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10:37 AM
Some short discussion on $2^{\mathbb N}$ in the main chat room.
in Mathematics, 1 hour ago, by BAYMAX
$X = \{(x_{n}) : x_{n} \in \{0,1\} n \in \mathbb{N} \}$
in Mathematics, 1 hour ago, by BAYMAX
Is this set uncountable ?
And this one is related to AC:
in Mathematics, 19 mins ago, by Serge Seredenko
Is it possible to explicitly construct a Hamel basis for R^inf or L_2 or l_2? I mean can I see such a set that spans a big space with finite linear combinations, or is this one of those things that just uselessly "exist" under AC?
I guess there must be a few posts about this on the main.
20
Q: Is there a constructive way to exhibit a basis for $\mathbb{R}^\mathbb{N}$?

F MAssuming the Axiom of Choice, every vector space has a basis, though it can be troublesome to show one explicitly. Is there any constructive way to exhibit a basis for $\mathbb{R}^\mathbb{N}$, the vector space of real sequences?

6
Q: What is the basis of the vector space $l^\infty$?

Prince KhanWe know that every vector space has a Hamel basis and also every normed space need not have a Schauder basis. As the normed space $l^\infty$ is not Separable so can't have the Schauder basis, but on the other side $l^\infty$ is also a vector space so what will be the Hamel basis of $l^\infty$?

13
Q: A Hamel basis for $l^{\,p}$?

Abhishek GuptaI am looking for an explicit example for a Hamel basis for $l^{\,p}$?. As we know that for a Banach space a Hamel basis has either finite or uncountably infinite cardinality and for such a basis one can express any element of the vector space as a finite linear combination of these. After some t...

This answers the question for all infinite-dimensional Banach spaces:
7
A: On every infinite-dimensional Banach space there exists a discontinuous linear functional.

Asaf KaragilaNo. There are models of $\mathsf{ZF+\lnot AC}$ in which every linear transformation from a Banach space to a normed space is automatically continuous. In particular this is true for linear functionals. In such models, it follows, every linear functional has to be continuous. An example for thes...

This seems somewhat related, too:
4
A: Pathological linear functionals and ZF

Asaf KaragilaOkay, here is something that I feel is worth posting. We say that a Banach space is a dream space if every linear operator into a normed space is continuous, in particular every linear functional of a dream space is continuous. This means that if we quotient a dream space by a dense subspace th...

 
$\left|\Bbb R^{\Bbb N}\right|=\left|\Bbb R\right|=2^{\aleph_0}$, correct?
 
@LegionMammal978 Yes.
 
10:53 AM
@MartinSleziak How would I go about proving that $\Bbb R$ and $\Bbb R^{\Bbb N}$ are equinumerous?
 
@LegionMammal978 Exactly the way you did, by calculating their cardinality.
You clearly have $\mathfrak c = |\mathbb R| \le |\mathbb R^{\mathbb N}|$.
 
Yes, there's a surjection
 
In fact, this is probably sufficient: $|\mathbb R^{\mathbb N}| = \mathfrak c^{\aleph_0} = (2^{\aleph_0})^{\aleph_0} = 2^{\aleph_0\cdot \aleph_0} = 2^{\aleph_0}$.
Using some basic rules of cardinal arithmetic (exponentiation) and $\aleph_0\cdot \aleph_0=\aleph_0$.
 
@MartinSleziak And so each of those equalities corresponds to a bijection, correct?
 
Yes.
There is also a post on the main asking for explicit bijection: Bijection from $\mathbb R$ to $\mathbb {R^N}$.
 
10:57 AM
Thanks!
 
But I suppose that for all practical purposes it is (usually) sufficient to know the cardinality (=to know that a bijection exists).
 
yawn Ima go try to understand Schröder–Bernstein
 

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