Scary Limit.

Calculus Level 5

Evaluate: lim m ( ( n = 1 m 1 n k = 1 n 1 ( 1 ) k k ) + log ( 2 ) H m ) \lim_{m\to\infty}\left(\left(\sum_{n=1}^{m}\frac{1}{n}\sum_{k=1}^{n-1}\frac{(-1)^k}{k}\right)+\log(2)H_m\right)

The answer is of the form: π A B ( log C ) D C \frac { { \pi }^{ A } }{ B } -\frac { { \left( \log { C } \right) }^{ D } }{ C } for positive integers A , B , C , D A,B,C,D .

Find A + B + C + D A+B+C+D .


The answer is 18.

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1 solution

Mark Hennings
Mar 15, 2016

We see that n = 1 m 1 n ( k = 1 n 1 ( 1 ) k k ) ln ( 2 ) H m = n = 1 m 1 n ( k = 1 n 1 ( 1 ) k k ln 2 ) = n = 1 m 1 n ( k = n ( 1 ) k 1 k ) = k = 1 ( 1 ) k 1 k ( n = 1 m i n ( k , m ) 1 n ) = k = 1 m ( 1 ) k 1 k H k + ( k = m + 1 ( 1 ) k 1 k ) H m \begin{array}{rcl} \displaystyle \sum_{n=1}^m \frac{1}{n} \left(\sum_{k=1}^{n-1} \frac{(-1)^k}{k}\right) - \ln(2)\, H_m & = & \displaystyle\sum_{n=1}^m \frac{1}{n}\left(\sum_{k=1}^{n-1} \frac{(-1)^k}{k} - \ln 2\right) \\ & = & \displaystyle \sum_{n=1}^m \frac{1}{n}\left(\sum_{k=n}^\infty \frac{(-1)^{k-1}}{k}\right) \; =\; \sum_{k=1}^\infty \frac{(-1)^{k-1}}{k}\left(\sum_{n=1}^{\mathrm{min}(k,m)} \frac{1}{n}\right) \\ & = & \displaystyle \sum_{k=1}^m \frac{(-1)^{k-1}}{k}H_k + \left(\sum_{k=m+1}^\infty \frac{(-1)^{k-1}}{k}\right)H_m \end{array} Now the second term is O ( ln m m ) O\big(\tfrac{\ln m}{m}\big) as m m \to \infty , and hence lim m n = 1 m 1 n ( k = 1 n 1 ( 1 ) k k ) ln ( 2 ) H m = k = 1 ( 1 ) k 1 k H k \lim_{m\to\infty} \sum_{n=1}^m \frac{1}{n} \left(\sum_{k=1}^{n-1} \frac{(-1)^k}{k}\right) - \ln(2) \,H_m \;= \; \sum_{k=1}^\infty \frac{(-1)^{k-1}}{k}H_k which I proved elsewhere to be equal to 1 2 ζ ( 2 ) 1 2 ( ln 2 ) 2 = 1 12 π 2 1 2 ( ln 2 ) 2 \tfrac12\zeta(2) - \tfrac12(\ln 2)^2 \; = \; \tfrac{1}{12}\pi^2 - \tfrac12(\ln 2)^2 which makes the answer 2 + 12 + 2 + 2 = 18 2 + 12 + 2 + 2 = 18 .

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