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Questions about abstract measure and Lebesgue integral theory. Also concerns such properties as measurability of maps and sets.

2 votes
1 answer
207 views

Expanding Measurable Sets

Let $S,T \subset \mathbb{R}^n$ be measurable sets, and suppose that there exists a measurable bijection $f\colon S\to T$ so that $$ \|f(x)-f(y)\| \;\geq\; \|x-y\| $$ for all $x,y \in S$. Does it foll …
Jim Belk's user avatar
  • 8,483
25 votes

Existence of a strange measure

Just about any form of the axiom of choice can be used to prove this. I like topology, so here's a proof using Tychonoff's theorem. Consider the space $2^{2^\mathbb{N}}$ of all functions $2^{\mathbb …
Jim Belk's user avatar
  • 8,483
21 votes
2 answers
916 views

Codimension of Measurable Sets

I am currently teaching an advanced undergraduate analysis class, and the following question came up. Intuition suggests that "most" subsets of $[0,1]$ are not Lebesgue measurable. However, the powe …
Jim Belk's user avatar
  • 8,483
10 votes
Accepted

Are finitely generated amenable groups positively finitely generated?

The answer is already no for $\mathbb{Z}$, assuming the question is whether this holds for every meaure. Let $n\in\mathbb{N}$, and let $$ S \,=\, \{(a_1,\ldots,a_n)\in\mathbb{Z}^n \mid \gcd(a_1,\ldot …
Jim Belk's user avatar
  • 8,483