Sum Fun

Calculus Level 4

n = 2 k = 1 ( 1 ) n ( k + n k ) \large \displaystyle \sum_{n=2}^\infty \sum_{k=1}^\infty \dfrac { (-1)^n }{ {k+n \choose k } }

If the above summation equals S S , what is the value of 10000 S \lfloor 10000 S \rfloor ?


The answer is 6931.

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1 solution

First we'll look at k = 1 1 ( k + n k ) = k = 1 1 ( k + n n ) = \displaystyle\sum_{k=1}^{\infty} \dfrac{1}{\dbinom{k + n}{k}} = \sum_{k=1}^{\infty} \dfrac{1}{\dbinom{k + n}{n}} =

k = 1 1 ( k + 1 ) ( k + 2 ) ( k + n ) n ! = \displaystyle\sum_{k=1}^{\infty} \dfrac{1}{\dfrac{(k + 1)(k + 2) \cdots (k + n)}{n!}} =

n ! k = 1 1 ( k + 1 ) ( k + 2 ) ( k + n ) = n! \displaystyle\sum_{k=1}^{\infty} \dfrac{1}{(k + 1)(k + 2) \cdots (k + n)} =

n ! n 1 k = 1 ( 1 ( k + 1 ) ( k + 2 ) ( k + n 1 ) 1 ( k + 2 ) ( k + 3 ) ( k + n ) ) = \dfrac{n!}{n - 1} \displaystyle\sum_{k=1}^{\infty} \left( \dfrac{1}{(k + 1)(k + 2) \cdots (k + n - 1)} - \dfrac{1}{(k + 2)(k + 3) \cdots (k + n)} \right)=

n ! n 1 ( 1 2 3 n 1 3 4 ( n + 1 ) + 1 3 4 ( n + 1 ) \dfrac{n!}{n - 1} ( \dfrac{1}{2 * 3 \cdots n} - \dfrac{1}{3 * 4 \cdots (n + 1)} + \dfrac{1}{3 * 4 \cdots (n + 1)} -

1 4 5 ( n + 2 ) + 1 4 5 ( n + 2 ) + ) = \dfrac{1}{4 * 5 \cdots (n + 2)} + \dfrac{1}{4 * 5 \cdots (n + 2)} -+ \cdots ) =

n ! n 1 1 n ! = 1 n 1 \dfrac{n!}{n - 1} * \dfrac{1}{n!} = \dfrac{1}{n - 1} ,

where in the next to last step we have formed a telescoping series with all the terms in the brackets canceling out in pairs except the first term. So now

S = n = 2 ( 1 ) n n 1 = 1 1 2 + 1 3 1 4 + = ln ( 2 ) = 0.693147181..... S = \displaystyle\sum_{n=2}^{\infty} \dfrac{(-1)^{n}}{n - 1} = 1 - \dfrac{1}{2} + \dfrac{1}{3} - \dfrac{1}{4} +- \cdots = \ln(2) = 0.693147181.....

Thus 10000 S = 6931 . \lfloor 10000*S \rfloor = \boxed{6931}.

Sir this is really beautiful Problem.Since Initially I thought It needs advanced calculus , but then It convert into interesting telescopic series , I really Love in solving such type sequences and series . Thanks for Posting such problems , and also I want to request you to Please Post such more sequence and series problems. I always waiting for such problems.

Also Sir One More request , It's long time you have not Posted Geometry Problems Like as your famous "Tight Fits" . We all are eagerly waiting for them , So sir Please post such more. Thanks Sir !

Deepanshu Gupta - 6 years, 3 months ago

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Thanks, Deepanshu. I'll try and come up with a geometry problem next. :)

Brian Charlesworth - 6 years, 3 months ago

Beta function can also be used!

Kartik Sharma - 6 years, 3 months ago

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Seems like Beta function is your fav ! You always have a Beta function solution ready :P

A Former Brilliant Member - 6 years, 3 months ago

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Yeah! Lol! :P

Kartik Sharma - 6 years, 3 months ago

Wow that's magnificent!

It's totally unexpected to transform that series into a telescoping one! Cheers :)

Hasan Kassim - 6 years, 3 months ago

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