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Convergence of $sumlimits_{n=1}^inftyleft{frac{1cdot 3dots 2n-1 }{2cdot 4dots 2n}cdotfrac{4n+3}{2n+2}right}^2$

Mathematics Asked by Charlie Chang on November 6, 2021

Show if the inf series
$sumlimits_{n=1}^inftyleft{frac{1cdot3dots2n-1 }{2cdot 4dots2n}cdotfrac{4n+3}{2n+2}right}^2$
converges.

My thought:

When $2n=2^k$, $frac{1cdot 3dots2n-1 }{2cdot 4dots2n}cdot frac{4n+3}{2n+2}
approx (1-1/4)^1cdot (1-1/8)^2dots(1-1/(2^k))^{2^{k-2}} approx (1-1/4)^{k-1},$

and so the $n$th item =
$((1-1/4)^{k-1})^2=(9/16)^{k-1}$.

When $2n neq 2^k$, we have $2^{k-1}<2n<2^k$,
begin{align*}
frac{1cdot 3dots2n-1 }{2cdot 4dots2n}cdot frac{4n+3}{2n+2}
approx (1-1/4)^1cdot (1-1/8)^2dots(1-1/(2^{k-1}))^{2^{k-3}}cdot(1-1/(2^k))^{n-(2^{k-2})}\
approx (1-1/4)^{k-2}cdot (1-1/(2^k))^{n-(2^{k-2})},
end{align*}

where $0<n-2^{k-2}<2^{k-2}$.

Then sum $(2^{k-2}+1)$ th to $n’$ th terms, for $2^{k-1}<2n'<=2^k$,
begin{align*}
sumlimits_{n=2^{k-2}+1}^{n’}left{frac{1cdot3dots2n-1 }{2. 4dots2n}cdot frac{4n+3}{2n+2}right}^2
approx sumlimits_{n=2^{k-2}+1}^{n’} (1-1/4)^{2(k-2)}.(1-1/(2^k))^{2(n-(2^{k-2}))}\
=(1-1/4)^{2(k-2)}cdot frac{(1-1/(2^k))^2}{1-(1-1/(2^k))^2}cdot(1-(1-1/(2^k))^2)^{n’-(2^{k-2})})\
=(9/16)^{(k-2}cdot (2^{k-1}-1)cdot (1-frac{n’-(2^{k-2})}{2^{k-1}}),$
end{align*}

which, when $2n’=2^k$, approximates
$(9/16)^{k-2}cdot (2^{k-1}-1)cdot(1-1/2)approx (9/16)^{k-2}cdot (2^{k-2}).$

(All the terms approximates $(9/16)^{k-1}$, and so the sum is roughly $(9/16)^{k-1}cdot2^{k-2}$. This gives a little larger estimation.)

Therefore the infinite series equals
$$sumlimits_{k=2}^{infty}sumlimits_{n=2^{k-2}+1}^{n’} approx sumlimits_{k=2}^{infty} (9/16)^{k-2}cdot (2^{k-2})=sumlimits_{k=2}^{infty} (9/8)^{k-2},$$
which diverges, and so the series possibly diverges.$

Given the final approximation being $sum a^n$ where a is near 1, a little difference in the approximations above could change the convergence. Were the series convergent and had the approximations made it not, which one causes that? Besides, are there other methods?

3 Answers

You can show the divergence of this series in a quite elementary way using a little trick by estimating

$$a_n:=frac 12cdot frac 34 cdot frac 56 cdots frac{2n-1 }{2n} geq frac 12cdot frac 23 cdot frac 45 cdots frac{2n-2}{2n-1} =: c_n $$

So, you have

$$a_n^2 geq a_nc_n = frac 1{4n}$$

Hence,

$$sum_{n=1}^{infty}left(a_nfrac{4n+3}{2n+2}right)^2geqsum_{n=1}^{infty}frac 1{4n}left(frac{4n+3}{2n+2}right)^2geq sum_{n=1}^{infty}frac 1{4n}$$

Answered by trancelocation on November 6, 2021

Initial series is equivalent to series $$sum_{n=1}^{infty}left( frac{1 cdot 3 cdot .. cdot(2n-1) }{2 cdot 4 cdot .. cdot (2n)}right)^2$$ So we have $$frac{a_n}{a_{n+1}} = left(1+ frac{1}{2n+1} right)^2 = 1+frac{1}{n} +frac{theta_n}{n^2}$$ Where $theta_n$ is bounded. So accordingly Gauss test series diverges.

Answered by zkutch on November 6, 2021

$1times3timesldotstimes (2n-1)=frac{1times2timesldotstimes 2n}{2times 4timesldotstimes 2n}=frac{(2n)!}{2^nn!}$. Thus $$ frac{1times3timesldotstimes(2n-1)}{2times4timesldotstimes 2n}=frac{(2n)!}{4^n(n!)^2} $$ Now, using Stirling's approximation $n!underset{nrightarrow +infty}{sim}sqrt{2pi n}left(frac{n}{e}right)^n$, we have $$ frac{1times3timesldotstimes(2n-1)}{2times4timesldotstimes 2n}frac{4n+3}{2n+2}underset{nrightarrow +infty}{sim} 2frac{sqrt{4pi n}left(frac{2n}{e}right)^{2n}}{4^ntimes 2pi nleft(frac{n}{e}right)^{2n}}underset{nrightarrow +infty}{sim}frac{2}{sqrt{npi}} $$ and thus your series diverges. Your approximations does not work since you find a sum of the form $sum a^n$ which can't lead to $frac{4ln n}{pi}$ (using $sum_{k=1}^nfrac{1}{k}underset{nrightarrow +infty}{sim} ln n$).

Answered by Tuvasbien on November 6, 2021

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