( n ! ) n 1 n + 1
Let R be the minimum value of the expression above where n is a positive integer.
Find the value of R 2 .
Extra credit : Prove that the value you found is indeed minimum.
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Great! It is important to find a value of n where the minimum is achieved.
I don't see how you proved that the minimum occurs at n = 1 . Though the only place where ( n ! ) n 1 = n is at n = 1 , note that n n + 1 decreases when n increases. Thus, you have proven that ( n ! ) n 1 n + 1 − n n + 1 increases as n increases, but not that ( n ! ) n 1 n + 1 increases as n increases.
Can we solve this question by differentiating and then putting dy/dx=0?
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
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L = n → ∞ lim ( n ! ) n 1 n + 1 = n → ∞ lim ( 2 π n n + 1 / 2 e − n ) 1 / n n + 1 = e > 2 By Stirling’s formula
Sum of first n positive integers is 2 n ( n + 1 ) .
By AM-GM inequality ,
n 1 + 2 + 3 ⋯ + ( n − 1 ) + n ≥ ( 1 ⋅ 2 ⋅ 3 ⋅ ⋯ ( n − 1 ) ⋅ n ) n 1
⟹ 2 n n ( n + 1 ) ≥ ( n ! ) n 1
⟹ 2 ( n + 1 ) ≥ ( n ! ) n 1
⟹ ( n ! ) n 1 ( n + 1 ) ≥ 2
⟹ minimum value of ( n ! ) n 1 ( n + 1 ) is 2 , hence our answer is 4 .
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. 👍😕😊
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Are you on google hangouts?
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Yep! But never used it .
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@A Former Brilliant Member – Can you use it? We have group on google hangouts where all brilliantians are there.
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@Harsh Shrivastava – Yes !,I can. Where to join
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@A Former Brilliant Member – Do ya' know my email-id?
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@Harsh Shrivastava – Knew it but can't remember is it harshshrivastav99
Can, you please sent the proof that the value is minimum!!!!!!!!!
Let x be the or representative of the given expression.
Then x n = n + 1 ( n + 1 ) n − 1 / n !
The value of ( n + 1 ) n − 1 / n ! will increase as with respect to n therefore to get minimum x we have to put minimum value for n which is 1.
Why? Where's your working?
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 .We could keep n as 0 , but then the power of 0 1 would not work.
So we keep n as 1 and 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 ! ) n 1 n + 1 is an increasing function over the positive integers, (induction, perhaps?), implying that min f ( n ) = f ( 1 ) = 2 .
Interestingly, by Stirling's approximation we see that lim n → ∞ f ( n ) = e .
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Awesome observation sir!👍
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I think it can be proved using logs too with some induction.
Prove that this is indeed the minimum😊
I will provide a solution later.
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@Harsh Shrivastava waiting for ur solution.
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( n ! ) n 1 = ( n ( n − 1 ) ( n − 2 ) . . . 1 ) n 1 ≤ ( n n ) n 1 = n ⟹ ( n ! ) n 1 n + 1 ≥ n n + 1 and minimum happens when ( n ! ) n 1 = n ⟹ n = 1 and ( n ! ) n 1 n + 1 = ( 1 ! ) 1 1 + 1 = 2 ⟹ 2 2 = 4 .