Reckon you can solve this recursion?

Calculus Level 5

Consider a sequence ( a n ) n = 1 (a_n)_{n=1}^\infty with a 1 = 1 , a 2 = 2 a_1 = 1, a_2 = 2 . Let S n = j = 1 n a n \displaystyle S_n = \sum_{j=1}^n a_n such that it satisfy the recurrence relation a n = n × S n 1 a_n = n\times S_{n-1} for n = 3 , 4 , 5 , n=3,4,5,\ldots .

Lastly, denote s s as the value of n = 1 n a n \displaystyle \sum_{n=1}^\infty \dfrac n{a_n} .

Find the last three digits of 1 0 8 × s \left\lfloor 10^8 \times s \right\rfloor .


The answer is 365.

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2 solutions

a 1 = 1 a 2 = 2 a 3 = 2 ( 3 2 ) 3 1 = 3 × 3 ! 2 a 4 = 3 × 3 ! 2 × 4 2 3 = 4 × 4 ! 2 a 5 = 4 × 4 ! 2 × 5 2 4 = 5 × 5 ! 2 a n = n × n ! 2 s = n = 1 n a n = 1 1 + 2 2 + n = 3 2 n n × n ! = 2 + n = 3 2 n ! = 2 + 2 n = 0 1 n ! 2 0 ! 2 1 ! 2 2 ! = 2 + 2 e 2 2 1 = 2 e 3 = 2.436563657 a_1 = 1 \quad a_2 = 2 \\ \Rightarrow a_3 = \dfrac {2(3^2)}{3-1} = \dfrac {3\times 3!}{2} \quad \Rightarrow a_4 = \dfrac {3\times 3!}{2} \times \dfrac{4^2}{3} = \dfrac {4\times 4!}{2} \\ \Rightarrow a_5 = \dfrac {4\times 4!}{2} \times \dfrac{5^2}{4} = \dfrac {5\times 5!}{2} \quad \Rightarrow a_n = \dfrac {n\times n!}{2} \\ \begin{aligned} \Rightarrow s & = \sum_{n=1}^\infty {\frac{n}{a_n}} = \frac{1}{1} + \frac{2}{2} + \sum_{n=3}^\infty {\frac{2n}{n\times n!}} = 2 + \sum_{n=3}^\infty {\frac{2}{n!}} \\ & = 2 + 2\sum_{n=0}^\infty {\frac{1}{n!}} - \frac{2}{0!} - \frac{2}{1!} - \frac{2}{2!} \\ & = 2 + 2e - 2 - 2 - 1 = 2e-3 = 2.436563657 \end{aligned}

1 0 8 × s = 243656 365 \Rightarrow \lfloor 10^8\times s \rfloor = 243656\boxed{365}

Shashwat Shukla
May 3, 2015

Define b n = a n n b_{n}=\frac{a_{n}}{n} .

The recurrence then becomes:

b n = n b n 1 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 ! ) b_{n}=c*n! \quad \rightarrow a_{n}=c(n*n!)

where c is an arbitrary constant to be determined from the initial conditions.

This c comes out to be 1 2 \frac{1}{2} and thus, a n = 1 2 ( n n ! ) a_{n}=\frac{1}{2}(n*n!)

The required summation is then trivial and evaluates to 2 ( e 1 ) 2(e-1) .

The required answer is then 365 \boxed{365} .

what's wrong in my approach ?

S n S n 1 = a n & a n = n S n 1 S n S n 1 = n + 1 S n S n 1 = ( n + 1 ) S n = ( n + 1 ) ! S n 1 = n ! n a n = 1 S n 1 n a n = 1 S n 1 = 1 n ! = e 1 A n s w e r = 182 ? \displaystyle{\because \quad { S }_{ n }-{ S }_{ n-1 }={ a }_{ n }\quad \& \quad { a }_{ n }=n{ S }_{ n-1 }\\ \therefore \quad \cfrac { { S }_{ n } }{ { S }_{ n-1 } } =n+1\\ \prod { \cfrac { { S }_{ n } }{ { S }_{ n-1 } } } =\prod { \left( n+1 \right) } \\ \Rightarrow { S }_{ n }=\left( n+1 \right) !\\ { S }_{ n-1 }=n!\\ \because \cfrac { n }{ { a }_{ n } } =\cfrac { 1 }{ { S }_{ n-1 } } \\ \sum { \cfrac { n }{ { a }_{ n } } } =\sum { \cfrac { 1 }{ { S }_{ n-1 } } } =\sum { \cfrac { 1 }{ n! } } =e-1\\ \boxed { Answer=182 } \quad ?\\ }

@Atul Antony Zachariahs @Shashwat Shukla

Nishu sharma - 6 years, 1 month ago

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@Nishu sharma S(2)/S(1)=3 So S(n)=(n+1) (n) (n-1)..3=(n+1)!/2

Atul Antony Zachariahs - 6 years, 1 month ago

There's nothing wrong with the approach..Just a minor detail... Π ( n + 1 ) = ( n + 1 ) ! 2 \Pi(n+1)=\frac{(n+1)!}{2} here as ( n + 1 ) (n+1) actually starts with 3 here(as n starts from 2).

Shashwat Shukla - 6 years, 1 month ago

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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 ! :)

Nishu sharma - 6 years, 1 month ago

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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. :)

Shashwat Shukla - 6 years, 1 month ago

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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 !

Nishu sharma - 6 years, 1 month ago

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