Open linear subspace of a Hilbert space.
$begingroup$
Does there exist any open linear (vector) subspace of a Hilbert space? I could not think of any example.
Actually, I was reading the book by Simmons, there almost in every theorem it assumed that "If M is a closed linear subspace".It seemed natural to me to think about subspaces which are not closed. I have an got an example which is not closed:
Take the Hilbert space H = C[0,1], with L^2 norm and the subspace set of all polynomials, it is not closed because it's closure is H and not open can be found here Set of all polynomials on [0, 1/2] is not open in C[0, 1/2]. Then I asked myself an example of to think of an open set. But I could lead myself nowhere, as I am not familiar with infinite dimensional vector space. Not closed does not necessarily mean open.
functional-analysis
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|
show 9 more comments
$begingroup$
Does there exist any open linear (vector) subspace of a Hilbert space? I could not think of any example.
Actually, I was reading the book by Simmons, there almost in every theorem it assumed that "If M is a closed linear subspace".It seemed natural to me to think about subspaces which are not closed. I have an got an example which is not closed:
Take the Hilbert space H = C[0,1], with L^2 norm and the subspace set of all polynomials, it is not closed because it's closure is H and not open can be found here Set of all polynomials on [0, 1/2] is not open in C[0, 1/2]. Then I asked myself an example of to think of an open set. But I could lead myself nowhere, as I am not familiar with infinite dimensional vector space. Not closed does not necessarily mean open.
functional-analysis
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3
$begingroup$
Linear span of a ball = ?
$endgroup$
– metamorphy
Jan 2 at 5:08
1
$begingroup$
@metamorphy If you take a ball in R^n, and take an open ball around a point. then it contains at n linearly independent vectors, span of those vectors give you whole R^n. We know whole space is an open set but my point was in asking a proper subspace. Or what else you wanted to say?
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– epsilon_delta
Jan 2 at 5:18
1
$begingroup$
I'm telling you that there's no proper subspace. A nonempty open set contains some ball, the linear span of which is the whole space (this is true for any dimensions, including infinite).
$endgroup$
– metamorphy
Jan 2 at 5:21
2
$begingroup$
Sorry, please disregard my close vote; I misread the question.
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– angryavian
Jan 2 at 5:27
1
$begingroup$
Nice question. Glad you didn't give up on it. :) +1.
$endgroup$
– DisintegratingByParts
Jan 2 at 6:02
|
show 9 more comments
$begingroup$
Does there exist any open linear (vector) subspace of a Hilbert space? I could not think of any example.
Actually, I was reading the book by Simmons, there almost in every theorem it assumed that "If M is a closed linear subspace".It seemed natural to me to think about subspaces which are not closed. I have an got an example which is not closed:
Take the Hilbert space H = C[0,1], with L^2 norm and the subspace set of all polynomials, it is not closed because it's closure is H and not open can be found here Set of all polynomials on [0, 1/2] is not open in C[0, 1/2]. Then I asked myself an example of to think of an open set. But I could lead myself nowhere, as I am not familiar with infinite dimensional vector space. Not closed does not necessarily mean open.
functional-analysis
$endgroup$
Does there exist any open linear (vector) subspace of a Hilbert space? I could not think of any example.
Actually, I was reading the book by Simmons, there almost in every theorem it assumed that "If M is a closed linear subspace".It seemed natural to me to think about subspaces which are not closed. I have an got an example which is not closed:
Take the Hilbert space H = C[0,1], with L^2 norm and the subspace set of all polynomials, it is not closed because it's closure is H and not open can be found here Set of all polynomials on [0, 1/2] is not open in C[0, 1/2]. Then I asked myself an example of to think of an open set. But I could lead myself nowhere, as I am not familiar with infinite dimensional vector space. Not closed does not necessarily mean open.
functional-analysis
functional-analysis
edited Jan 2 at 20:40
epsilon_delta
asked Jan 2 at 5:02
epsilon_deltaepsilon_delta
286
286
3
$begingroup$
Linear span of a ball = ?
$endgroup$
– metamorphy
Jan 2 at 5:08
1
$begingroup$
@metamorphy If you take a ball in R^n, and take an open ball around a point. then it contains at n linearly independent vectors, span of those vectors give you whole R^n. We know whole space is an open set but my point was in asking a proper subspace. Or what else you wanted to say?
$endgroup$
– epsilon_delta
Jan 2 at 5:18
1
$begingroup$
I'm telling you that there's no proper subspace. A nonempty open set contains some ball, the linear span of which is the whole space (this is true for any dimensions, including infinite).
$endgroup$
– metamorphy
Jan 2 at 5:21
2
$begingroup$
Sorry, please disregard my close vote; I misread the question.
$endgroup$
– angryavian
Jan 2 at 5:27
1
$begingroup$
Nice question. Glad you didn't give up on it. :) +1.
$endgroup$
– DisintegratingByParts
Jan 2 at 6:02
|
show 9 more comments
3
$begingroup$
Linear span of a ball = ?
$endgroup$
– metamorphy
Jan 2 at 5:08
1
$begingroup$
@metamorphy If you take a ball in R^n, and take an open ball around a point. then it contains at n linearly independent vectors, span of those vectors give you whole R^n. We know whole space is an open set but my point was in asking a proper subspace. Or what else you wanted to say?
$endgroup$
– epsilon_delta
Jan 2 at 5:18
1
$begingroup$
I'm telling you that there's no proper subspace. A nonempty open set contains some ball, the linear span of which is the whole space (this is true for any dimensions, including infinite).
$endgroup$
– metamorphy
Jan 2 at 5:21
2
$begingroup$
Sorry, please disregard my close vote; I misread the question.
$endgroup$
– angryavian
Jan 2 at 5:27
1
$begingroup$
Nice question. Glad you didn't give up on it. :) +1.
$endgroup$
– DisintegratingByParts
Jan 2 at 6:02
3
3
$begingroup$
Linear span of a ball = ?
$endgroup$
– metamorphy
Jan 2 at 5:08
$begingroup$
Linear span of a ball = ?
$endgroup$
– metamorphy
Jan 2 at 5:08
1
1
$begingroup$
@metamorphy If you take a ball in R^n, and take an open ball around a point. then it contains at n linearly independent vectors, span of those vectors give you whole R^n. We know whole space is an open set but my point was in asking a proper subspace. Or what else you wanted to say?
$endgroup$
– epsilon_delta
Jan 2 at 5:18
$begingroup$
@metamorphy If you take a ball in R^n, and take an open ball around a point. then it contains at n linearly independent vectors, span of those vectors give you whole R^n. We know whole space is an open set but my point was in asking a proper subspace. Or what else you wanted to say?
$endgroup$
– epsilon_delta
Jan 2 at 5:18
1
1
$begingroup$
I'm telling you that there's no proper subspace. A nonempty open set contains some ball, the linear span of which is the whole space (this is true for any dimensions, including infinite).
$endgroup$
– metamorphy
Jan 2 at 5:21
$begingroup$
I'm telling you that there's no proper subspace. A nonempty open set contains some ball, the linear span of which is the whole space (this is true for any dimensions, including infinite).
$endgroup$
– metamorphy
Jan 2 at 5:21
2
2
$begingroup$
Sorry, please disregard my close vote; I misread the question.
$endgroup$
– angryavian
Jan 2 at 5:27
$begingroup$
Sorry, please disregard my close vote; I misread the question.
$endgroup$
– angryavian
Jan 2 at 5:27
1
1
$begingroup$
Nice question. Glad you didn't give up on it. :) +1.
$endgroup$
– DisintegratingByParts
Jan 2 at 6:02
$begingroup$
Nice question. Glad you didn't give up on it. :) +1.
$endgroup$
– DisintegratingByParts
Jan 2 at 6:02
|
show 9 more comments
2 Answers
2
active
oldest
votes
$begingroup$
No.
Let $N$ be a normed space and
$M subsetneq N tag 1$
a proper subspace. Then $M$ contains no nonempty open set. For if
$emptyset ne U subset M tag 2$
were open, with
$M ni m in U, tag 3$
we could find $rho > 0$ such that the open ball
$B(m, rho) subset U; tag 4$
then picking any
$0 ne v in N setminus M tag 5$
the vector
$m + alpha (v - m) in B(m, rho) tag 6$
if $0 ne alpha in Bbb R$ is sufficiently small, since
$Vert (m + alpha (v - m)) - m Vert = Vert alpha (v - m) Vert = vert alpha vert Vert v - m Vert < rho tag 7$
for
$vert alpha vert < dfrac{rho}{Vert v - m Vert}; tag 8$
but then
$m + alpha(v - m) in M, tag 9$
whence
$alpha(v - m) = m + alpha(v - m) - m in M, tag{10}$
whence
$v - m in M, tag{11}$
whence
$v = v - m + m in M, tag{12}$
in contradiction to (5); therefore no $B(m, rho)$ as in (5) can exist, and $M$ cannot be open, since it contains no open set.
$endgroup$
add a comment |
$begingroup$
If $M leq mathcal{H}$ a subspace of a Hilbert space (or generally any normed space) is open, then it contains a ball around the origin $0 in B_r(0) subset M$, but for every (none-zero) vector $v in mathcal{H}$, we have
$$ frac{r}{2Vert v Vert} v in B_r(0) subset M $$
But M is a linear subspace so $ v in M $. Thus the only open subspaces of $ mathcal{H} $ are $ mathcal{H} $ itself.
$endgroup$
add a comment |
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2 Answers
2
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2 Answers
2
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oldest
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active
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$begingroup$
No.
Let $N$ be a normed space and
$M subsetneq N tag 1$
a proper subspace. Then $M$ contains no nonempty open set. For if
$emptyset ne U subset M tag 2$
were open, with
$M ni m in U, tag 3$
we could find $rho > 0$ such that the open ball
$B(m, rho) subset U; tag 4$
then picking any
$0 ne v in N setminus M tag 5$
the vector
$m + alpha (v - m) in B(m, rho) tag 6$
if $0 ne alpha in Bbb R$ is sufficiently small, since
$Vert (m + alpha (v - m)) - m Vert = Vert alpha (v - m) Vert = vert alpha vert Vert v - m Vert < rho tag 7$
for
$vert alpha vert < dfrac{rho}{Vert v - m Vert}; tag 8$
but then
$m + alpha(v - m) in M, tag 9$
whence
$alpha(v - m) = m + alpha(v - m) - m in M, tag{10}$
whence
$v - m in M, tag{11}$
whence
$v = v - m + m in M, tag{12}$
in contradiction to (5); therefore no $B(m, rho)$ as in (5) can exist, and $M$ cannot be open, since it contains no open set.
$endgroup$
add a comment |
$begingroup$
No.
Let $N$ be a normed space and
$M subsetneq N tag 1$
a proper subspace. Then $M$ contains no nonempty open set. For if
$emptyset ne U subset M tag 2$
were open, with
$M ni m in U, tag 3$
we could find $rho > 0$ such that the open ball
$B(m, rho) subset U; tag 4$
then picking any
$0 ne v in N setminus M tag 5$
the vector
$m + alpha (v - m) in B(m, rho) tag 6$
if $0 ne alpha in Bbb R$ is sufficiently small, since
$Vert (m + alpha (v - m)) - m Vert = Vert alpha (v - m) Vert = vert alpha vert Vert v - m Vert < rho tag 7$
for
$vert alpha vert < dfrac{rho}{Vert v - m Vert}; tag 8$
but then
$m + alpha(v - m) in M, tag 9$
whence
$alpha(v - m) = m + alpha(v - m) - m in M, tag{10}$
whence
$v - m in M, tag{11}$
whence
$v = v - m + m in M, tag{12}$
in contradiction to (5); therefore no $B(m, rho)$ as in (5) can exist, and $M$ cannot be open, since it contains no open set.
$endgroup$
add a comment |
$begingroup$
No.
Let $N$ be a normed space and
$M subsetneq N tag 1$
a proper subspace. Then $M$ contains no nonempty open set. For if
$emptyset ne U subset M tag 2$
were open, with
$M ni m in U, tag 3$
we could find $rho > 0$ such that the open ball
$B(m, rho) subset U; tag 4$
then picking any
$0 ne v in N setminus M tag 5$
the vector
$m + alpha (v - m) in B(m, rho) tag 6$
if $0 ne alpha in Bbb R$ is sufficiently small, since
$Vert (m + alpha (v - m)) - m Vert = Vert alpha (v - m) Vert = vert alpha vert Vert v - m Vert < rho tag 7$
for
$vert alpha vert < dfrac{rho}{Vert v - m Vert}; tag 8$
but then
$m + alpha(v - m) in M, tag 9$
whence
$alpha(v - m) = m + alpha(v - m) - m in M, tag{10}$
whence
$v - m in M, tag{11}$
whence
$v = v - m + m in M, tag{12}$
in contradiction to (5); therefore no $B(m, rho)$ as in (5) can exist, and $M$ cannot be open, since it contains no open set.
$endgroup$
No.
Let $N$ be a normed space and
$M subsetneq N tag 1$
a proper subspace. Then $M$ contains no nonempty open set. For if
$emptyset ne U subset M tag 2$
were open, with
$M ni m in U, tag 3$
we could find $rho > 0$ such that the open ball
$B(m, rho) subset U; tag 4$
then picking any
$0 ne v in N setminus M tag 5$
the vector
$m + alpha (v - m) in B(m, rho) tag 6$
if $0 ne alpha in Bbb R$ is sufficiently small, since
$Vert (m + alpha (v - m)) - m Vert = Vert alpha (v - m) Vert = vert alpha vert Vert v - m Vert < rho tag 7$
for
$vert alpha vert < dfrac{rho}{Vert v - m Vert}; tag 8$
but then
$m + alpha(v - m) in M, tag 9$
whence
$alpha(v - m) = m + alpha(v - m) - m in M, tag{10}$
whence
$v - m in M, tag{11}$
whence
$v = v - m + m in M, tag{12}$
in contradiction to (5); therefore no $B(m, rho)$ as in (5) can exist, and $M$ cannot be open, since it contains no open set.
answered Jan 2 at 6:15
Robert LewisRobert Lewis
48.5k23167
48.5k23167
add a comment |
add a comment |
$begingroup$
If $M leq mathcal{H}$ a subspace of a Hilbert space (or generally any normed space) is open, then it contains a ball around the origin $0 in B_r(0) subset M$, but for every (none-zero) vector $v in mathcal{H}$, we have
$$ frac{r}{2Vert v Vert} v in B_r(0) subset M $$
But M is a linear subspace so $ v in M $. Thus the only open subspaces of $ mathcal{H} $ are $ mathcal{H} $ itself.
$endgroup$
add a comment |
$begingroup$
If $M leq mathcal{H}$ a subspace of a Hilbert space (or generally any normed space) is open, then it contains a ball around the origin $0 in B_r(0) subset M$, but for every (none-zero) vector $v in mathcal{H}$, we have
$$ frac{r}{2Vert v Vert} v in B_r(0) subset M $$
But M is a linear subspace so $ v in M $. Thus the only open subspaces of $ mathcal{H} $ are $ mathcal{H} $ itself.
$endgroup$
add a comment |
$begingroup$
If $M leq mathcal{H}$ a subspace of a Hilbert space (or generally any normed space) is open, then it contains a ball around the origin $0 in B_r(0) subset M$, but for every (none-zero) vector $v in mathcal{H}$, we have
$$ frac{r}{2Vert v Vert} v in B_r(0) subset M $$
But M is a linear subspace so $ v in M $. Thus the only open subspaces of $ mathcal{H} $ are $ mathcal{H} $ itself.
$endgroup$
If $M leq mathcal{H}$ a subspace of a Hilbert space (or generally any normed space) is open, then it contains a ball around the origin $0 in B_r(0) subset M$, but for every (none-zero) vector $v in mathcal{H}$, we have
$$ frac{r}{2Vert v Vert} v in B_r(0) subset M $$
But M is a linear subspace so $ v in M $. Thus the only open subspaces of $ mathcal{H} $ are $ mathcal{H} $ itself.
edited Jan 2 at 7:02
Henno Brandsma
114k348124
114k348124
answered Jan 2 at 6:16
pitariverpitariver
444213
444213
add a comment |
add a comment |
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3
$begingroup$
Linear span of a ball = ?
$endgroup$
– metamorphy
Jan 2 at 5:08
1
$begingroup$
@metamorphy If you take a ball in R^n, and take an open ball around a point. then it contains at n linearly independent vectors, span of those vectors give you whole R^n. We know whole space is an open set but my point was in asking a proper subspace. Or what else you wanted to say?
$endgroup$
– epsilon_delta
Jan 2 at 5:18
1
$begingroup$
I'm telling you that there's no proper subspace. A nonempty open set contains some ball, the linear span of which is the whole space (this is true for any dimensions, including infinite).
$endgroup$
– metamorphy
Jan 2 at 5:21
2
$begingroup$
Sorry, please disregard my close vote; I misread the question.
$endgroup$
– angryavian
Jan 2 at 5:27
1
$begingroup$
Nice question. Glad you didn't give up on it. :) +1.
$endgroup$
– DisintegratingByParts
Jan 2 at 6:02