinom MN = \dfrac {M!}{N! (M-N)!} denotes the binomial coefficient .


Inspiration .


The answer is 32581.

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1 solution

Mark Hennings
Mar 15, 2017

Since 0 1 x k ln x d x = 1 ( k + 1 ) 2 k 0 \int_0^1 x^k \ln x\,dx \; = \; -\frac{1}{(k+1)^2} \hspace{2cm} k \ge 0 we see that J r = k = 0 r 1 0 1 x k ln x d x = 0 1 1 x r 1 x ln x d x r N J_r \; = \; -\sum_{k=0}^{r-1} \int_0^1 x^k \ln x\,dx \; = \; -\int_0^1 \frac{1-x^r}{1-x} \ln x\,dx \hspace{2cm} r \in \mathbb{N} and hence, for N N N \in \mathbb{N} , K N = r = 1 N ( N r ) ( 1 ) r 1 J r = 0 1 r = 1 N ( N r ) ( 1 ) r ( 1 x r ) ln x 1 x d x = 0 1 { ( 1 1 ) N 1 ( 1 x ) N + 1 } ln x 1 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 1 j x j ) d x = j = 1 1 j ( j + N ) = 1 N j = 1 ( 1 j 1 j + N ) = 1 N H N \begin{aligned} K_N & = \sum_{r=1}^N \binom{N}{r} (-1)^{r-1}J_r \; = \; \int_0^1 \sum_{r=1}^N \binom{N}{r}(-1)^r (1-x^r) \frac{\ln x}{1-x}\,dx \\ & = \int_0^1 \left\{ (1-1)^N - 1 - (1-x)^N + 1\right\} \frac{\ln x}{1-x}\,dx \; =\; -\int_0^1 (1-x)^{N-1} \ln x \,dx \\ & = -\int_0^1 x^{N-1} \ln(1-x)\,dx \; = \; \int_0^1 x^{N-1} \left(\sum_{j=1}^\infty \frac{1}{j}x^j\right)\,dx \\ & = \sum_{j=1}^\infty \frac{1}{j(j+N)} \; = \; \frac{1}{N}\sum_{j=1}^\infty \left(\frac{1}{j} - \frac{1}{j+N}\right) \\ & = \frac{1}{N}H_N \end{aligned} where H N H_N is the N N th Harmonic number. We want K 10 = 1 10 H 10 = 7381 25200 K_{10} = \tfrac{1}{10}H_{10} = \frac{7381}{25200} , making the answer 32581 \boxed{32581} .

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)

Tapas Mazumdar - 4 years, 2 months ago

How did transit from the first to second line of K N K_N ?

Changing integral and summation seems tricky, could you link some readings ?

Vishal Yadav - 4 years, 2 months ago

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The first to the second line in K N 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.

Mark Hennings - 4 years, 2 months ago

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