Is a connected set always an uncountably infinite set?
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I'm trying to understand the concept of a connected set. The classic example which is presented is that of $Bbb R$ or any interval of $Bbb R$ with the usual topology.
Moreover, I heard that to a certain extent connected sets can be considered opposite to discrete sets. They are sometimes indicated as representing the idea of a continuum. So, intuitively, shouldn't they always be uncountable sets?
real-analysis general-topology connectedness
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add a comment |
$begingroup$
I'm trying to understand the concept of a connected set. The classic example which is presented is that of $Bbb R$ or any interval of $Bbb R$ with the usual topology.
Moreover, I heard that to a certain extent connected sets can be considered opposite to discrete sets. They are sometimes indicated as representing the idea of a continuum. So, intuitively, shouldn't they always be uncountable sets?
real-analysis general-topology connectedness
$endgroup$
6
$begingroup$
It's plainly false that every subset of $mathbb{R}$ is connected. The connected subsets of $mathbb{R}$ are the empty set, the singletons and all intervals. But $[0,1]cup[2,3]$ is obviously disconnected.
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– egreg
Feb 3 at 19:14
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@egreg You're right. I meant any interval :)
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– Gabriele Scarlatti
Feb 3 at 23:21
1
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@GabrieleScarlatti You might want to edit your question accordingly.
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– Luke
Feb 5 at 19:08
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@Luke I"ve edited it :)
$endgroup$
– Gabriele Scarlatti
Feb 6 at 12:03
add a comment |
$begingroup$
I'm trying to understand the concept of a connected set. The classic example which is presented is that of $Bbb R$ or any interval of $Bbb R$ with the usual topology.
Moreover, I heard that to a certain extent connected sets can be considered opposite to discrete sets. They are sometimes indicated as representing the idea of a continuum. So, intuitively, shouldn't they always be uncountable sets?
real-analysis general-topology connectedness
$endgroup$
I'm trying to understand the concept of a connected set. The classic example which is presented is that of $Bbb R$ or any interval of $Bbb R$ with the usual topology.
Moreover, I heard that to a certain extent connected sets can be considered opposite to discrete sets. They are sometimes indicated as representing the idea of a continuum. So, intuitively, shouldn't they always be uncountable sets?
real-analysis general-topology connectedness
real-analysis general-topology connectedness
edited Feb 6 at 11:59
Gabriele Scarlatti
asked Feb 3 at 17:58
Gabriele ScarlattiGabriele Scarlatti
380212
380212
6
$begingroup$
It's plainly false that every subset of $mathbb{R}$ is connected. The connected subsets of $mathbb{R}$ are the empty set, the singletons and all intervals. But $[0,1]cup[2,3]$ is obviously disconnected.
$endgroup$
– egreg
Feb 3 at 19:14
$begingroup$
@egreg You're right. I meant any interval :)
$endgroup$
– Gabriele Scarlatti
Feb 3 at 23:21
1
$begingroup$
@GabrieleScarlatti You might want to edit your question accordingly.
$endgroup$
– Luke
Feb 5 at 19:08
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@Luke I"ve edited it :)
$endgroup$
– Gabriele Scarlatti
Feb 6 at 12:03
add a comment |
6
$begingroup$
It's plainly false that every subset of $mathbb{R}$ is connected. The connected subsets of $mathbb{R}$ are the empty set, the singletons and all intervals. But $[0,1]cup[2,3]$ is obviously disconnected.
$endgroup$
– egreg
Feb 3 at 19:14
$begingroup$
@egreg You're right. I meant any interval :)
$endgroup$
– Gabriele Scarlatti
Feb 3 at 23:21
1
$begingroup$
@GabrieleScarlatti You might want to edit your question accordingly.
$endgroup$
– Luke
Feb 5 at 19:08
$begingroup$
@Luke I"ve edited it :)
$endgroup$
– Gabriele Scarlatti
Feb 6 at 12:03
6
6
$begingroup$
It's plainly false that every subset of $mathbb{R}$ is connected. The connected subsets of $mathbb{R}$ are the empty set, the singletons and all intervals. But $[0,1]cup[2,3]$ is obviously disconnected.
$endgroup$
– egreg
Feb 3 at 19:14
$begingroup$
It's plainly false that every subset of $mathbb{R}$ is connected. The connected subsets of $mathbb{R}$ are the empty set, the singletons and all intervals. But $[0,1]cup[2,3]$ is obviously disconnected.
$endgroup$
– egreg
Feb 3 at 19:14
$begingroup$
@egreg You're right. I meant any interval :)
$endgroup$
– Gabriele Scarlatti
Feb 3 at 23:21
$begingroup$
@egreg You're right. I meant any interval :)
$endgroup$
– Gabriele Scarlatti
Feb 3 at 23:21
1
1
$begingroup$
@GabrieleScarlatti You might want to edit your question accordingly.
$endgroup$
– Luke
Feb 5 at 19:08
$begingroup$
@GabrieleScarlatti You might want to edit your question accordingly.
$endgroup$
– Luke
Feb 5 at 19:08
$begingroup$
@Luke I"ve edited it :)
$endgroup$
– Gabriele Scarlatti
Feb 6 at 12:03
$begingroup$
@Luke I"ve edited it :)
$endgroup$
– Gabriele Scarlatti
Feb 6 at 12:03
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
First of all, $emptyset$ is connected. And so is every singleton.
On the other hand, it is true that every connected metric space with more than one point is uncountable. But there are countable connected topological spaces.
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I've heard some argue that the empty set is not connected, in a similar sense to 1 not being a prime number. See ncatlab.org/nlab/show/too+simple+to+be+simple for related thoughts.
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– Ben Millwood
Feb 4 at 12:54
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@BenMillwood, to me, the empty set is probably connected, but not $0$-connected (because $0$-connected implies $(-1)$-connected, and the latter is equivalent to being non-empty).
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– Carsten S
Feb 4 at 14:14
add a comment |
$begingroup$
Consider the space $X = {a, b}$ equipped with the trivial topology (i.e. the only open sets are $X$ and $varnothing$). Then, the space is connected, but there are only a finite number of points in this example.
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9
$begingroup$
Even cooler is the pseudocircle. It's the smallest multiconnected space.
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– PyRulez
Feb 3 at 23:10
add a comment |
$begingroup$
Well, there's the empty set, as others have noted. Also, every singleton is connected. But you can say a bit more if you assume some separation.
- Any finite $T_1$ space is discrete, so it is connected if and only if it has only one point, so if you assume $T_1$, there are no nontrivial connected finite sets or spaces.
- There are also connected $T_0$ spaces of any size you want (including any finite): if you take a nonempty set $X$ and fix $xin X$, then if you declare open sets to be exactly those containing $x$, then the resulting topology is $T_0$ and connected (all points except for $x$ are closed).
- On any infinite set (possibly countable), the cofinite topology is $T_1$ and connected.
- There are connected countably infinite Hausdorff spaces. (and even $T_{2frac{1}{2}}$-spaces).
- A connected $T_4$ space with more than one point is necessarily uncountable. This follows easily from Urysohn's lemma (in fact, it follows that a nontrivial connected $T_4$ space has at least the cardinality of the continuum).
- One can show that a countable $T_3$ space is $T_4$, so by the preceding point, a connected $T_3$ space cannot be countably infinite.
Incidentally, in topology, a continuum is a kind of connected set (namely one which is compact Hausdorff, or even metrisable, depending on the author).
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add a comment |
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3 Answers
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active
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3 Answers
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$begingroup$
First of all, $emptyset$ is connected. And so is every singleton.
On the other hand, it is true that every connected metric space with more than one point is uncountable. But there are countable connected topological spaces.
$endgroup$
$begingroup$
I've heard some argue that the empty set is not connected, in a similar sense to 1 not being a prime number. See ncatlab.org/nlab/show/too+simple+to+be+simple for related thoughts.
$endgroup$
– Ben Millwood
Feb 4 at 12:54
$begingroup$
@BenMillwood, to me, the empty set is probably connected, but not $0$-connected (because $0$-connected implies $(-1)$-connected, and the latter is equivalent to being non-empty).
$endgroup$
– Carsten S
Feb 4 at 14:14
add a comment |
$begingroup$
First of all, $emptyset$ is connected. And so is every singleton.
On the other hand, it is true that every connected metric space with more than one point is uncountable. But there are countable connected topological spaces.
$endgroup$
$begingroup$
I've heard some argue that the empty set is not connected, in a similar sense to 1 not being a prime number. See ncatlab.org/nlab/show/too+simple+to+be+simple for related thoughts.
$endgroup$
– Ben Millwood
Feb 4 at 12:54
$begingroup$
@BenMillwood, to me, the empty set is probably connected, but not $0$-connected (because $0$-connected implies $(-1)$-connected, and the latter is equivalent to being non-empty).
$endgroup$
– Carsten S
Feb 4 at 14:14
add a comment |
$begingroup$
First of all, $emptyset$ is connected. And so is every singleton.
On the other hand, it is true that every connected metric space with more than one point is uncountable. But there are countable connected topological spaces.
$endgroup$
First of all, $emptyset$ is connected. And so is every singleton.
On the other hand, it is true that every connected metric space with more than one point is uncountable. But there are countable connected topological spaces.
answered Feb 3 at 18:02
José Carlos SantosJosé Carlos Santos
169k23132237
169k23132237
$begingroup$
I've heard some argue that the empty set is not connected, in a similar sense to 1 not being a prime number. See ncatlab.org/nlab/show/too+simple+to+be+simple for related thoughts.
$endgroup$
– Ben Millwood
Feb 4 at 12:54
$begingroup$
@BenMillwood, to me, the empty set is probably connected, but not $0$-connected (because $0$-connected implies $(-1)$-connected, and the latter is equivalent to being non-empty).
$endgroup$
– Carsten S
Feb 4 at 14:14
add a comment |
$begingroup$
I've heard some argue that the empty set is not connected, in a similar sense to 1 not being a prime number. See ncatlab.org/nlab/show/too+simple+to+be+simple for related thoughts.
$endgroup$
– Ben Millwood
Feb 4 at 12:54
$begingroup$
@BenMillwood, to me, the empty set is probably connected, but not $0$-connected (because $0$-connected implies $(-1)$-connected, and the latter is equivalent to being non-empty).
$endgroup$
– Carsten S
Feb 4 at 14:14
$begingroup$
I've heard some argue that the empty set is not connected, in a similar sense to 1 not being a prime number. See ncatlab.org/nlab/show/too+simple+to+be+simple for related thoughts.
$endgroup$
– Ben Millwood
Feb 4 at 12:54
$begingroup$
I've heard some argue that the empty set is not connected, in a similar sense to 1 not being a prime number. See ncatlab.org/nlab/show/too+simple+to+be+simple for related thoughts.
$endgroup$
– Ben Millwood
Feb 4 at 12:54
$begingroup$
@BenMillwood, to me, the empty set is probably connected, but not $0$-connected (because $0$-connected implies $(-1)$-connected, and the latter is equivalent to being non-empty).
$endgroup$
– Carsten S
Feb 4 at 14:14
$begingroup$
@BenMillwood, to me, the empty set is probably connected, but not $0$-connected (because $0$-connected implies $(-1)$-connected, and the latter is equivalent to being non-empty).
$endgroup$
– Carsten S
Feb 4 at 14:14
add a comment |
$begingroup$
Consider the space $X = {a, b}$ equipped with the trivial topology (i.e. the only open sets are $X$ and $varnothing$). Then, the space is connected, but there are only a finite number of points in this example.
$endgroup$
9
$begingroup$
Even cooler is the pseudocircle. It's the smallest multiconnected space.
$endgroup$
– PyRulez
Feb 3 at 23:10
add a comment |
$begingroup$
Consider the space $X = {a, b}$ equipped with the trivial topology (i.e. the only open sets are $X$ and $varnothing$). Then, the space is connected, but there are only a finite number of points in this example.
$endgroup$
9
$begingroup$
Even cooler is the pseudocircle. It's the smallest multiconnected space.
$endgroup$
– PyRulez
Feb 3 at 23:10
add a comment |
$begingroup$
Consider the space $X = {a, b}$ equipped with the trivial topology (i.e. the only open sets are $X$ and $varnothing$). Then, the space is connected, but there are only a finite number of points in this example.
$endgroup$
Consider the space $X = {a, b}$ equipped with the trivial topology (i.e. the only open sets are $X$ and $varnothing$). Then, the space is connected, but there are only a finite number of points in this example.
answered Feb 3 at 18:01
Kurtland ChuaKurtland Chua
3541212
3541212
9
$begingroup$
Even cooler is the pseudocircle. It's the smallest multiconnected space.
$endgroup$
– PyRulez
Feb 3 at 23:10
add a comment |
9
$begingroup$
Even cooler is the pseudocircle. It's the smallest multiconnected space.
$endgroup$
– PyRulez
Feb 3 at 23:10
9
9
$begingroup$
Even cooler is the pseudocircle. It's the smallest multiconnected space.
$endgroup$
– PyRulez
Feb 3 at 23:10
$begingroup$
Even cooler is the pseudocircle. It's the smallest multiconnected space.
$endgroup$
– PyRulez
Feb 3 at 23:10
add a comment |
$begingroup$
Well, there's the empty set, as others have noted. Also, every singleton is connected. But you can say a bit more if you assume some separation.
- Any finite $T_1$ space is discrete, so it is connected if and only if it has only one point, so if you assume $T_1$, there are no nontrivial connected finite sets or spaces.
- There are also connected $T_0$ spaces of any size you want (including any finite): if you take a nonempty set $X$ and fix $xin X$, then if you declare open sets to be exactly those containing $x$, then the resulting topology is $T_0$ and connected (all points except for $x$ are closed).
- On any infinite set (possibly countable), the cofinite topology is $T_1$ and connected.
- There are connected countably infinite Hausdorff spaces. (and even $T_{2frac{1}{2}}$-spaces).
- A connected $T_4$ space with more than one point is necessarily uncountable. This follows easily from Urysohn's lemma (in fact, it follows that a nontrivial connected $T_4$ space has at least the cardinality of the continuum).
- One can show that a countable $T_3$ space is $T_4$, so by the preceding point, a connected $T_3$ space cannot be countably infinite.
Incidentally, in topology, a continuum is a kind of connected set (namely one which is compact Hausdorff, or even metrisable, depending on the author).
$endgroup$
add a comment |
$begingroup$
Well, there's the empty set, as others have noted. Also, every singleton is connected. But you can say a bit more if you assume some separation.
- Any finite $T_1$ space is discrete, so it is connected if and only if it has only one point, so if you assume $T_1$, there are no nontrivial connected finite sets or spaces.
- There are also connected $T_0$ spaces of any size you want (including any finite): if you take a nonempty set $X$ and fix $xin X$, then if you declare open sets to be exactly those containing $x$, then the resulting topology is $T_0$ and connected (all points except for $x$ are closed).
- On any infinite set (possibly countable), the cofinite topology is $T_1$ and connected.
- There are connected countably infinite Hausdorff spaces. (and even $T_{2frac{1}{2}}$-spaces).
- A connected $T_4$ space with more than one point is necessarily uncountable. This follows easily from Urysohn's lemma (in fact, it follows that a nontrivial connected $T_4$ space has at least the cardinality of the continuum).
- One can show that a countable $T_3$ space is $T_4$, so by the preceding point, a connected $T_3$ space cannot be countably infinite.
Incidentally, in topology, a continuum is a kind of connected set (namely one which is compact Hausdorff, or even metrisable, depending on the author).
$endgroup$
add a comment |
$begingroup$
Well, there's the empty set, as others have noted. Also, every singleton is connected. But you can say a bit more if you assume some separation.
- Any finite $T_1$ space is discrete, so it is connected if and only if it has only one point, so if you assume $T_1$, there are no nontrivial connected finite sets or spaces.
- There are also connected $T_0$ spaces of any size you want (including any finite): if you take a nonempty set $X$ and fix $xin X$, then if you declare open sets to be exactly those containing $x$, then the resulting topology is $T_0$ and connected (all points except for $x$ are closed).
- On any infinite set (possibly countable), the cofinite topology is $T_1$ and connected.
- There are connected countably infinite Hausdorff spaces. (and even $T_{2frac{1}{2}}$-spaces).
- A connected $T_4$ space with more than one point is necessarily uncountable. This follows easily from Urysohn's lemma (in fact, it follows that a nontrivial connected $T_4$ space has at least the cardinality of the continuum).
- One can show that a countable $T_3$ space is $T_4$, so by the preceding point, a connected $T_3$ space cannot be countably infinite.
Incidentally, in topology, a continuum is a kind of connected set (namely one which is compact Hausdorff, or even metrisable, depending on the author).
$endgroup$
Well, there's the empty set, as others have noted. Also, every singleton is connected. But you can say a bit more if you assume some separation.
- Any finite $T_1$ space is discrete, so it is connected if and only if it has only one point, so if you assume $T_1$, there are no nontrivial connected finite sets or spaces.
- There are also connected $T_0$ spaces of any size you want (including any finite): if you take a nonempty set $X$ and fix $xin X$, then if you declare open sets to be exactly those containing $x$, then the resulting topology is $T_0$ and connected (all points except for $x$ are closed).
- On any infinite set (possibly countable), the cofinite topology is $T_1$ and connected.
- There are connected countably infinite Hausdorff spaces. (and even $T_{2frac{1}{2}}$-spaces).
- A connected $T_4$ space with more than one point is necessarily uncountable. This follows easily from Urysohn's lemma (in fact, it follows that a nontrivial connected $T_4$ space has at least the cardinality of the continuum).
- One can show that a countable $T_3$ space is $T_4$, so by the preceding point, a connected $T_3$ space cannot be countably infinite.
Incidentally, in topology, a continuum is a kind of connected set (namely one which is compact Hausdorff, or even metrisable, depending on the author).
edited Feb 4 at 1:39
answered Feb 4 at 1:19
tomasztomasz
23.9k23482
23.9k23482
add a comment |
add a comment |
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6
$begingroup$
It's plainly false that every subset of $mathbb{R}$ is connected. The connected subsets of $mathbb{R}$ are the empty set, the singletons and all intervals. But $[0,1]cup[2,3]$ is obviously disconnected.
$endgroup$
– egreg
Feb 3 at 19:14
$begingroup$
@egreg You're right. I meant any interval :)
$endgroup$
– Gabriele Scarlatti
Feb 3 at 23:21
1
$begingroup$
@GabrieleScarlatti You might want to edit your question accordingly.
$endgroup$
– Luke
Feb 5 at 19:08
$begingroup$
@Luke I"ve edited it :)
$endgroup$
– Gabriele Scarlatti
Feb 6 at 12:03