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Tuesday, May 1, 2012

Some results in measure theory

Proposition. Let \mu be a positive Borel measure on \mathbb{R} and \epsilon > 0. Then for almost every x, we have \int_\mathbb{R} \frac{d\mu(t)}{|x - t|^{1 + \epsilon}} = +\infty.
Proof: For each positive integer N, define E_N = \{x \in \mathbb{R}: \int_\mathbb{R} d\mu(t) / |x - t|^{1 + \epsilon} \leq N\}. Let I be any bounded closed interval with E_N \cap I nonempty. Pick x \in E_N \cap I and write I = [x - \delta_1, x + \delta_2]. We have N \geq \int_\mathbb{R} \frac{d\mu(t)}{|x - t|^{1 + \epsilon}} \geq \int_I \frac{d\mu(t)}{|x - t|^{1 + \epsilon}} \geq \frac{\mu(I)}{\max\{\delta_1, \delta_2\}^{1 + \epsilon}} \geq \frac{\mu(E_N \cap I)}{(\delta_1 + \delta_2)^{1 + \epsilon}}, i.e. \mu(E_N \cap I) \leq Nm(I)^{1 + \epsilon} where m(I) is the length of the interval I. Now let [a, b] \subseteq \mathbb{R} and n \in \mathbb{N}. Decompose [a, b] into a union of n closed subintervals of length (b - a)/n, then \mu(E_N \cap [a, b]) \leq nN\left(\frac{b - a}{n}\right)^{1 + \epsilon} \rightarrow 0 as n \rightarrow \infty. Hence \mu(E_N \cap [a, b]) = 0. It follows that \mu(E_N) = 0 for all N \in \mathbb{N} and our result follows.

(To be continued)

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