Double sum inverse product-1

Calculus Level 4

2021 ! k = 0 l = 0 1 α = 1 2022 ( k + l + α ) = 1 A 2021!\sum _{ k=0 }^{ \infty }{ \sum _{ l=0 }^{ \infty }{ \frac { 1 }{ \displaystyle\prod _{ \alpha =1 }^{ 2022 }{ \left( k+l+\alpha \right) } } } } =\frac { 1 }{ A }

Find the value of A A .

Note: This is an advanced version of the problem Double sum inverse product .


The answer is 2020.

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1 solution

Syed Shahabudeen
Jan 16, 2020

I have already given a proof in Double sum inverse product in which I have generalized the given sum as k = 0 m = 0 1 α = 1 n ( k + m + α ) = 1 ( n 2 ) ( n 1 ) ! \boxed{\sum _{ k=0 }^{ \infty }{ \sum _{ m=0 }^{ \infty }{ \frac { 1 }{ \displaystyle\prod _{ \alpha =1 }^{ n }{ \left( k+m+\alpha \right) } } } } =\frac { 1 }{ \left( n-2 \right) \left( n-1 \right) ! } } therefore when n = 2022 n=2022 and by multiplying both sides by 2021 ! 2021! \quad we get 2021 ! k = 0 l = 0 1 α = 1 2022 ( k + l + α ) = 1 2020 2021!\sum _{ k=0 }^{ \infty }{ \sum _{ l=0 }^{ \infty }{ \frac { 1 }{ \displaystyle\prod _{ \alpha =1 }^{ 2022 }{ \left( k+l+\alpha \right) } } } } =\frac { 1 }{ 2020 } this implies A = 2020 \boxed{A=2020}

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