Does it converge?

Compute 1 2 + 1 4 + 2 8 + 3 16 + 5 32 + + F k 2 k + \frac{1}{2} + \frac{1}{4} + \frac{2}{8} + \frac{3}{16} + \frac{5}{32} + \dots + \frac{F_k}{2^k} + \dots where F k F_k represents the the k th k^\text{th} term of the Fibonacci sequence: 1 , 1 , 2 , 3 , 5 , 8 , 13 , . . . 1, 1, 2, 3, 5, 8, 13, ...


The answer is 2.

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3 solutions

Discussions for this problem are now closed

Dhruva Patil
Nov 25, 2014

S = 1 2 + 1 4 + 2 8 + 3 16 + 5 32 + 8 64 . . . S = 1 2 ( 1 + 1 2 + 2 4 + 3 8 + 5 16 + 8 32 . . . ) 2 S = 1 + 1 2 + 2 4 + 3 8 + 5 16 + 8 32 . . . 2 S S = 1 + 1 4 + 1 8 + 2 16 + 3 32 . . . S = 1 + 1 2 ( 1 2 + 1 4 + 2 8 + 3 16 + 5 32 + 8 64 . . ) S = 1 + S 2 S 2 = 1 S = 2 S=\frac { 1 }{ 2 } +\frac { 1 }{ 4 } +\frac { 2 }{ 8 } +\frac { 3 }{ 16 } +\frac { 5 }{ 32 } +\frac { 8 }{ 64 } ...\\ S=\frac { 1 }{ 2 } (1+\frac { 1 }{ 2 } +\frac { 2 }{ 4 } +\frac { 3 }{ 8 } +\frac { 5 }{ 16 } +\frac { 8 }{ 32 } ...)\\ 2S=1+\frac { 1 }{ 2 } +\frac { 2 }{ 4 } +\frac { 3 }{ 8 } +\frac { 5 }{ 16 } +\frac { 8 }{ 32 } ...\\ 2S-S=1+\frac { 1 }{ 4 } +\frac { 1 }{ 8 } +\frac { 2 }{ 16 } +\frac { 3 }{ 32 } ...\\ S=1+\frac { 1 }{ 2 } (\frac { 1 }{ 2 } +\frac { 1 }{ 4 } +\frac { 2 }{ 8 } +\frac { 3 }{ 16 } +\frac { 5 }{ 32 } +\frac { 8 }{ 64 } ..)\\ S=1+\frac { S }{ 2 } \\ \frac { S }{ 2 } =1\\ S=2

Moderator note:

Very nice! But you need to prove that S S is finite else you would be performing arithmetic on infinities.

I solved the problem this way. By the way, how does this problem have any calculus at all?

bobby jim - 6 years, 5 months ago

Series is a calculus topic but you don't have to use calculus to solve it. Also, I saw this exact same problem posted by someone else, and that problem is Level 2, but this is Level 4. There's some discrepancy here.

Hobart Pao - 6 years, 5 months ago

Yes indeed. Very famous problem from the Inequalities Chapter of Problem Solving Strategies.

Utkarsh Gupta - 6 years, 5 months ago

We can also use generating functions for this problem.

Letting x = 1 2 \frac{1}{2}

S = x + x 2 + 2 x 3 + 3 x 4 + 5 x 5 + 8 x 6 + 13 x 7 + 21 x 8 + 34 x 9 + . . . S = x + x^{2} + 2 x^{3} + 3 x^{4} + 5 x^{5} + 8 x^{6} + 13 x^{7} + 21 x^{8} + 34 x^{9} + ...

x S = 0 x + 1 x 2 + 1 x 3 + 2 x 4 + 3 x 5 + 5 x 6 + 8 x 7 + 13 x 8 + 21 x 9 + . . . x * S = 0 x + 1 x^{2} + 1 x^{3} + 2 x^{4} + 3 x^{5} + 5 x^{6} + 8 x^{7} + 13 x^{8} + 21 x^{9} + ...

x 2 S = 0 x + 0 x 2 + 1 x 3 + 1 x 4 + 2 x 5 + 3 x 6 + 5 x 7 + 8 x 8 + 13 x 9 + . . . x^{2} * S = 0 x + 0 x^{2} + 1 x^{3} + 1 x^{4} + 2 x^{5} + 3 x^{6} + 5 x^{7} + 8 x^{8} + 13 x^{9} + ...

Now: ( 1 x x 2 ) S = x (1 - x - x^{2}) S = x

S = x 1 x x 2 S = \frac{x}{1 - x - x^{2}}

Substituting x = 1 2 \frac{1}{2} , we get S = 2 2

Anatoliy Razin
Nov 17, 2014

S = n = 1 ( 1 5 ( 1 + 5 4 ) n 1 5 ( 1 5 4 ) n ) S = \displaystyle \sum_{n=1}^\infty (\frac{1}{\sqrt{5}} \cdot (\frac{1 + \sqrt{5}}{4})^n - \frac{1}{\sqrt{5}} \cdot (\frac{1 - \sqrt{5}}{4})^n)

Let φ 1 = 1 + 5 4 \varphi_1 = \frac{1 + \sqrt{5}}{4} and φ 2 = 1 5 4 \varphi_2 = \frac{1 - \sqrt{5}}{4}

Then S = 1 5 ( φ 1 ( 1 φ 1 ) φ 2 ( 1 φ 2 ) ) = 1 5 ( 1 + 5 3 5 1 5 3 + 5 ) S = \frac{1}{\sqrt{5}} \cdot (\frac{\varphi_1}{(1 - \varphi_1)} - \frac{\varphi_2}{(1 - \varphi_2)}) = \frac{1}{\sqrt{5}} \cdot (\frac{1 + \sqrt{5}}{3 - \sqrt{5}} - \frac{1 - \sqrt{5}}{3 + \sqrt{5}})

Therefore S = 1 5 ( 2 + 5 ( 2 5 ) ) = 2 S = \frac{1}{\sqrt{5}} \cdot ({2 + \sqrt{5}} - (2 - \sqrt{5})) = \boxed{2}

great solution @AnatoliyRazin .... but how did that complicated factorization came into your mind

Ayush Choubey - 6 years, 6 months ago

It's called Binet's formula for the Fibonacci sequence

Brian Kelly - 6 years, 4 months ago

I did it this way. It isn't complicated. Anybody who has studied algebra in depth will come across the closed form for the fibonacci sequence and try to prove it. It was a result that stuck and reused. However I like Dhruva's solution more because surds introduce mistakes very easily.

A Former Brilliant Member - 6 years, 5 months ago

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