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Banach spaces, function spaces, real functions, integral transforms, theory of distributions, measure theory.

1 vote
1 answer
171 views

Extension of power bounded operators over a finite subspace

Suppose $Y$ is a Banach space and $X$ is a finite-dimensional subspace of $Y$. Further assume $T:X \rightarrow X$ is a linear operator which is power bounded from above and below, in other words ther …
Jason Rute's user avatar
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4 votes
2 answers
505 views

Martingale-cotype vs cotype on super-reflexive spaces

I'm have difficultly nailing down the direction of some implications. For $2 \leq q < \infty$, there are (at least) two ways to say that a Banach space $B$ has "cotype $q$". $B$ has cotype q. $B$ is …
Jason Rute's user avatar
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3 votes
Accepted

Martingale-cotype vs cotype on super-reflexive spaces

I'll attempt to answer this with what I've found: (1) The answer to my main question is that it is not true. In this document of Pisier's, he states It is possible to find a uniformly convex s …
Jason Rute's user avatar
  • 6,287
5 votes
2 answers
434 views

Sz.-Nagy dilation for uniformly convex Banach spaces

The Sz.-Nagy dilation theorem says that for a Hilbert space $H$ with nonexpansive operator $T$, there is a larger space $H'$ containing $H$ and a unitary operator $U$ on $H'$ such that for all $x \in …
Jason Rute's user avatar
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4 votes
0 answers
309 views

Conditional expectation with respect to random closed sets

Short question If $X$ is a random closed set, and $Y$ is an integrable random variable, I would like a definition of the "conditional expectation" $$\mathbf{E}[Y \mid X\ni x].$$ Has this been worked …
Jason Rute's user avatar
  • 6,287
2 votes
0 answers
169 views

Classify spaces that make extension theorems hold

Recall a Polish space is a completely metrizable separable space. Say a Polish space $Y$ is a terminal space if for any Polish space $X$ and any closed $C \subseteq X$, one can extend a continuous m …
Jason Rute's user avatar
  • 6,287
15 votes
2 answers
3k views

Generalizations of the Tietze extension theorem (and Lusin's theorem)

I am reasking a year-old math.stackexchange.com question asked by someone else. (For my needs every space $X$ and $Y$ will be Polish---that is a completely separably metrizable space.) The Tietze ex …
Jason Rute's user avatar
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5 votes
2 answers
639 views

Is the Hausdorff metric on sub-$\sigma$-fields separable?

Let $(X,\mu,\mathcal{F})$ be a probability space. The paper Equiconvergence of Martingales by Edward Boylan introduced a pseudometric on sub-$\sigma$-fields (sub-$\sigma$-algebras) of $\mathcal{F}$ a …
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9 votes
2 answers
535 views

What mode of convergence is this?

I'm interested in a new (to me) mode of convergence which is stronger than convergence in measure/probability. I want to know if it has a name and if it is used much in the literature. I will write …
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8 votes
0 answers
382 views

What is the name for a Banach space property closed under ultraproducts?

In Banach space theory, a super-property is a property of a Banach space that is preserved under ultrapowers. (Update (2015-09-28): The property must also be closed under isometric embeddings.) (Sup …
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13 votes
1 answer
3k views

Does this metric have an official name? Lévy metric? Ky Fan metric?

Let $X$ and $Y$ be random variables taking values in a separable metric space $(S,d)$. The metric I have in mind is $$\rho(X,Y) = \mathbb{E}[\min\{d(X,Y),1\}]$$ if $X$ and $Y$ take values in the a me …
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