A not so familiar summation

Calculus Level 5

lim n ( ln ( 1 + 1 n ) 1 + ln ( 1 + 2 n ) 2 + ln ( 1 + 3 n ) 3 + + ln 2 n ) \large \lim\limits_{n \to \infty} \left( \frac{ \ln ( 1 + \frac{1}{n}) }{1} + \frac{ \ln (1 + \frac{2}{n})}{2} + \frac{ \ln(1 + \frac{3}{n})}{3} + \cdots + \frac{ \ln 2}{n} \right )

If the limit above equals to A A , find 1 0 4 A \lfloor 10^4 A \rfloor .


Notation: \lfloor \cdot \rfloor denotes the floor function .


Inspiration .


The answer is 8224.

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1 solution

Efren Medallo
May 8, 2017

The summation could be expressed as

A = lim n k = 1 n ln ( 1 + k n ) k A = \large \lim\limits_{n \to \infty} \sum\limits_{k=1}^{n} \frac{ \ln (1+ \frac{k}{n})}{k}

We could modify a little bit of the RHS and get

A = lim n 1 n k = 1 n ln ( 1 + k n ) k n A = \large \lim\limits_{n \to \infty} \frac{1}{n} \sum\limits_{k=1}^{n} \frac{ \ln (1+ \frac{k}{n})}{\frac{k}{n}}

Well, this is actually the Riemann Sum representation of the integral

A = 0 1 ln ( 1 + x ) x d x A = \large \int_0^1 \frac{ \ln(1+x) }{x} \mathrm{d}x

Which, at first glance, isn't computable.

Let us turn to power series for help.

Note that ln ( 1 + x ) = x x 2 2 + x 3 3 x 4 4 + x 5 5 . . . \ln (1+x) = x - \frac{x^2}{2} + \frac{x^3}{3} - \frac{x^4}{4} + \frac{x^5}{5}...

ln ( 1 + x ) = x x 2 2 + x 3 3 x 4 4 + x 5 5 . . . \ln (1+x) = x - \frac{x^2}{2} + \frac{x^3}{3} - \frac{x^4}{4} + \frac{x^5}{5}...

ln ( 1 + x ) x = 1 x 2 + x 2 3 x 3 4 + x 4 5 . . . \frac{\ln (1+x)}{x} = 1 - \frac{x}{2} + \frac{x^2}{3} - \frac{x^3}{4} + \frac{x^4}{5}...

0 x ln ( 1 + x ) x = x x 2 4 + x 3 9 x 4 16 + x 5 25 . . . \int_0^x \frac{\ln (1+x)}{x} = x - \frac{x^2}{4} + \frac{x^3}{9} - \frac{x^4}{16} + \frac{x^5}{25}...

Substituting x = 1 x=1 to the above equation, we get an infinite sum equating to A A .

Now, we know of this sum

π 2 6 = 1 + 1 4 + 1 9 + 1 16 + 1 25 + 1 36 + . . . \frac{\pi^2}{6} = 1 + \frac{1}{4} + \frac{1}{9} + \frac{1}{16} + \frac{1}{25} + \frac{1}{36} +...

π 2 12 = 1 2 + 1 8 + 1 18 + 1 32 + . . . \frac{\pi^2}{12} = \frac{1}{2} + \frac{1}{8} + \frac{1}{18} + \frac{1}{32} +...

π 2 6 π 2 12 = 1 + ( 1 4 1 2 ) + 1 9 + ( 1 16 1 8 ) + 1 25 + ( 1 36 1 18 ) + . . . \frac{\pi^2}{6} - \frac{\pi^2}{12} = 1 + (\frac{1}{4} - \frac{1}{2}) + \frac{1}{9} + (\frac{1}{16} - \frac{1}{8}) + \frac{1}{25} + ( \frac{1}{36} - \frac{1}{18} ) + ...

π 2 12 = 1 1 4 + 1 9 1 16 + 1 25 1 36 + . . . = A \frac{\pi^2}{12} = 1 - \frac{1}{4} + \frac{1}{9} - \frac{1}{16} + \frac{1}{25} - \frac{1}{36} +... = A

So that's it! A = π 2 12 A = \frac{\pi^2}{12} , such that

1 0 4 A = 8224 \lfloor 10^4 A \rfloor = \boxed{8224}

Same way!!!

A Former Brilliant Member - 3 years, 9 months ago

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