Evaluate the infinite product

Calculus Level 3

Evaluate the limit lim m n = 1 m e 1 / ( 2 n 1 ) n = 1 m e 1 / ( 2 n ) \lim_{m\to\infty} \dfrac{\displaystyle \prod_{n=1}^m e^{1/(2n-1)}}{\displaystyle \prod_{n=1}^m e^{1/(2n)}}

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2 solutions

Ron Gallagher
Dec 30, 2020

By expanding prior to taking the limit, we find:

exp(1/1 + 1/3 + 1/5 + ... + 1/(2*m-1)) / exp(1/2 + 1/4 + ... + 1/2m) =

exp(1 - 1/2 + 1/3 - 1/4 + .... - 1/2m).

As m-> infinity, the quantity in parenthesis is well known to approach ln(2) (Google "sum of alternating harmonic series for an explanation"). Therefore, the limit is:

exp(ln(2)) = 2

Dwaipayan Shikari
Dec 30, 2020

n = 1 e 1 ( 2 n 1 ) = e n = 1 1 ( 2 n 1 ) = α \prod_{n=1}^∞ e^{\frac{1}{(2n-1)}} = e^{\sum_{n=1}^∞ \frac{1}{(2n-1)} } =\alpha n = 1 e 1 2 n = e n = 1 1 2 n = β \prod_{n=1}^∞ e^{\frac{1}{2n}} = e^{\sum_{n=1}^∞ \frac{1}{2n} } =\beta So α β = e n = 1 1 ( 2 n 1 ) 1 2 n = e log ( 2 ) = 2 \frac{\alpha}{\beta} = e^{\sum_{n=1}^∞ \frac{1}{(2n-1)}-\frac{1}{2n} } = e^{\log(2)}=2

Note \color{#20A900}\textrm{Note}

n = 1 1 2 n 1 1 2 n = 1 1 2 + 1 3 . . = log ( 2 ) \sum_{n=1}^∞ \frac{1}{2n-1} -\frac{1}{2n} = 1-\frac{1}{2} +\frac{1}{3} -.. =\log(2)

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