Disguised as something?

n + 1 ( n ! ) 1 n \large \dfrac{n+1}{(n!)^{\frac{1}{n}}}

Let R R be the minimum value of the expression above where n n is a positive integer.

Find the value of R R 2 ^{2} .

Extra credit : Prove that the value you found is indeed minimum.


The answer is 4.

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5 solutions

Chew-Seong Cheong
Jul 10, 2015

( n ! ) 1 n = ( n ( n 1 ) ( n 2 ) . . . 1 ) 1 n ( n n ) 1 n = n (n!)^{\frac{1}{n}} = (n(n-1)(n-2)...1)^{\frac{1}{n}} \le \left(n^n\right)^{\frac{1}{n}} = n n + 1 ( n ! ) 1 n n + 1 n \implies \dfrac{n+1}{(n!)^{\frac{1}{n}}} \ge \dfrac{n+1}{n} \space and minimum happens when ( n ! ) 1 n = n n = 1 (n!)^{\frac{1}{n}} = n \implies n = 1\space and n + 1 ( n ! ) 1 n = 1 + 1 ( 1 ! ) 1 = 2 2 2 = 4 \dfrac{n+1}{(n!)^{\frac{1}{n}}} = \dfrac{1+1}{(1!)^1} = 2 \implies 2^2 = \boxed{4} .

Moderator note:

Great! It is important to find a value of n n where the minimum is achieved.

I don't see how you proved that the minimum occurs at n = 1 n=1 . Though the only place where ( n ! ) 1 n = n (n!)^\frac{1}{n}=n is at n = 1 n=1 , note that n + 1 n \frac{n+1}{n} decreases when n n increases. Thus, you have proven that n + 1 ( n ! ) 1 n n + 1 n \dfrac{n+1}{(n!)^\frac{1}{n}}-\dfrac{n+1}{n} increases as n n increases, but not that n + 1 ( n ! ) 1 n \dfrac{n+1}{(n!)^\frac{1}{n}} increases as n n increases.

Jason Zou - 5 years, 11 months ago

Can we solve this question by differentiating and then putting dy/dx=0?

Samarth Agarwal - 5 years, 11 months ago

But sir what if I put a very very large integer close to infinity In that case we have to take limiting value But when I did the above mentioned thing I got minimum 1

Ayush Sharma - 4 years ago

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L = lim n n + 1 ( n ! ) 1 n By Stirling’s formula = lim n n + 1 ( 2 π n n + 1 / 2 e n ) 1 / n = e > 2 \begin{aligned} L & = \lim_{n \to \infty} \frac {n+1}{({\color{#3D99F6}n!})^\frac 1n} & \small \color{#3D99F6} \text{By Stirling's formula} \\ &= \lim_{n \to \infty} \frac {n+1}{\left({\color{#3D99F6}\sqrt{2\pi}n^{n+1/2}e^{-n}}\right)^{1/n}} \\ & = e > 2 \end{aligned}

Chew-Seong Cheong - 4 years ago

Sum of first n n positive integers is n ( n + 1 ) 2 \dfrac{n(n+1)}{2} .

By AM-GM inequality ,

1 + 2 + 3 + ( n 1 ) + n n ( 1 2 3 ( n 1 ) n ) 1 n \dfrac{1 + 2+ 3 \cdots + (n-1) + n}{n} \geq (1\cdot 2\cdot 3\cdot \cdots (n-1) \cdot n )^{\frac{1}{n}}

n ( n + 1 ) 2 n ( n ! ) 1 n \implies \dfrac{n(n+1)}{2n} \geq (n!)^{\frac{1}{n}}

( n + 1 ) 2 ( n ! ) 1 n \implies \dfrac{(n+1)}{2} \geq (n!)^{\frac{1}{n}}

( n + 1 ) ( n ! ) 1 n 2 \implies \dfrac{(n+1)}{(n!)^{\frac{1}{n}}} \geq 2

\implies minimum value of ( n + 1 ) ( n ! ) 1 n \dfrac{(n+1)}{(n!)^{\frac{1}{n}}} is 2 2 , hence our answer is 4 \boxed{4} .

Moderator note:

You have not proven that this is the minimum value.

You still need to find an instance when it is achieved.

Ohhh! How could I forget A-M& G.M your speciality. 👍😕😊

A Former Brilliant Member - 6 years, 2 months ago

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Are you on google hangouts?

Harsh Shrivastava - 6 years, 2 months ago

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Yep! But never used it .

A Former Brilliant Member - 6 years, 2 months ago

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@A Former Brilliant Member Can you use it? We have group on google hangouts where all brilliantians are there.

Harsh Shrivastava - 6 years, 2 months ago

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@Harsh Shrivastava Yes !,I can. Where to join

A Former Brilliant Member - 6 years, 2 months ago

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@A Former Brilliant Member Do ya' know my email-id?

Harsh Shrivastava - 6 years, 2 months ago

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@Harsh Shrivastava Knew it but can't remember is it harshshrivastav99

A Former Brilliant Member - 6 years, 2 months ago

Can, you please sent the proof that the value is minimum!!!!!!!!!

Abhisek Mohanty - 6 years, 2 months ago

Let x x be the or representative of the given expression.

Then x n = n + 1 ( n + 1 ) n 1 / n ! x^{n} = n+1(n+1)^{n-1}/n!

The value of ( n + 1 ) n 1 / n ! (n+1)^{n-1}/n! will increase as with respect to n n therefore to get minimum x we have to put minimum value for n which is 1.

Rama Devi
May 14, 2015

Moderator note:

Why? Where's your working?

Vaibhav Prasad
Apr 4, 2015

Harsh Shrivastava I know that you can provide a better solution. ;)

We can't have negative factorials here, that's for sure.

After, negative integers, comes 0 0 .We could keep n n as 0 0 , but then the power of 1 0 \frac{1}{0} would not work.

So we keep n n as 1 \boxed {1} and Bingo ! \large \displaystyle \text {Bingo !}

I bet everyone who solved this till 4/4/15 used this method hence nobody has written a solution till now.

I think the best approach would be to show that f ( n ) = n + 1 ( n ! ) 1 n f(n) = \dfrac{n + 1}{(n!)^{\frac{1}{n}}} is an increasing function over the positive integers, (induction, perhaps?), implying that min f ( n ) = f ( 1 ) = 2. \min{f(n)} = f(1) = 2.

Interestingly, by Stirling's approximation we see that lim n f ( n ) = e . \lim_{n \rightarrow \infty} f(n) = e.

Brian Charlesworth - 6 years, 2 months ago

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Awesome observation sir!👍

Harsh Shrivastava - 6 years, 2 months ago

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I think it can be proved using logs too with some induction.

A Former Brilliant Member - 6 years, 2 months ago

Prove that this is indeed the minimum😊

I will provide a solution later.

Harsh Shrivastava - 6 years, 2 months ago

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@Harsh Shrivastava waiting for ur solution.

A Former Brilliant Member - 6 years, 2 months ago

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Posted! @Kalash Verma

Harsh Shrivastava - 6 years, 2 months ago

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