Infinite Infinities

Algebra Level 4

2 n = 0 [ 2 [ 2 n = 0 [ 2 [ 2 n = 0 [ 2 [ . . . 2013 ] n ] 1 2013 ] n ] 1 2013 ] n ] 1 2\sum _{ n=0 }^{ \infty }{ { \left[ { 2\left[ \frac { 2 \sum _{ n=0 }^{ \infty }{ { \left[ { 2\left[ \frac { 2 \sum _{ n=0 }^{ \infty }{ { \left[ { 2\left[ \frac { ... }{ 2013 } \right] }^{ n } \right] }^{ -1 } } }{ 2013 } \right] }^{ n } \right] }^{ -1 } } }{ 2013 } \right] }^{ n } \right] }^{ -1 } }

What is the value of the nested summation above?


This might look daunting at first but the solution will come out very nicely and easily.


The answer is 2014.

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2 solutions

I have written the solution on a paper. Please see the image .

Though the answer comes out correct, their is a mistake towards the end of the solution where 2013 should also be raised to the power n:

n = 0 201 3 n x n = x \sum _{n=0}^{\infty } \frac{2013^n}{x^n}=x

Abdul Hafiz Kaissi - 6 years, 3 months ago
Julian Poon
Aug 13, 2014

Since this goes on for infinity, this can be written as:

2 n = 0 [ 2 [ x 2013 ] n ] 1 = x 2\sum _{ n=0 }^{ \infty }{ { \left[ { 2\left[ \frac { x }{ 2013 } \right] }^{ n } \right] }^{ -1 } } =x

Now let:

2 n = 0 [ 2 [ x 2013 ] n ] 1 = S 2\sum _{ n=0 }^{ \infty }{ { \left[ { 2\left[ \frac { x }{ 2013 } \right] }^{ n } \right] }^{ -1 } } =S

n = 0 [ 2 [ x 2013 ] n ] 1 = S 2 \sum _{ n=0 }^{ \infty }{ { \left[ { 2\left[ \frac { x }{ 2013 } \right] }^{ n } \right] }^{ -1 } } =\frac { S }{ 2 }

n = 1 [ 2 [ x 2013 ] n ] 1 = S 2 1 [ x 2013 ] n = 0 [ 2 [ x 2013 ] n ] 1 n = 1 [ 2 [ x 2013 ] n ] 1 = [ 2 [ x 2013 ] 0 ] 1 = 1 2 = S 2 S 2 1 [ x 2013 ] = S x 2013 S 2 x \sum _{ n=1 }^{ \infty }{ { \left[ { 2\left[ \frac { x }{ 2013 } \right] }^{ n } \right] }^{ -1 } } =\frac { S }{ 2 } \frac { 1 }{ \left[ \frac { x }{ 2013 } \right] } \\ \therefore \sum _{ n=0 }^{ \infty }{ { \left[ { 2\left[ \frac { x }{ 2013 } \right] }^{ n } \right] }^{ -1 } } -\sum _{ n=1 }^{ \infty }{ { \left[ { 2\left[ \frac { x }{ 2013 } \right] }^{ n } \right] }^{ -1 } } ={ \left[ { 2\left[ \frac { x }{ 2013 } \right] }^{ 0 } \right] }^{ -1 }=\frac { 1 }{ 2 } \\ =\frac { S }{ 2 } -\frac { S }{ 2 } \frac { 1 }{ \left[ \frac { x }{ 2013 } \right] } =\frac { Sx-2013S }{ 2x }

Solving for S S gives: S = x x 2013 S=\frac { x }{ x-2013 }

And since 2 n = 0 [ 2 [ x 2013 ] n ] 1 = x 2\sum _{ n=0 }^{ \infty }{ { \left[ { 2\left[ \frac { x }{ 2013 } \right] }^{ n } \right] }^{ -1 } } =x , x x 2013 = x \frac { x }{ x-2013 } =x

Solving for x x gives x = 2014 o r 0 x=2014\quad or\quad 0

Since x > 0 x>0 , x = 2014 x=\boxed { 2014 }

By the way, can anyone tell me what this method is called?

Call it Poonisation.

Jake Lai - 6 years, 4 months ago

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How 'bout... no.

Julian Poon - 6 years, 4 months ago

I just applied the infinite g.p. formula after putting in x in first step.

Aakarshit Uppal - 6 years, 3 months ago

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Actually, this is deriving infinite g.p, so its the same thing actually.

Julian Poon - 6 years, 3 months ago

I solved it in similar way , nut don't know what it is called .

Shriram Lokhande - 6 years, 10 months ago

One of my teacher calls it infinite resistance. Since its similar to the way we solve resistors connected up to infinity

Lavisha Parab - 6 years, 3 months ago

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