Processing math: 2%

Monday, June 3, 2013

1.34

I'll prove such a function exists by actually constructing one.

Let F:[0,1] \to [0,1] such that:

F(x) = \Bigg\lbrace \begin{array}{cc} f(x) & x \in \mathcal{C}_\epsilon & \\ g(x) & x \in [0,1]\cap \mathcal{C}_\epsilon^c \\ \end{array}


Our mapping f(x):\mathcal{C}_\epsilon \to \mathcal{C}_\gamma is constructed by simply reassinging x \in \mathcal{C}_\epsilon's binary expansion to \mathcal{C}_\gamma's. To do this, simply observe that for any x in \mathcal{C}_\epsilon,

x = \sum_{n=1}^{\infty} \alpha_n \epsilon^n where \hspace{0.25cm} \alpha_n \in \lbrace 0, 2 \rbrace, and is unique to x.

Thus,

f(x) = \sum_{n=1}^{\infty} \alpha_n \gamma^n


Next, we need to construct g(x). These functions are going to be continuous (linear) functions that map open disjoint intervals to corresponding open disjoint intervals.

First, lets define the set \beta_{n,k} as the "binary representation of the integer k with a string of length n". For example: \beta_{5,3} = (0,0,1,0,1) Next, augment \beta with a 1 on the rightmost-side of the string. Call this set \phi_{n,k}. I.e. \phi_{5,3} = (0,0,1,0,1,1) Now, let \phi_{n,k}(i) be the i^{th} element of the string, from the left.
(I.e. \phi_{5,3}(3) = 1.)

Moving forward, we'll use \phi_{n,k} to define the sets \mathcal{O}_{n,k}^\epsilon (and similarly for \mathcal{O}_{n,k}^\gamma) such that: \bigcup_{n = 0}^\infty \bigcup_{k = 0}^{2^n - 1} \mathcal{O}_{n,k}^\epsilon = \hspace{0.25cm} [0,1]\cap \mathcal{C}_\epsilon^c Without further ado, \mathcal{O}_{n,k}^\epsilon = \Bigg( \sum_{i=1}^{n+1} \Big(\frac{(1-\epsilon)^{i-1}(1+\epsilon)}{2^i}\phi_{n,k}(i)\Big)-\epsilon \Big(\frac{(1-\epsilon)}{2}\Big)^n, \sum_{i=1}^{n+1} \Big(\frac{(1-\epsilon)^{i-1}(1+\epsilon)}{2^i}\phi_{n,k}(i)\Big) \Bigg) While the expressions are regrettably unpleasant, notice that for each n,k, \epsilon, it successfully reproduces the correct (k+1)^{th} open interval for the (n+1)^{th} generation of an \epsilon-cantor set. For example, we know that the second interval in the third generation of the standard (\epsilon = \frac{1}{3}) Cantor set is \big(\frac{7}{27}, \frac{8}{27}\big). We can easily check...

First, lets calculate: \sum_{i=1}^{4} \Big(\frac{(\frac{2}{3})^{i-1}(\frac{4}{3})}{2^i}\phi_{3,2}(i)\Big) = \frac{2}{9} + \frac{2}{27} = \frac{8}{27} With that out of the way, it's easy to check now that: \mathcal{O}_{2,1}^{\frac{1}{3}} = \Big(\frac{7}{27}, \frac{8}{27} \Big) So... it works! This was constructed so that the collection of all \mathcal{O}_{n,k}^\epsilon's are pairwise-disjoint. From here, it's actually easy to define a linear function from \mathcal{O}_{n,k}^\epsilon \to \mathcal{O}_{n,k}^\gamma. The slope from one set to the other is (obviously) \big(\frac{\gamma}{\epsilon}\big)^{n+1}. The y-intercept b_{n,k} is fairly straightforward to calculate... (just pick the middle points of the corresponding intervals, and evaluate.)

Thus, we can finally fully define g as follows: g(x) = \sum_{n=0}^{\infty} \sum_{k = 0}^{2^n - 1} \Big(\big(\frac{\gamma}{\epsilon}\big)^{n+1} x + b_{n,k}\Big)\chi_{\mathcal{O}_{n,k}^\epsilon} Finally, we have our function F(x) = f(x) + g(x), which satisfies all three properties outlined in the exercise.

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