Summation Kills

n 60 = 0 2 n 59 = 0 n 60 n 2 = 0 n 3 n 1 = 0 n 2 n 0 = 0 n 1 1 = ? \Large \displaystyle \sum_{n_{60}=0}^{2} \sum^{n_{60}}_{n_{59}=0} \cdots \sum_{n_2 = 0}^{n_3} \sum_{n_1 = 0}^{n_2} \sum_{n_0 = 0}^{n_1} 1 = \, ?


The answer is 1953.00.

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1 solution

Rishabh Jain
Jun 17, 2016

Using repeatedly the Hockey Stick identity : n 60 = 0 2 n 59 = 0 n 60 n 2 = 0 n 3 n 1 = 0 n 2 n 0 = 0 n 1 ( 1 ) \Large\displaystyle\sum_{\color{#D61F06}{n_{60}}=0}^{2}\sum_{\color{#D61F06}{n_{59}}=0}^{n_{60}}\cdots\sum_{\color{#D61F06}{n_2}=0}^{n_3}\sum_{\color{#D61F06}{n_1}=0}^{n_2}\sum_{\color{#D61F06}{n_0}=0}^{n_1}(1) = n 60 = 0 2 n 59 = 0 n 60 n 2 = 0 n 3 n 1 = 0 n 2 ( n 1 + 1 1 ) \Large =\displaystyle\sum_{\color{#D61F06}{n_{60}}=0}^{2}\sum_{\color{#D61F06}{n_{59}}=0}^{n_{60}}\cdots\sum_{\color{#D61F06}{n_2}=0}^{n_3}\sum_{\color{#D61F06}{n_1}=0}^{n_2}\dbinom{n_1+1}1 = n 60 = 0 2 n 59 = 0 n 60 n 2 = 0 n 3 ( n 2 + 2 2 ) \Large =\displaystyle\sum_{\color{#D61F06}{n_{60}}=0}^{2}\sum_{\color{#D61F06}{n_{59}}=0}^{n_{60}}\cdots\sum_{\color{#D61F06}{n_2}=0}^{n_3}\dbinom{n_2+2}{2} \cdots\\\cdots = n 60 = 0 2 n 59 = 0 n 60 ( n 59 + 59 59 ) \Large =\displaystyle\sum_{\color{#D61F06}{n_{60}}=0}^{2}\sum_{\color{#D61F06}{n_{59}}=0}^{n_{60}}\dbinom{n_{59}+59}{59} = n 60 = 0 2 ( n 60 + 60 60 ) = ( 2 + 61 61 ) \Large =\displaystyle\sum_{\color{#D61F06}{n_{60}}=0}^{2}\binom{n_{60}+60}{60}=\binom{2+61}{61} ( 63 61 ) = 1953 \Large\therefore \dbinom{63}{61}=\boxed{\color{#007fff}{1953}}


If you're looking for a proof here are some cute proofs posted... :-)

Cool! Thanks for a nice solution. (+1)

Samara Simha Reddy - 4 years, 12 months ago

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My pleasure.... :-)

Rishabh Jain - 4 years, 12 months ago

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