Help limits

Can someone please prove that

$\lim _{ N \to \infty }{ \frac { N! }{ \left( \frac { N+m }{ 2 } \right) !\left( \frac { N-m }{ 2 } \right) ! } \quad } =\quad { 2 }^{ N }{ e }^{ -\frac { { m }^{ 2 } }{ 2N } }$

I tried to use strilings approximation (simple form) and arrived here

$\displaystyle\lim _{ N \to \infty }{ \frac { N! }{ \left( \frac { N+m }{ 2 } \right) !\left( \frac { N-m }{ 2 } \right) ! } \quad } \simeq \quad \frac { { N }^{ N } }{ { e }^{ N } } \frac { { e }^{ (N+m)/2\quad +\quad (N-m)/2 }{ 2 }^{ N+m)/2\quad +\quad (N-m)/2 } }{ { (N+m) }^{ (N+m)/2 }\left( N-m \right) ^{ (N-m)/2 } } ={ 2 }^{ N }\frac { { N }^{ N } }{ { (N+m) }^{ (N+m)/2 }\left( N-m \right) ^{ (N-m)/2 } }$

but how to go beyond?

#Combinatorics #Calculus #Limits #Statisticalmechanics

Easy Math Editor

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Comments

Well it's easy from here rewrite it as :

$\displaystyle L = \lim _{ n\rightarrow \infty }{\dfrac { { 2 }^{ N } }{ { (1+\frac { m }{ N } ) }^{ N/2 }{ (1-\frac { m }{ N } ) }^{ N/2 }{ (\frac { N+m }{ N-m } ) }^{ m/2 } }}$

$\displaystyle \Rightarrow \lim _{ n\rightarrow \infty }{ \frac { { 2 }^{ N } }{ { (1-\frac { { m }^{ 2 } }{ { N }^{ 2 } } ) }^{ N/2 }{ (\frac { N+m }{ N-m } ) }^{ m/2 } } }$

$\displaystyle \Rightarrow L = \lim _{ n\rightarrow \infty }{ \frac { { 2 }^{ N }{ { ((1-\frac { { m }^{ 2 } }{ { N }^{ 2 } } ) }^{ { N }^{ 2 }/{ m }^{ 2 } }) }^{ { -m }^{ 2 }/2N } }{ { (\frac { N+m }{ N-m } ) }^{ m/2 } } }$

Now $\displaystyle \lim _{ n\rightarrow \infty }{ { ((1-\frac { { m }^{ 2 } }{ { N }^{ 2 } } ) }^{ { N }^{ 2 }/{ m }^{ 2 } } } =e$ and

$\displaystyle \lim _{ n\rightarrow \infty }{ { (\frac { N+m }{ N-m } ) }^{ m/2 } } =1$

Resulting in $\displaystyle L ={ 2 }^{ N }{ e }^{ { -m }^{ 2 }/2N }$

@Mvs Saketh

Ronak Agarwal - 6 years, 3 months ago

Thankyou , that was awesome :)

Mvs Saketh - 6 years, 3 months ago

Are you learning statistical mechanics. @Mvs Saketh

Ronak Agarwal - 6 years, 3 months ago

@Ronak Agarwal – indeed , and thanks for helping :)

Mvs Saketh - 6 years, 3 months ago

@Ronak Agarwal – @Ronak Agarwal - sorry but one small doubt again, there is a minus sign there,

i mean (1+x)^(1/x) = e so here it should be (1-x)^(1/x) =1/e right, why did you write e ?

Mvs Saketh - 6 years, 3 months ago

@Mvs Saketh – Even I am wondering about this, this should be $\frac{1}{e}$ but then the identity remains unproven.

Ronak Agarwal - 6 years, 3 months ago

@Ronak Agarwal – but it is definitely e because function must attain maxima at m=0, and also be positive on either side of m=0, and hence e^(-x^2) is only suitable

Mvs Saketh - 6 years, 3 months ago

please help @Ronak Agarwal @Shashwat Shukla @Deepanshu Gupta @Pratik Shastri and any one else who can

Mvs Saketh - 6 years, 3 months ago

You're missing a $\sqrt{\frac{\pi n}{2}}$ in the denominator.

It can be shown (using the central limit theorem) that :

$\binom{n}{k}=\frac{2^n}{\sqrt{\frac{n\pi}{2}}}e^{-\frac{(k-\frac{n}{2})^2}{\frac{n}{2}}}$ (As $n\rightarrow \infty$ )

Here, setting $k=\frac{n \pm m}{2}$ yields the desired result.

Mvs Saketh

Shashwat Shukla - 6 years, 3 months ago

yes thats why i said that i have used the simpler form of stirlings theorem, without involving the pi terms,

The real expression is the one you said but i was studying just the introductory parts of the statistical limits and there the simpler form without the pi part was given, yours is indeed the more accurate one

and yes thanks for giving the general limit for nCr, that is helpful, thanks

Mvs Saketh - 6 years, 3 months ago

You're welcome :)...Are you studying Boltzmann's law for entropy by any chance? This expression looks like one for the multiplicity of a system...

Shashwat Shukla - 6 years, 3 months ago

@Shashwat Shukla – Yes, indeed , i was studying the second law of thermodynamics, and the mathematical meaning of entropy

Mvs Saketh - 6 years, 3 months ago

@Mvs Saketh – Cool :)

Shashwat Shukla - 6 years, 3 months ago

Math	Appears as
Remember to wrap math in `\(` ... `\)` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$