Consider a sequence ( a n ) n = 1 ∞ with a 1 = 1 , a 2 = 2 . Let S n = j = 1 ∑ n a n such that it satisfy the recurrence relation a n = n × S n − 1 for n = 3 , 4 , 5 , … .
Lastly, denote s as the value of n = 1 ∑ ∞ a n n .
Find the last three digits of ⌊ 1 0 8 × s ⌋ .
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Define b n = n a n .
The recurrence then becomes:
b n = n ∗ b n − 1
This is obviously the recurrence relation for the factorial function.
Thus, we have,
b n = c ∗ n ! → a n = c ( n ∗ n ! )
where c is an arbitrary constant to be determined from the initial conditions.
This c comes out to be 2 1 and thus, a n = 2 1 ( n ∗ n ! )
The required summation is then trivial and evaluates to 2 ( e − 1 ) .
The required answer is then 3 6 5 .
what's wrong in my approach ?
∵ S n − S n − 1 = a n & a n = n S n − 1 ∴ S n − 1 S n = n + 1 ∏ S n − 1 S n = ∏ ( n + 1 ) ⇒ S n = ( n + 1 ) ! S n − 1 = n ! ∵ a n n = S n − 1 1 ∑ a n n = ∑ S n − 1 1 = ∑ n ! 1 = e − 1 A n s w e r = 1 8 2 ?
@Atul Antony Zachariahs @Shashwat Shukla
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@Nishu sharma S(2)/S(1)=3 So S(n)=(n+1) (n) (n-1)..3=(n+1)!/2
There's nothing wrong with the approach..Just a minor detail... Π ( n + 1 ) = 2 ( n + 1 ) ! here as ( n + 1 ) actually starts with 3 here(as n starts from 2).
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Ahh , yes It is ! Actually I did all such calculation orally , So gotta silly mistake !
Thanks for notifying ! :)
Btw , I'am new to brilliant , And I found you are good user at this site , I like your problem solving skill , Btw How much r u getting in Mains , if you don't mind tell ! :)
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@Nishu Sharma – Hey, welcome to the community :) ...Mains was a big let down for me as I made horrifying careless mistakes...As a result I'm only getting 280...Your score of 321 is very impressive.
Best of luck with Advanced. :)
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@Shashwat Shukla – Thanks ! Don't worry , mains is just practice paper , Real thing is advance ! Hope we both don't do mistakes in that . Best of luck to you too !
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a 1 = 1 a 2 = 2 ⇒ a 3 = 3 − 1 2 ( 3 2 ) = 2 3 × 3 ! ⇒ a 4 = 2 3 × 3 ! × 3 4 2 = 2 4 × 4 ! ⇒ a 5 = 2 4 × 4 ! × 4 5 2 = 2 5 × 5 ! ⇒ a n = 2 n × n ! ⇒ s = n = 1 ∑ ∞ a n n = 1 1 + 2 2 + n = 3 ∑ ∞ n × n ! 2 n = 2 + n = 3 ∑ ∞ n ! 2 = 2 + 2 n = 0 ∑ ∞ n ! 1 − 0 ! 2 − 1 ! 2 − 2 ! 2 = 2 + 2 e − 2 − 2 − 1 = 2 e − 3 = 2 . 4 3 6 5 6 3 6 5 7
⇒ ⌊ 1 0 8 × s ⌋ = 2 4 3 6 5 6 3 6 5