# Fibonacci decimal

• Jun 23rd 2007, 02:16 PM
Soroban
Fibonacci decimal
Find the value of this infinite decimal:
Code:

      0.01       0.001       0.0002       0.00003       0.000005       0.0000008       0.00000013       0.000000021     +    ...     ----------------- S  =  0.011235955 ...
That is: . $\displaystyle S \;=\;\sum^{\infty}_{n=1} \frac{F_n}{10^{n+1}}$

. . where $\displaystyle F_n$ is the $\displaystyle n^{th}$ Fibonacci number.

• Jun 23rd 2007, 02:46 PM
topsquark
Quote:

Originally Posted by Soroban
Find the value of this infinite decimal:
Code:

      0.01       0.001       0.0002       0.00003       0.000005       0.0000008       0.00000013       0.000000021     +    ...     ----------------- S  =  0.011235955 ...
That is: . $\displaystyle S \;=\;\sum^{\infty}_{n=1} \frac{F_n}{10^{n+1}}$

. . where $\displaystyle F_n$ is the $\displaystyle n^{th}$ Fibonacci number.

Well,
$\displaystyle F_n = \frac{1}{\sqrt{5}} \left ( \frac{1 + \sqrt{5}}{2} \right ) ^n - \frac{1}{\sqrt{5}} \left ( \frac{1 - \sqrt{5}}{2} \right ) ^n$

So S is the difference of two geometric series:
$\displaystyle S = \frac{1}{10\sqrt{5}}\sum_{n = 1}^{\infty} \left ( \frac{1 + \sqrt{5}}{2 \cdot 10} \right ) ^n - \frac{1}{10\sqrt{5}}\sum_{n = 1}^{\infty} \left ( \frac{1 - \sqrt{5}}{2 \cdot 10} \right ) ^n$

I have to get to dinner, so I'll let someone else finish. It's trivial from here anyway.

-Dan
• Jun 23rd 2007, 03:30 PM
galactus
Hey Soroban. I think I may have this one. Let's see. I encountered this proof as a homework assignment back in college. In a Seminars in Mathematics course.

[tex]\frac{1}{10}\sum_{n=1}^{\infty}\frac{F_{n}}{10^{n} }[\math]

Let $\displaystyle d=0.11235955056........=\frac{F_{1}}{10}+\frac{F_{ 2}}{10^{2}}+\frac{F_{3}}{10^{3}}+.........$

Now use $\displaystyle 10d+d$ and we have:

$\displaystyle 11d=F_{1}+\frac{F_{2}+F_{1}}{10}+\frac{F_{3}+F_{2} }{10}+\frac{F_{4}+F_{3}}{10}+....$

$\displaystyle =1+\frac{F_{3}}{10}+\frac{F_{4}}{10^{2}}+\frac{F_{ 5}}{10^{3}}+..........$

$\displaystyle =1+100d-10F_{1}-F_{2}$

$\displaystyle =1+100d-10-1$

$\displaystyle =100d-10$

So, we have:

$\displaystyle 11d=100d-10$

$\displaystyle 89d=10$

$\displaystyle d=\frac{10}{89}$

Don't forget to multiply by our 1/10 and get:

$\displaystyle \frac{1}{89}$
• Jun 23rd 2007, 06:02 PM
ThePerfectHacker
Welcome back, you were away for a long time!
• Jun 24th 2007, 02:33 AM
galactus
Thanks PH. Yeah, it has been a while. I see you're up to 6200. Very prolific:)

How's the academic world treating you?.
• Jun 24th 2007, 06:04 AM
ThePerfectHacker
Quote:

Originally Posted by galactus
How's the academic world treating you?.

I am on vacation now. 2 Months to go.
• Jun 24th 2007, 04:13 PM
Goistein
Quote

http://www.mathhelpforum.com/math-he...94c654a9-1.gif
/Quote

I'm pretty sure that F(4)/10 should be F(4)/100, etc.;)
• Jun 25th 2007, 10:09 AM
galactus
You're so right, G. Thanks for the catch. I fix.
• Jun 25th 2007, 12:42 PM
topsquark
Quote:

Originally Posted by topsquark
Well,
$\displaystyle F_n = \frac{1}{\sqrt{5}} \left ( \frac{1 + \sqrt{5}}{2} \right ) ^n - \frac{1}{\sqrt{5}} \left ( \frac{1 - \sqrt{5}}{2} \right ) ^n$

So S is the difference of two geometric series:
$\displaystyle S = \frac{1}{10\sqrt{5}}\sum_{n = 1}^{\infty} \left ( \frac{1 + \sqrt{5}}{2 \cdot 10} \right ) ^n - \frac{1}{10\sqrt{5}}\sum_{n = 1}^{\infty} \left ( \frac{1 - \sqrt{5}}{2 \cdot 10} \right ) ^n$

I have to get to dinner, so I'll let someone else finish. It's trivial from here anyway.

-Dan

Okay, I'll finish. :)

We have
$\displaystyle \sum_{n = 0}^{\infty}ar^n = a + \sum_{n = 1}^{\infty}ar^n = \frac{a}{1 - r}$

So
$\displaystyle \sum_{n = 1}^{\infty}ar^n = \frac{a}{1 - r} - a$

Thus
$\displaystyle \sum_{n = 1}^{\infty} \left ( \frac{1 + \sqrt{5}}{2 \cdot 10} \right ) ^n = \frac{1}{1 - \left ( \frac{1 + \sqrt{5}}{2 \cdot 10} \right )} - 1$

and
$\displaystyle \sum_{n = 1}^{\infty} \left ( \frac{1 - \sqrt{5}}{2 \cdot 10} \right ) ^n = \frac{1}{1 - \left ( \frac{1 - \sqrt{5}}{2 \cdot 10} \right )} - 1$

So
$\displaystyle S = \frac{1}{10\sqrt{5}}\sum_{n = 1}^{\infty} \left ( \frac{1 + \sqrt{5}}{2 \cdot 10} \right ) ^n - \frac{1}{10\sqrt{5}}\sum_{n = 1}^{\infty} \left ( \frac{1 - \sqrt{5}}{2 \cdot 10} \right ) ^n$ $\displaystyle = \frac{1}{10\sqrt{5}} \left ( \frac{1}{1 - \left ( \frac{1 + \sqrt{5}}{2 \cdot 10} \right )} - \frac{1}{1 - \left ( \frac{1 - \sqrt{5}}{2 \cdot 10} \right )} \right )$

$\displaystyle S = \frac{1}{10\sqrt{5}} \left ( \frac{20}{19 - \sqrt{5}} - \frac{20}{19 + \sqrt{5}} \right )$

$\displaystyle S = \frac{2}{\sqrt{5}} \left ( \frac{19 + \sqrt{5}}{19^2 - 5} - \frac{19 - \sqrt{5}}{19^2 - 5} \right )$

$\displaystyle S = \frac{2}{\sqrt{5}} \left ( \frac{2 \sqrt{5}}{356} \right )$

$\displaystyle S = \frac{4}{356} = \frac{1}{89}$

as galactus already informed us. ;)

-Dan
• Jun 25th 2007, 01:06 PM
galactus
Cool, Topsquark. That's a good one.

One could also just use the closed-form for the Fibonacci numbers with x=1/10.

$\displaystyle \frac{x}{1-x-x^{2}}\Rightarrow\frac{\frac{1}{10}}{1-(\frac{1}{10})-(\frac{1}{10})^{2}}=\frac{10}{89}$

Multiply by our 1/10 and we get 1/89.