Does a random sequence of vectors span a Hilbert space? Planned maintenance scheduled April...
Does a random sequence of vectors span a Hilbert space?
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Let $mathcal{H}$ be a separable Hilbert space. Let $v$ be a random variable taking values in $mathcal{H}$ such that $P(v perp h) < 1$ for all $h in mathcal{H}.$ Suppose we sample an infinite sequence $v_1, v_2, ldots.$ Is it the case that, almost surely, the closed span of $v_1, v_2, ldots$ is all of $mathcal{H}?$
reference-request fa.functional-analysis pr.probability operator-theory hilbert-spaces
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|
show 4 more comments
$begingroup$
Let $mathcal{H}$ be a separable Hilbert space. Let $v$ be a random variable taking values in $mathcal{H}$ such that $P(v perp h) < 1$ for all $h in mathcal{H}.$ Suppose we sample an infinite sequence $v_1, v_2, ldots.$ Is it the case that, almost surely, the closed span of $v_1, v_2, ldots$ is all of $mathcal{H}?$
reference-request fa.functional-analysis pr.probability operator-theory hilbert-spaces
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$begingroup$
and if they are iid the probability of being in a closed hyperplane $(h)^perp$ is $P(v_kperp h, k=1,2,dots)=0$
$endgroup$
– Pietro Majer
8 hours ago
3
$begingroup$
@Pietro Majer: this is the probability that the vectors all lie in a given closed hyperplane.
$endgroup$
– Anthony Quas
8 hours ago
2
$begingroup$
Whoever voted to close this, I'm pretty sure you don't understand the question. This is subtle and interesting.
$endgroup$
– Anthony Quas
8 hours ago
1
$begingroup$
Maybe I'm missing something obvious, but is it even clear that the event you are interested in is measurable?
$endgroup$
– Jochen Glueck
8 hours ago
1
$begingroup$
@JochenGlueck: Yes it's measurable: Let $(y_n)$ be a dense sequence in $mathcal H$. Then the event is: for all $m>0$, for all $n>0$, there exist $k>0$ and rational $t_1,ldots,t_k$ such that $|t_1v_1+ldots+t_kv_k-y_n|<1/m$.
$endgroup$
– Anthony Quas
6 hours ago
|
show 4 more comments
$begingroup$
Let $mathcal{H}$ be a separable Hilbert space. Let $v$ be a random variable taking values in $mathcal{H}$ such that $P(v perp h) < 1$ for all $h in mathcal{H}.$ Suppose we sample an infinite sequence $v_1, v_2, ldots.$ Is it the case that, almost surely, the closed span of $v_1, v_2, ldots$ is all of $mathcal{H}?$
reference-request fa.functional-analysis pr.probability operator-theory hilbert-spaces
$endgroup$
Let $mathcal{H}$ be a separable Hilbert space. Let $v$ be a random variable taking values in $mathcal{H}$ such that $P(v perp h) < 1$ for all $h in mathcal{H}.$ Suppose we sample an infinite sequence $v_1, v_2, ldots.$ Is it the case that, almost surely, the closed span of $v_1, v_2, ldots$ is all of $mathcal{H}?$
reference-request fa.functional-analysis pr.probability operator-theory hilbert-spaces
reference-request fa.functional-analysis pr.probability operator-theory hilbert-spaces
asked 9 hours ago
J. E. PascoeJ. E. Pascoe
570316
570316
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and if they are iid the probability of being in a closed hyperplane $(h)^perp$ is $P(v_kperp h, k=1,2,dots)=0$
$endgroup$
– Pietro Majer
8 hours ago
3
$begingroup$
@Pietro Majer: this is the probability that the vectors all lie in a given closed hyperplane.
$endgroup$
– Anthony Quas
8 hours ago
2
$begingroup$
Whoever voted to close this, I'm pretty sure you don't understand the question. This is subtle and interesting.
$endgroup$
– Anthony Quas
8 hours ago
1
$begingroup$
Maybe I'm missing something obvious, but is it even clear that the event you are interested in is measurable?
$endgroup$
– Jochen Glueck
8 hours ago
1
$begingroup$
@JochenGlueck: Yes it's measurable: Let $(y_n)$ be a dense sequence in $mathcal H$. Then the event is: for all $m>0$, for all $n>0$, there exist $k>0$ and rational $t_1,ldots,t_k$ such that $|t_1v_1+ldots+t_kv_k-y_n|<1/m$.
$endgroup$
– Anthony Quas
6 hours ago
|
show 4 more comments
$begingroup$
and if they are iid the probability of being in a closed hyperplane $(h)^perp$ is $P(v_kperp h, k=1,2,dots)=0$
$endgroup$
– Pietro Majer
8 hours ago
3
$begingroup$
@Pietro Majer: this is the probability that the vectors all lie in a given closed hyperplane.
$endgroup$
– Anthony Quas
8 hours ago
2
$begingroup$
Whoever voted to close this, I'm pretty sure you don't understand the question. This is subtle and interesting.
$endgroup$
– Anthony Quas
8 hours ago
1
$begingroup$
Maybe I'm missing something obvious, but is it even clear that the event you are interested in is measurable?
$endgroup$
– Jochen Glueck
8 hours ago
1
$begingroup$
@JochenGlueck: Yes it's measurable: Let $(y_n)$ be a dense sequence in $mathcal H$. Then the event is: for all $m>0$, for all $n>0$, there exist $k>0$ and rational $t_1,ldots,t_k$ such that $|t_1v_1+ldots+t_kv_k-y_n|<1/m$.
$endgroup$
– Anthony Quas
6 hours ago
$begingroup$
and if they are iid the probability of being in a closed hyperplane $(h)^perp$ is $P(v_kperp h, k=1,2,dots)=0$
$endgroup$
– Pietro Majer
8 hours ago
$begingroup$
and if they are iid the probability of being in a closed hyperplane $(h)^perp$ is $P(v_kperp h, k=1,2,dots)=0$
$endgroup$
– Pietro Majer
8 hours ago
3
3
$begingroup$
@Pietro Majer: this is the probability that the vectors all lie in a given closed hyperplane.
$endgroup$
– Anthony Quas
8 hours ago
$begingroup$
@Pietro Majer: this is the probability that the vectors all lie in a given closed hyperplane.
$endgroup$
– Anthony Quas
8 hours ago
2
2
$begingroup$
Whoever voted to close this, I'm pretty sure you don't understand the question. This is subtle and interesting.
$endgroup$
– Anthony Quas
8 hours ago
$begingroup$
Whoever voted to close this, I'm pretty sure you don't understand the question. This is subtle and interesting.
$endgroup$
– Anthony Quas
8 hours ago
1
1
$begingroup$
Maybe I'm missing something obvious, but is it even clear that the event you are interested in is measurable?
$endgroup$
– Jochen Glueck
8 hours ago
$begingroup$
Maybe I'm missing something obvious, but is it even clear that the event you are interested in is measurable?
$endgroup$
– Jochen Glueck
8 hours ago
1
1
$begingroup$
@JochenGlueck: Yes it's measurable: Let $(y_n)$ be a dense sequence in $mathcal H$. Then the event is: for all $m>0$, for all $n>0$, there exist $k>0$ and rational $t_1,ldots,t_k$ such that $|t_1v_1+ldots+t_kv_k-y_n|<1/m$.
$endgroup$
– Anthony Quas
6 hours ago
$begingroup$
@JochenGlueck: Yes it's measurable: Let $(y_n)$ be a dense sequence in $mathcal H$. Then the event is: for all $m>0$, for all $n>0$, there exist $k>0$ and rational $t_1,ldots,t_k$ such that $|t_1v_1+ldots+t_kv_k-y_n|<1/m$.
$endgroup$
– Anthony Quas
6 hours ago
|
show 4 more comments
2 Answers
2
active
oldest
votes
$begingroup$
(This may turn out to be a simplified version of J. E. Pascoe's answer).
With probability one, the closure of the random set $V = {v_1, v_2, ldots}$ is equal to $operatorname{supp} v$, the support of the distribution of $v$ ($operatorname{supp} v$ is the set of those $h$ such that $P(v in B(h, varepsilon)) > 0$ for every $varepsilon > 0$).
For every $h$, we have $P(h perp v) < 1$, and therefore $h$ is not orthogonal to $operatorname{supp} v$. It follows that the closed span of $operatorname{supp} v$ is $mathcal{H}$.
It remains to note that the closed span of $V$ is the same as the closed span of the closure of $V$, equal to $operatorname{supp} v$ with probability one.
$endgroup$
add a comment |
$begingroup$
Another Try
We say a $mathcal{H}$-valued random variable $h$ is a random vector if $P(h perp g)<1$ for all $gin mathcal{H}.$
If $h_1, h_2, ldots$ is a sequence independent identically distributed of random vectors,
then, almost surely, the closed span of the $h_i$ is equal to $mathcal{H}.$
First we will need a lemma.
Lemma 1
Let $h$ be a random vector.
There is a countable subset $A$ of $mathcal{H}$ such that the closed span of the elements of $A$ is equal to $mathcal{H}$
and for every point $ain A,$ $P(hin U)>0$ for any neighborhood $U$ of $a.$
Proof
For any subset $A$ such that for every point $ain A,$ $P(hin U)>0$ for any neighborhood $U$ of $a,$ and
the closed span of the elements of $A$ is not equal to $mathcal{H},$
we will show that we can grow $A$ by a single element which is not in closed span of the elements of $A.$
We can only do this a countable number of times because the Hilbert space dimension of $mathcal{H}$ is countable.
(Otherwise, via Gram-Schmidt, we could construct an uncountable orthonormal set by transfinite induction.)
Choose $g$ such that $g perp a$ for all $ain A.$ Now, $P(h perp g)<1.$ So there must be a point $b$ such that
$P(hin U) >0$ for every neighborhood of $b$ and $b$ is not perpendicular to $g,$ therefore, $b$ is not in the span of the elements of $A.$ QED
Suppose $h_1, h_2, ldots$ is a sequence independent identically distributed of random vectors.
Let $A$ be as in Lemma 1. Index $A$ a a sequence $a_n.$
Let $B_{m,n}$ be a ball of radius $1/m$ centered at $a_n$
Almost surely, the sequence $h_i$ must visit $B_{m,n}$ infinitely often,
as $P(h_iin B_{m,n})>0$. Therefore $A$ is a subset of the closure of the values of the sequence. (We have essentially the fact that a random function $f:mathbb{N}rightarrow mathbb{N}^2$ is surjective with infinite multiplicity.)
$endgroup$
$begingroup$
The point is that you "keep going" by transfinite induction. The process must stop at some countable ordinal before $omega_1$ as the space is countable dimensional.
$endgroup$
– J. E. Pascoe
6 hours ago
$begingroup$
That is for each $alpha < omega_1$ there would be $A_alpha$ such that if $alpha < beta,$ $A_alpha^perp cap A_beta neq {0}.$
$endgroup$
– J. E. Pascoe
5 hours ago
$begingroup$
"Let $A$ be as in Lemma 1" ... "we may take $A$ to consist of only isolated points, as in a Polish space a countable closed set is equal to the closure of its isolated points." But Lemma 1 does not guarantee that $A$ is closed.
$endgroup$
– Iosif Pinelis
5 hours ago
$begingroup$
That does seem to be a gap @IosifPinelis . Ideas for closing it?
$endgroup$
– J. E. Pascoe
5 hours ago
$begingroup$
Could you elaborate a bit further on why the set $A$ in Lemma 1 is countable? Of course, there exist linearly independent sets in $H$ that are uncountable -- just choose your favourite Hamel basis of $H$.
$endgroup$
– Jochen Glueck
5 hours ago
|
show 8 more comments
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
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$begingroup$
(This may turn out to be a simplified version of J. E. Pascoe's answer).
With probability one, the closure of the random set $V = {v_1, v_2, ldots}$ is equal to $operatorname{supp} v$, the support of the distribution of $v$ ($operatorname{supp} v$ is the set of those $h$ such that $P(v in B(h, varepsilon)) > 0$ for every $varepsilon > 0$).
For every $h$, we have $P(h perp v) < 1$, and therefore $h$ is not orthogonal to $operatorname{supp} v$. It follows that the closed span of $operatorname{supp} v$ is $mathcal{H}$.
It remains to note that the closed span of $V$ is the same as the closed span of the closure of $V$, equal to $operatorname{supp} v$ with probability one.
$endgroup$
add a comment |
$begingroup$
(This may turn out to be a simplified version of J. E. Pascoe's answer).
With probability one, the closure of the random set $V = {v_1, v_2, ldots}$ is equal to $operatorname{supp} v$, the support of the distribution of $v$ ($operatorname{supp} v$ is the set of those $h$ such that $P(v in B(h, varepsilon)) > 0$ for every $varepsilon > 0$).
For every $h$, we have $P(h perp v) < 1$, and therefore $h$ is not orthogonal to $operatorname{supp} v$. It follows that the closed span of $operatorname{supp} v$ is $mathcal{H}$.
It remains to note that the closed span of $V$ is the same as the closed span of the closure of $V$, equal to $operatorname{supp} v$ with probability one.
$endgroup$
add a comment |
$begingroup$
(This may turn out to be a simplified version of J. E. Pascoe's answer).
With probability one, the closure of the random set $V = {v_1, v_2, ldots}$ is equal to $operatorname{supp} v$, the support of the distribution of $v$ ($operatorname{supp} v$ is the set of those $h$ such that $P(v in B(h, varepsilon)) > 0$ for every $varepsilon > 0$).
For every $h$, we have $P(h perp v) < 1$, and therefore $h$ is not orthogonal to $operatorname{supp} v$. It follows that the closed span of $operatorname{supp} v$ is $mathcal{H}$.
It remains to note that the closed span of $V$ is the same as the closed span of the closure of $V$, equal to $operatorname{supp} v$ with probability one.
$endgroup$
(This may turn out to be a simplified version of J. E. Pascoe's answer).
With probability one, the closure of the random set $V = {v_1, v_2, ldots}$ is equal to $operatorname{supp} v$, the support of the distribution of $v$ ($operatorname{supp} v$ is the set of those $h$ such that $P(v in B(h, varepsilon)) > 0$ for every $varepsilon > 0$).
For every $h$, we have $P(h perp v) < 1$, and therefore $h$ is not orthogonal to $operatorname{supp} v$. It follows that the closed span of $operatorname{supp} v$ is $mathcal{H}$.
It remains to note that the closed span of $V$ is the same as the closed span of the closure of $V$, equal to $operatorname{supp} v$ with probability one.
answered 4 hours ago
Mateusz KwaśnickiMateusz Kwaśnicki
4,7421619
4,7421619
add a comment |
add a comment |
$begingroup$
Another Try
We say a $mathcal{H}$-valued random variable $h$ is a random vector if $P(h perp g)<1$ for all $gin mathcal{H}.$
If $h_1, h_2, ldots$ is a sequence independent identically distributed of random vectors,
then, almost surely, the closed span of the $h_i$ is equal to $mathcal{H}.$
First we will need a lemma.
Lemma 1
Let $h$ be a random vector.
There is a countable subset $A$ of $mathcal{H}$ such that the closed span of the elements of $A$ is equal to $mathcal{H}$
and for every point $ain A,$ $P(hin U)>0$ for any neighborhood $U$ of $a.$
Proof
For any subset $A$ such that for every point $ain A,$ $P(hin U)>0$ for any neighborhood $U$ of $a,$ and
the closed span of the elements of $A$ is not equal to $mathcal{H},$
we will show that we can grow $A$ by a single element which is not in closed span of the elements of $A.$
We can only do this a countable number of times because the Hilbert space dimension of $mathcal{H}$ is countable.
(Otherwise, via Gram-Schmidt, we could construct an uncountable orthonormal set by transfinite induction.)
Choose $g$ such that $g perp a$ for all $ain A.$ Now, $P(h perp g)<1.$ So there must be a point $b$ such that
$P(hin U) >0$ for every neighborhood of $b$ and $b$ is not perpendicular to $g,$ therefore, $b$ is not in the span of the elements of $A.$ QED
Suppose $h_1, h_2, ldots$ is a sequence independent identically distributed of random vectors.
Let $A$ be as in Lemma 1. Index $A$ a a sequence $a_n.$
Let $B_{m,n}$ be a ball of radius $1/m$ centered at $a_n$
Almost surely, the sequence $h_i$ must visit $B_{m,n}$ infinitely often,
as $P(h_iin B_{m,n})>0$. Therefore $A$ is a subset of the closure of the values of the sequence. (We have essentially the fact that a random function $f:mathbb{N}rightarrow mathbb{N}^2$ is surjective with infinite multiplicity.)
$endgroup$
$begingroup$
The point is that you "keep going" by transfinite induction. The process must stop at some countable ordinal before $omega_1$ as the space is countable dimensional.
$endgroup$
– J. E. Pascoe
6 hours ago
$begingroup$
That is for each $alpha < omega_1$ there would be $A_alpha$ such that if $alpha < beta,$ $A_alpha^perp cap A_beta neq {0}.$
$endgroup$
– J. E. Pascoe
5 hours ago
$begingroup$
"Let $A$ be as in Lemma 1" ... "we may take $A$ to consist of only isolated points, as in a Polish space a countable closed set is equal to the closure of its isolated points." But Lemma 1 does not guarantee that $A$ is closed.
$endgroup$
– Iosif Pinelis
5 hours ago
$begingroup$
That does seem to be a gap @IosifPinelis . Ideas for closing it?
$endgroup$
– J. E. Pascoe
5 hours ago
$begingroup$
Could you elaborate a bit further on why the set $A$ in Lemma 1 is countable? Of course, there exist linearly independent sets in $H$ that are uncountable -- just choose your favourite Hamel basis of $H$.
$endgroup$
– Jochen Glueck
5 hours ago
|
show 8 more comments
$begingroup$
Another Try
We say a $mathcal{H}$-valued random variable $h$ is a random vector if $P(h perp g)<1$ for all $gin mathcal{H}.$
If $h_1, h_2, ldots$ is a sequence independent identically distributed of random vectors,
then, almost surely, the closed span of the $h_i$ is equal to $mathcal{H}.$
First we will need a lemma.
Lemma 1
Let $h$ be a random vector.
There is a countable subset $A$ of $mathcal{H}$ such that the closed span of the elements of $A$ is equal to $mathcal{H}$
and for every point $ain A,$ $P(hin U)>0$ for any neighborhood $U$ of $a.$
Proof
For any subset $A$ such that for every point $ain A,$ $P(hin U)>0$ for any neighborhood $U$ of $a,$ and
the closed span of the elements of $A$ is not equal to $mathcal{H},$
we will show that we can grow $A$ by a single element which is not in closed span of the elements of $A.$
We can only do this a countable number of times because the Hilbert space dimension of $mathcal{H}$ is countable.
(Otherwise, via Gram-Schmidt, we could construct an uncountable orthonormal set by transfinite induction.)
Choose $g$ such that $g perp a$ for all $ain A.$ Now, $P(h perp g)<1.$ So there must be a point $b$ such that
$P(hin U) >0$ for every neighborhood of $b$ and $b$ is not perpendicular to $g,$ therefore, $b$ is not in the span of the elements of $A.$ QED
Suppose $h_1, h_2, ldots$ is a sequence independent identically distributed of random vectors.
Let $A$ be as in Lemma 1. Index $A$ a a sequence $a_n.$
Let $B_{m,n}$ be a ball of radius $1/m$ centered at $a_n$
Almost surely, the sequence $h_i$ must visit $B_{m,n}$ infinitely often,
as $P(h_iin B_{m,n})>0$. Therefore $A$ is a subset of the closure of the values of the sequence. (We have essentially the fact that a random function $f:mathbb{N}rightarrow mathbb{N}^2$ is surjective with infinite multiplicity.)
$endgroup$
$begingroup$
The point is that you "keep going" by transfinite induction. The process must stop at some countable ordinal before $omega_1$ as the space is countable dimensional.
$endgroup$
– J. E. Pascoe
6 hours ago
$begingroup$
That is for each $alpha < omega_1$ there would be $A_alpha$ such that if $alpha < beta,$ $A_alpha^perp cap A_beta neq {0}.$
$endgroup$
– J. E. Pascoe
5 hours ago
$begingroup$
"Let $A$ be as in Lemma 1" ... "we may take $A$ to consist of only isolated points, as in a Polish space a countable closed set is equal to the closure of its isolated points." But Lemma 1 does not guarantee that $A$ is closed.
$endgroup$
– Iosif Pinelis
5 hours ago
$begingroup$
That does seem to be a gap @IosifPinelis . Ideas for closing it?
$endgroup$
– J. E. Pascoe
5 hours ago
$begingroup$
Could you elaborate a bit further on why the set $A$ in Lemma 1 is countable? Of course, there exist linearly independent sets in $H$ that are uncountable -- just choose your favourite Hamel basis of $H$.
$endgroup$
– Jochen Glueck
5 hours ago
|
show 8 more comments
$begingroup$
Another Try
We say a $mathcal{H}$-valued random variable $h$ is a random vector if $P(h perp g)<1$ for all $gin mathcal{H}.$
If $h_1, h_2, ldots$ is a sequence independent identically distributed of random vectors,
then, almost surely, the closed span of the $h_i$ is equal to $mathcal{H}.$
First we will need a lemma.
Lemma 1
Let $h$ be a random vector.
There is a countable subset $A$ of $mathcal{H}$ such that the closed span of the elements of $A$ is equal to $mathcal{H}$
and for every point $ain A,$ $P(hin U)>0$ for any neighborhood $U$ of $a.$
Proof
For any subset $A$ such that for every point $ain A,$ $P(hin U)>0$ for any neighborhood $U$ of $a,$ and
the closed span of the elements of $A$ is not equal to $mathcal{H},$
we will show that we can grow $A$ by a single element which is not in closed span of the elements of $A.$
We can only do this a countable number of times because the Hilbert space dimension of $mathcal{H}$ is countable.
(Otherwise, via Gram-Schmidt, we could construct an uncountable orthonormal set by transfinite induction.)
Choose $g$ such that $g perp a$ for all $ain A.$ Now, $P(h perp g)<1.$ So there must be a point $b$ such that
$P(hin U) >0$ for every neighborhood of $b$ and $b$ is not perpendicular to $g,$ therefore, $b$ is not in the span of the elements of $A.$ QED
Suppose $h_1, h_2, ldots$ is a sequence independent identically distributed of random vectors.
Let $A$ be as in Lemma 1. Index $A$ a a sequence $a_n.$
Let $B_{m,n}$ be a ball of radius $1/m$ centered at $a_n$
Almost surely, the sequence $h_i$ must visit $B_{m,n}$ infinitely often,
as $P(h_iin B_{m,n})>0$. Therefore $A$ is a subset of the closure of the values of the sequence. (We have essentially the fact that a random function $f:mathbb{N}rightarrow mathbb{N}^2$ is surjective with infinite multiplicity.)
$endgroup$
Another Try
We say a $mathcal{H}$-valued random variable $h$ is a random vector if $P(h perp g)<1$ for all $gin mathcal{H}.$
If $h_1, h_2, ldots$ is a sequence independent identically distributed of random vectors,
then, almost surely, the closed span of the $h_i$ is equal to $mathcal{H}.$
First we will need a lemma.
Lemma 1
Let $h$ be a random vector.
There is a countable subset $A$ of $mathcal{H}$ such that the closed span of the elements of $A$ is equal to $mathcal{H}$
and for every point $ain A,$ $P(hin U)>0$ for any neighborhood $U$ of $a.$
Proof
For any subset $A$ such that for every point $ain A,$ $P(hin U)>0$ for any neighborhood $U$ of $a,$ and
the closed span of the elements of $A$ is not equal to $mathcal{H},$
we will show that we can grow $A$ by a single element which is not in closed span of the elements of $A.$
We can only do this a countable number of times because the Hilbert space dimension of $mathcal{H}$ is countable.
(Otherwise, via Gram-Schmidt, we could construct an uncountable orthonormal set by transfinite induction.)
Choose $g$ such that $g perp a$ for all $ain A.$ Now, $P(h perp g)<1.$ So there must be a point $b$ such that
$P(hin U) >0$ for every neighborhood of $b$ and $b$ is not perpendicular to $g,$ therefore, $b$ is not in the span of the elements of $A.$ QED
Suppose $h_1, h_2, ldots$ is a sequence independent identically distributed of random vectors.
Let $A$ be as in Lemma 1. Index $A$ a a sequence $a_n.$
Let $B_{m,n}$ be a ball of radius $1/m$ centered at $a_n$
Almost surely, the sequence $h_i$ must visit $B_{m,n}$ infinitely often,
as $P(h_iin B_{m,n})>0$. Therefore $A$ is a subset of the closure of the values of the sequence. (We have essentially the fact that a random function $f:mathbb{N}rightarrow mathbb{N}^2$ is surjective with infinite multiplicity.)
edited 4 hours ago
answered 6 hours ago
J. E. PascoeJ. E. Pascoe
570316
570316
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The point is that you "keep going" by transfinite induction. The process must stop at some countable ordinal before $omega_1$ as the space is countable dimensional.
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– J. E. Pascoe
6 hours ago
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That is for each $alpha < omega_1$ there would be $A_alpha$ such that if $alpha < beta,$ $A_alpha^perp cap A_beta neq {0}.$
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– J. E. Pascoe
5 hours ago
$begingroup$
"Let $A$ be as in Lemma 1" ... "we may take $A$ to consist of only isolated points, as in a Polish space a countable closed set is equal to the closure of its isolated points." But Lemma 1 does not guarantee that $A$ is closed.
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– Iosif Pinelis
5 hours ago
$begingroup$
That does seem to be a gap @IosifPinelis . Ideas for closing it?
$endgroup$
– J. E. Pascoe
5 hours ago
$begingroup$
Could you elaborate a bit further on why the set $A$ in Lemma 1 is countable? Of course, there exist linearly independent sets in $H$ that are uncountable -- just choose your favourite Hamel basis of $H$.
$endgroup$
– Jochen Glueck
5 hours ago
|
show 8 more comments
$begingroup$
The point is that you "keep going" by transfinite induction. The process must stop at some countable ordinal before $omega_1$ as the space is countable dimensional.
$endgroup$
– J. E. Pascoe
6 hours ago
$begingroup$
That is for each $alpha < omega_1$ there would be $A_alpha$ such that if $alpha < beta,$ $A_alpha^perp cap A_beta neq {0}.$
$endgroup$
– J. E. Pascoe
5 hours ago
$begingroup$
"Let $A$ be as in Lemma 1" ... "we may take $A$ to consist of only isolated points, as in a Polish space a countable closed set is equal to the closure of its isolated points." But Lemma 1 does not guarantee that $A$ is closed.
$endgroup$
– Iosif Pinelis
5 hours ago
$begingroup$
That does seem to be a gap @IosifPinelis . Ideas for closing it?
$endgroup$
– J. E. Pascoe
5 hours ago
$begingroup$
Could you elaborate a bit further on why the set $A$ in Lemma 1 is countable? Of course, there exist linearly independent sets in $H$ that are uncountable -- just choose your favourite Hamel basis of $H$.
$endgroup$
– Jochen Glueck
5 hours ago
$begingroup$
The point is that you "keep going" by transfinite induction. The process must stop at some countable ordinal before $omega_1$ as the space is countable dimensional.
$endgroup$
– J. E. Pascoe
6 hours ago
$begingroup$
The point is that you "keep going" by transfinite induction. The process must stop at some countable ordinal before $omega_1$ as the space is countable dimensional.
$endgroup$
– J. E. Pascoe
6 hours ago
$begingroup$
That is for each $alpha < omega_1$ there would be $A_alpha$ such that if $alpha < beta,$ $A_alpha^perp cap A_beta neq {0}.$
$endgroup$
– J. E. Pascoe
5 hours ago
$begingroup$
That is for each $alpha < omega_1$ there would be $A_alpha$ such that if $alpha < beta,$ $A_alpha^perp cap A_beta neq {0}.$
$endgroup$
– J. E. Pascoe
5 hours ago
$begingroup$
"Let $A$ be as in Lemma 1" ... "we may take $A$ to consist of only isolated points, as in a Polish space a countable closed set is equal to the closure of its isolated points." But Lemma 1 does not guarantee that $A$ is closed.
$endgroup$
– Iosif Pinelis
5 hours ago
$begingroup$
"Let $A$ be as in Lemma 1" ... "we may take $A$ to consist of only isolated points, as in a Polish space a countable closed set is equal to the closure of its isolated points." But Lemma 1 does not guarantee that $A$ is closed.
$endgroup$
– Iosif Pinelis
5 hours ago
$begingroup$
That does seem to be a gap @IosifPinelis . Ideas for closing it?
$endgroup$
– J. E. Pascoe
5 hours ago
$begingroup$
That does seem to be a gap @IosifPinelis . Ideas for closing it?
$endgroup$
– J. E. Pascoe
5 hours ago
$begingroup$
Could you elaborate a bit further on why the set $A$ in Lemma 1 is countable? Of course, there exist linearly independent sets in $H$ that are uncountable -- just choose your favourite Hamel basis of $H$.
$endgroup$
– Jochen Glueck
5 hours ago
$begingroup$
Could you elaborate a bit further on why the set $A$ in Lemma 1 is countable? Of course, there exist linearly independent sets in $H$ that are uncountable -- just choose your favourite Hamel basis of $H$.
$endgroup$
– Jochen Glueck
5 hours ago
|
show 8 more comments
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$begingroup$
and if they are iid the probability of being in a closed hyperplane $(h)^perp$ is $P(v_kperp h, k=1,2,dots)=0$
$endgroup$
– Pietro Majer
8 hours ago
3
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@Pietro Majer: this is the probability that the vectors all lie in a given closed hyperplane.
$endgroup$
– Anthony Quas
8 hours ago
2
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Whoever voted to close this, I'm pretty sure you don't understand the question. This is subtle and interesting.
$endgroup$
– Anthony Quas
8 hours ago
1
$begingroup$
Maybe I'm missing something obvious, but is it even clear that the event you are interested in is measurable?
$endgroup$
– Jochen Glueck
8 hours ago
1
$begingroup$
@JochenGlueck: Yes it's measurable: Let $(y_n)$ be a dense sequence in $mathcal H$. Then the event is: for all $m>0$, for all $n>0$, there exist $k>0$ and rational $t_1,ldots,t_k$ such that $|t_1v_1+ldots+t_kv_k-y_n|<1/m$.
$endgroup$
– Anthony Quas
6 hours ago