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?
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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