# TIFR2019-B-5

792 views

Stirling’s approximation for $n!$ states for some constants $c_1,c_2$

$$c_1 n^{n+\frac{1}{2}}e^{-n} \leq n! \leq c_2 n^{n+\frac{1}{2}}e^{-n}.$$

What are the tightest asymptotic bounds that can be placed on $n!$ $?$

1. $n! = \Omega(n^n) \text{ and } n! = \mathcal{O}(n^{n+\frac{1}{2}})$
2. $n! = \Theta(n^{n+\frac{1}{2}})$
3. $n! =\Theta((\frac{n}{e})^n)$
4. $n! =\Theta((\frac{n}{e})^{n+\frac{1}{2}})$
5. $n! =\Theta(n^{n+\frac{1}{2}}2^{-n})$

edited
1
Marked D. Is it correct?
2
yes
1
okay!!!
1
why not C)?
1
because in power $n+\frac{1}{2}=n$

right?
1
why not E ??? anyone ??
2
0

this stackoverflow answer explains this problem very well.

Given that -

$c_1n^{n+\frac{1}{2}}e^{-n}\leq n!\leq c_2n^{n+\frac{1}{2}}e^{-n}$.

$n!=\Theta\left(\frac{n^{n+1/2}}{e^n}\right)$

We can divide it by $e^{1/2}$  (multiplying or dividing by a +ve constant doesn't affect the asymptotic nature of a function)

$\implies n!=\Theta\left(\frac{n^{n+1/2}}{e^{n+1/2}}\right)=\Theta\left(\left(\frac{n}{e}\right)^{n+\frac{1}{2}}\right)$

edited
0

@Verma Ashish

C) and D) asymptotically same, na? Then why not C) too ans?

2

@srestha

They are not asymptomatically same..

$n^n$ vs $n^{n+1/2}$ , $n^{n+1/2}$ is $\theta (n^n\times \sqrt n)$

3

here my soln

from the range we can write:

n! = theta(n^(n+1/2) x e^-n)

now

we know that

2^n = e^nln2

so, 2^-n = e^-nln2     neglecting ln2 as it is a constant

replace this e^-n  with 2^-n

finally,

n! = theta(n^(n+1/2) x 2^-n)

that's the option E.

and

D is also correct.

why they had written " what are the asymptotic bounds...." in the question ,that means multiple solution possible?

0

@shaktisingh you seems correct..

but in my opinion removing constant from power is not a good choice...

$2^n=3^{n\log_32}$

​​​​​​after removing $log_32$, 2^n=O(3^n) but 2^n can't be theta(3^n)

1
Constant does not have any effect in this case.becuz e^-ln2 does not have much contribution.

I think answers can be multiple but I have checked the answer key there Only D is the answer
0

But We Know already that Asymptotically [ 2^n == 2^(n+1) ] are same which is also, [ 2^n == 2*(2^n) ].

is this your contradiction to my doubt, that '2' is Constant and 'n' is not a constant?

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