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Crossfader
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1. Homework Statement [/b]
Let [itex]f:ℝ\rightarrowℝ[/itex] be measureable and [itex]A_{k}=\left\{x\inℝ:2^{k-1}<\left|f(x)\right|≤2^{k}\right\}[/itex], [itex]k\in[/itex] [itex]\mathbb{Z}[/itex].
Show that [itex]f[/itex] is integrable only if [itex]\sum^{∞}_{k=-∞}2^{k}m(A_{k}) < ∞ [/itex].
By the definition [itex]f[/itex] is integrable in ℝ if and only if [itex]f[/itex] is measurable and [itex]∫_{ℝ}\left|f\right|<∞[/itex].
Now we know that [itex]f[/itex] is measureable, thus we should show that [itex]∫_{E}\left|f\right|≤ \sum^{∞}_{k=-∞}2^{k}m(A_{k}) < ∞ [/itex].
Let [itex]f = \sum^{∞}_{k=-∞}f_{k}[/itex], we get [itex] \left|f \right| = \left| \sum^{∞}_{k=-∞} f_{k} \right| \Rightarrow \int_{ℝ}\left|f\right| = \int_{ℝ}\left|\sum^{∞}_{k=-∞}f_{k}\right| [/itex]
(?)We also notice that [itex]\int_{ℝ}\left|\sum^{∞}_{k=-∞}f_{k}\right| = \sum^{∞}_{k=-∞}\int_{ℝ}\left|f_{k}\right| = 2 \sum^{∞}_{k=0}\int_{ℝ}\left|f_{k}\right| [/itex] (Beppo Levi's lemma?) (?)
Let [itex]I_{k} = [2^{k-1}, 2^{k}][/itex]. The length of this interval is [itex]\ell(I_{k}) = 2^{k}-2^{k-1}=2^{k}(1-2^{-1}) = (1/2)2^{k}[/itex]
We get [itex]m(A_{k})≤\ell(I_{k}) \Rightarrow \sum^{∞}_{k=-∞}m(A_{k})≤\sum^{∞}_{k=-∞}\ell(I_{k}) = \frac{1}{2}\sum^{∞}_{k=-∞}2^{k}[/itex].
And thereby [itex]2\sum^{∞}_{k=-∞}m(A_{k}) = \sum^{∞}_{k=-∞}2m(A_{k}) ≤ \sum^{∞}_{k=-∞}2^{k}[/itex].
Does my inference make any sense?
Let [itex]f:ℝ\rightarrowℝ[/itex] be measureable and [itex]A_{k}=\left\{x\inℝ:2^{k-1}<\left|f(x)\right|≤2^{k}\right\}[/itex], [itex]k\in[/itex] [itex]\mathbb{Z}[/itex].
Show that [itex]f[/itex] is integrable only if [itex]\sum^{∞}_{k=-∞}2^{k}m(A_{k}) < ∞ [/itex].
Homework Equations
By the definition [itex]f[/itex] is integrable in ℝ if and only if [itex]f[/itex] is measurable and [itex]∫_{ℝ}\left|f\right|<∞[/itex].
Now we know that [itex]f[/itex] is measureable, thus we should show that [itex]∫_{E}\left|f\right|≤ \sum^{∞}_{k=-∞}2^{k}m(A_{k}) < ∞ [/itex].
The Attempt at a Solution
Let [itex]f = \sum^{∞}_{k=-∞}f_{k}[/itex], we get [itex] \left|f \right| = \left| \sum^{∞}_{k=-∞} f_{k} \right| \Rightarrow \int_{ℝ}\left|f\right| = \int_{ℝ}\left|\sum^{∞}_{k=-∞}f_{k}\right| [/itex]
(?)We also notice that [itex]\int_{ℝ}\left|\sum^{∞}_{k=-∞}f_{k}\right| = \sum^{∞}_{k=-∞}\int_{ℝ}\left|f_{k}\right| = 2 \sum^{∞}_{k=0}\int_{ℝ}\left|f_{k}\right| [/itex] (Beppo Levi's lemma?) (?)
Let [itex]I_{k} = [2^{k-1}, 2^{k}][/itex]. The length of this interval is [itex]\ell(I_{k}) = 2^{k}-2^{k-1}=2^{k}(1-2^{-1}) = (1/2)2^{k}[/itex]
We get [itex]m(A_{k})≤\ell(I_{k}) \Rightarrow \sum^{∞}_{k=-∞}m(A_{k})≤\sum^{∞}_{k=-∞}\ell(I_{k}) = \frac{1}{2}\sum^{∞}_{k=-∞}2^{k}[/itex].
And thereby [itex]2\sum^{∞}_{k=-∞}m(A_{k}) = \sum^{∞}_{k=-∞}2m(A_{k}) ≤ \sum^{∞}_{k=-∞}2^{k}[/itex].
Does my inference make any sense?