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Notes on a few abnormalities in analysis (2022)

39 点作者 dante44超过 1 年前

3 条评论

fiforpg超过 1 年前
Gelbaum-Olmsted <i>Counterexamples in analysis</i> is a standard reference for various pathological examples in analysis:<p><a href="https:&#x2F;&#x2F;archive.org&#x2F;details&#x2F;counterexamplesi0000gelb" rel="nofollow">https:&#x2F;&#x2F;archive.org&#x2F;details&#x2F;counterexamplesi0000gelb</a>
xyzzyz超过 1 年前
Cantor&#x27;s set always messes with my mind.<p>Say you start with the standard construction: take closed interval [0, 1], then remove open middle third, then remove two open middle thirds from two remaining segments, then remove 4 open middle thirds from 4 remaining segments, etc countably many times (more formally, intersect all of the sets you obtain in each of the step).<p>Intuitively, what remains should be the end points of the removed intervals, but this is not true: since at every step you removed finitely many intervals, and there were countably many steps, you removed finite<i>countable &lt;= countable</i>countable = countably many intervals, so there are only countably many many ends of removed intervals, but the Cantor&#x27;s set is uncountable. So where do the remaining points come from?
blt超过 1 年前
&gt; <i>Things fall apart when the space is not compact. Your sequences may converge but not to an element in your space.</i><p>This is about closedness only. ℝ is not compact but contains its limit points.
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