It's All Bases.

Level 2

For each positive integer n n let ϕ n = n 2 + 4 + n 2 \phi_{n} = \dfrac{\sqrt{n^2 + 4} + n}{2} .

Which is a representation of ϕ n n n \dfrac{\phi_{n} - n}{n} in base ϕ n \phi_{n} .

0.001001001001 ... 0.010101010101 ... 0.101010101010 ... 0.100100100100 ...

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1 solution

Rocco Dalto
Feb 9, 2019

Let ϕ n = n 2 + 4 + n 2 \phi_{n} = \dfrac{\sqrt{n^2 + 4} + n}{2} .

ϕ n \phi_{n} is one solution to x 2 n x 1 = 0 x^2 - nx - 1 = 0

ϕ n 2 = n ϕ n + 1 ϕ n 1 n ϕ n 1 ϕ n 2 1 = 1 n ϕ n = ( n 2 + 4 n 2 ) n \implies \phi_{n}^2 = n\phi_{n} + 1 \implies \phi_{n} - 1 - n\phi_{n} \implies \dfrac{1}{\phi_{n}^2 - 1} = \dfrac{1}{n\phi_{n}} = \dfrac{(\dfrac{\sqrt{n^2 + 4} - n}{2})}{n} = ϕ n n n ϕ n n n = 1 ϕ n 2 1 = 1 ϕ n 2 1 1 ϕ n 2 = = \dfrac{\phi_{n} - n}{n} \implies \dfrac{\phi_{n} - n}{n} = \dfrac{1}{\phi_{n}^2 - 1} = \dfrac{\dfrac{1}{\phi_{n}^2}}{1 - \dfrac{1}{\phi_{n}^2}} = m = 1 ( 1 ϕ n 2 ) m = ( 0.010101010101... ) ϕ n \sum_{m = 1}^{\infty} (\dfrac{1}{\phi_{n}^2})^{m} = \boxed{(0.010101010101 ...)_{\phi_{n}}} .

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