Pre-image of Hausdorff space is Hausdorff
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Let $X, Y$ be topological spaces, with Y a Hausdorff space. Prove that if there exists an injective and continuous function $f: X rightarrow Y$, then $X$ is Hausdorff.
Here's my idea:
Since $Y$ is Hausdorff, different points $x,y in Y$ have disjoint neighborhood $U subset tau_x,V subset tau_y$.
But $f$ is also continuous, so the pre-image of these sets is an open set in $X$, say $f^{-1}(U)=A$, $f^{-1}(V)=B$. Now, because $f$ is injective, I got that $A cap B = emptyset$, and $f^{-1}(x)$, $f^{-1}(y)$ have disjoint neighborhoods.
general-topology proof-verification separation-axioms
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add a comment |
$begingroup$
Let $X, Y$ be topological spaces, with Y a Hausdorff space. Prove that if there exists an injective and continuous function $f: X rightarrow Y$, then $X$ is Hausdorff.
Here's my idea:
Since $Y$ is Hausdorff, different points $x,y in Y$ have disjoint neighborhood $U subset tau_x,V subset tau_y$.
But $f$ is also continuous, so the pre-image of these sets is an open set in $X$, say $f^{-1}(U)=A$, $f^{-1}(V)=B$. Now, because $f$ is injective, I got that $A cap B = emptyset$, and $f^{-1}(x)$, $f^{-1}(y)$ have disjoint neighborhoods.
general-topology proof-verification separation-axioms
$endgroup$
add a comment |
$begingroup$
Let $X, Y$ be topological spaces, with Y a Hausdorff space. Prove that if there exists an injective and continuous function $f: X rightarrow Y$, then $X$ is Hausdorff.
Here's my idea:
Since $Y$ is Hausdorff, different points $x,y in Y$ have disjoint neighborhood $U subset tau_x,V subset tau_y$.
But $f$ is also continuous, so the pre-image of these sets is an open set in $X$, say $f^{-1}(U)=A$, $f^{-1}(V)=B$. Now, because $f$ is injective, I got that $A cap B = emptyset$, and $f^{-1}(x)$, $f^{-1}(y)$ have disjoint neighborhoods.
general-topology proof-verification separation-axioms
$endgroup$
Let $X, Y$ be topological spaces, with Y a Hausdorff space. Prove that if there exists an injective and continuous function $f: X rightarrow Y$, then $X$ is Hausdorff.
Here's my idea:
Since $Y$ is Hausdorff, different points $x,y in Y$ have disjoint neighborhood $U subset tau_x,V subset tau_y$.
But $f$ is also continuous, so the pre-image of these sets is an open set in $X$, say $f^{-1}(U)=A$, $f^{-1}(V)=B$. Now, because $f$ is injective, I got that $A cap B = emptyset$, and $f^{-1}(x)$, $f^{-1}(y)$ have disjoint neighborhoods.
general-topology proof-verification separation-axioms
general-topology proof-verification separation-axioms
edited Dec 12 '18 at 17:09
José Carlos Santos
159k22126231
159k22126231
asked Aug 22 '17 at 21:50
VoBVoB
729413
729413
add a comment |
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3 Answers
3
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$begingroup$
Let $x_1,x_2 in X$ such that $x_1 neq x_2$
Then $f(x_1),f(x_2) in Y$ and $f(x_1) neq f(x_2)$ because $f$ is injective.
Then there exist two disjoint open sets $U_1,U_2 in mathcal{T_Y}$ such that $f(x_1) in U_1$ and $f(x_2) in U_2$
Thus $$x_1 in f^{-1}(U_1)$$ $$x_2 in f^{-1}(U_2)$$ where $ f^{-1}(U_1),f^{-1}(U_2)$ are open in $X$ because $f$ is continuous and $U_1,U_2$ are open in $Y$.
Put $V_1=f^{-1}(U_1)$ and $V_2=f^{-1}(U_2)$ and you have: $$V_1 cap V_2 =f^{-1}(U_1) cap f^{-1}(U_2)=f^{-1}(U_1 cap U_2)=f^{-1}(emptyset)=emptyset$$
Therefore $X$ is Hausdorf.
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add a comment |
$begingroup$
There's a jump in your proof: you write that $x$ and $y$ have disjoint neighborhoods $Uintau_x$ and $Vintau_y$ (note that you wrote $Usubsettau_x$ and $Vsubsettau_y$; this is wrong), and, in the next sentence, you write that the pre-images of $U$ and $V$ are open sets. Why?
There are two ways of solving this:
- Use the fact that the pre-image of a neighborhood by a continuous function is again a neighborhood.
- You can assume without loss of generality that $U$ and $V$ are open sets.
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I said that $U, V subset Y$ are open sets just because $f$ is continuous.
$endgroup$
– VoB
Aug 22 '17 at 22:14
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The existence of such neighborhoods have nothing to do with $f$. In my opinion, you should have started with two neighboroods $U^star$ and $V^star$ of $f(x)$ and of $f(y)$ respectively such that $U^starcap V^star=emptyset$ and then say that there are open sets $Uintau_x$ and $Vintau_y$ such that $xin Usubset U^star$ and $yin Vsubset V^star$. Since $U^starcap V^star=emptyset$, $Ucap V=emptyset$.
$endgroup$
– José Carlos Santos
Aug 22 '17 at 22:20
add a comment |
$begingroup$
Take distinct $x,y in X$ then $f(x), f(y)$ are distinct points in $Y$ (by injectivity) and so we have disjoint open sets $U$ and $V$ containing $f(x)$ and $f(y)$, respectively. Now their preimages $f^{-1}(U)$ and $f^{-1}(V)$ are open (by continuity), disjoint and contain $x$ and $y$.
$endgroup$
add a comment |
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3 Answers
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3 Answers
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active
oldest
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$begingroup$
Let $x_1,x_2 in X$ such that $x_1 neq x_2$
Then $f(x_1),f(x_2) in Y$ and $f(x_1) neq f(x_2)$ because $f$ is injective.
Then there exist two disjoint open sets $U_1,U_2 in mathcal{T_Y}$ such that $f(x_1) in U_1$ and $f(x_2) in U_2$
Thus $$x_1 in f^{-1}(U_1)$$ $$x_2 in f^{-1}(U_2)$$ where $ f^{-1}(U_1),f^{-1}(U_2)$ are open in $X$ because $f$ is continuous and $U_1,U_2$ are open in $Y$.
Put $V_1=f^{-1}(U_1)$ and $V_2=f^{-1}(U_2)$ and you have: $$V_1 cap V_2 =f^{-1}(U_1) cap f^{-1}(U_2)=f^{-1}(U_1 cap U_2)=f^{-1}(emptyset)=emptyset$$
Therefore $X$ is Hausdorf.
$endgroup$
add a comment |
$begingroup$
Let $x_1,x_2 in X$ such that $x_1 neq x_2$
Then $f(x_1),f(x_2) in Y$ and $f(x_1) neq f(x_2)$ because $f$ is injective.
Then there exist two disjoint open sets $U_1,U_2 in mathcal{T_Y}$ such that $f(x_1) in U_1$ and $f(x_2) in U_2$
Thus $$x_1 in f^{-1}(U_1)$$ $$x_2 in f^{-1}(U_2)$$ where $ f^{-1}(U_1),f^{-1}(U_2)$ are open in $X$ because $f$ is continuous and $U_1,U_2$ are open in $Y$.
Put $V_1=f^{-1}(U_1)$ and $V_2=f^{-1}(U_2)$ and you have: $$V_1 cap V_2 =f^{-1}(U_1) cap f^{-1}(U_2)=f^{-1}(U_1 cap U_2)=f^{-1}(emptyset)=emptyset$$
Therefore $X$ is Hausdorf.
$endgroup$
add a comment |
$begingroup$
Let $x_1,x_2 in X$ such that $x_1 neq x_2$
Then $f(x_1),f(x_2) in Y$ and $f(x_1) neq f(x_2)$ because $f$ is injective.
Then there exist two disjoint open sets $U_1,U_2 in mathcal{T_Y}$ such that $f(x_1) in U_1$ and $f(x_2) in U_2$
Thus $$x_1 in f^{-1}(U_1)$$ $$x_2 in f^{-1}(U_2)$$ where $ f^{-1}(U_1),f^{-1}(U_2)$ are open in $X$ because $f$ is continuous and $U_1,U_2$ are open in $Y$.
Put $V_1=f^{-1}(U_1)$ and $V_2=f^{-1}(U_2)$ and you have: $$V_1 cap V_2 =f^{-1}(U_1) cap f^{-1}(U_2)=f^{-1}(U_1 cap U_2)=f^{-1}(emptyset)=emptyset$$
Therefore $X$ is Hausdorf.
$endgroup$
Let $x_1,x_2 in X$ such that $x_1 neq x_2$
Then $f(x_1),f(x_2) in Y$ and $f(x_1) neq f(x_2)$ because $f$ is injective.
Then there exist two disjoint open sets $U_1,U_2 in mathcal{T_Y}$ such that $f(x_1) in U_1$ and $f(x_2) in U_2$
Thus $$x_1 in f^{-1}(U_1)$$ $$x_2 in f^{-1}(U_2)$$ where $ f^{-1}(U_1),f^{-1}(U_2)$ are open in $X$ because $f$ is continuous and $U_1,U_2$ are open in $Y$.
Put $V_1=f^{-1}(U_1)$ and $V_2=f^{-1}(U_2)$ and you have: $$V_1 cap V_2 =f^{-1}(U_1) cap f^{-1}(U_2)=f^{-1}(U_1 cap U_2)=f^{-1}(emptyset)=emptyset$$
Therefore $X$ is Hausdorf.
edited Aug 22 '17 at 23:07
answered Aug 22 '17 at 22:17
Marios GretsasMarios Gretsas
8,48011437
8,48011437
add a comment |
add a comment |
$begingroup$
There's a jump in your proof: you write that $x$ and $y$ have disjoint neighborhoods $Uintau_x$ and $Vintau_y$ (note that you wrote $Usubsettau_x$ and $Vsubsettau_y$; this is wrong), and, in the next sentence, you write that the pre-images of $U$ and $V$ are open sets. Why?
There are two ways of solving this:
- Use the fact that the pre-image of a neighborhood by a continuous function is again a neighborhood.
- You can assume without loss of generality that $U$ and $V$ are open sets.
$endgroup$
$begingroup$
I said that $U, V subset Y$ are open sets just because $f$ is continuous.
$endgroup$
– VoB
Aug 22 '17 at 22:14
$begingroup$
The existence of such neighborhoods have nothing to do with $f$. In my opinion, you should have started with two neighboroods $U^star$ and $V^star$ of $f(x)$ and of $f(y)$ respectively such that $U^starcap V^star=emptyset$ and then say that there are open sets $Uintau_x$ and $Vintau_y$ such that $xin Usubset U^star$ and $yin Vsubset V^star$. Since $U^starcap V^star=emptyset$, $Ucap V=emptyset$.
$endgroup$
– José Carlos Santos
Aug 22 '17 at 22:20
add a comment |
$begingroup$
There's a jump in your proof: you write that $x$ and $y$ have disjoint neighborhoods $Uintau_x$ and $Vintau_y$ (note that you wrote $Usubsettau_x$ and $Vsubsettau_y$; this is wrong), and, in the next sentence, you write that the pre-images of $U$ and $V$ are open sets. Why?
There are two ways of solving this:
- Use the fact that the pre-image of a neighborhood by a continuous function is again a neighborhood.
- You can assume without loss of generality that $U$ and $V$ are open sets.
$endgroup$
$begingroup$
I said that $U, V subset Y$ are open sets just because $f$ is continuous.
$endgroup$
– VoB
Aug 22 '17 at 22:14
$begingroup$
The existence of such neighborhoods have nothing to do with $f$. In my opinion, you should have started with two neighboroods $U^star$ and $V^star$ of $f(x)$ and of $f(y)$ respectively such that $U^starcap V^star=emptyset$ and then say that there are open sets $Uintau_x$ and $Vintau_y$ such that $xin Usubset U^star$ and $yin Vsubset V^star$. Since $U^starcap V^star=emptyset$, $Ucap V=emptyset$.
$endgroup$
– José Carlos Santos
Aug 22 '17 at 22:20
add a comment |
$begingroup$
There's a jump in your proof: you write that $x$ and $y$ have disjoint neighborhoods $Uintau_x$ and $Vintau_y$ (note that you wrote $Usubsettau_x$ and $Vsubsettau_y$; this is wrong), and, in the next sentence, you write that the pre-images of $U$ and $V$ are open sets. Why?
There are two ways of solving this:
- Use the fact that the pre-image of a neighborhood by a continuous function is again a neighborhood.
- You can assume without loss of generality that $U$ and $V$ are open sets.
$endgroup$
There's a jump in your proof: you write that $x$ and $y$ have disjoint neighborhoods $Uintau_x$ and $Vintau_y$ (note that you wrote $Usubsettau_x$ and $Vsubsettau_y$; this is wrong), and, in the next sentence, you write that the pre-images of $U$ and $V$ are open sets. Why?
There are two ways of solving this:
- Use the fact that the pre-image of a neighborhood by a continuous function is again a neighborhood.
- You can assume without loss of generality that $U$ and $V$ are open sets.
edited Apr 14 '18 at 21:34
answered Aug 22 '17 at 22:09
José Carlos SantosJosé Carlos Santos
159k22126231
159k22126231
$begingroup$
I said that $U, V subset Y$ are open sets just because $f$ is continuous.
$endgroup$
– VoB
Aug 22 '17 at 22:14
$begingroup$
The existence of such neighborhoods have nothing to do with $f$. In my opinion, you should have started with two neighboroods $U^star$ and $V^star$ of $f(x)$ and of $f(y)$ respectively such that $U^starcap V^star=emptyset$ and then say that there are open sets $Uintau_x$ and $Vintau_y$ such that $xin Usubset U^star$ and $yin Vsubset V^star$. Since $U^starcap V^star=emptyset$, $Ucap V=emptyset$.
$endgroup$
– José Carlos Santos
Aug 22 '17 at 22:20
add a comment |
$begingroup$
I said that $U, V subset Y$ are open sets just because $f$ is continuous.
$endgroup$
– VoB
Aug 22 '17 at 22:14
$begingroup$
The existence of such neighborhoods have nothing to do with $f$. In my opinion, you should have started with two neighboroods $U^star$ and $V^star$ of $f(x)$ and of $f(y)$ respectively such that $U^starcap V^star=emptyset$ and then say that there are open sets $Uintau_x$ and $Vintau_y$ such that $xin Usubset U^star$ and $yin Vsubset V^star$. Since $U^starcap V^star=emptyset$, $Ucap V=emptyset$.
$endgroup$
– José Carlos Santos
Aug 22 '17 at 22:20
$begingroup$
I said that $U, V subset Y$ are open sets just because $f$ is continuous.
$endgroup$
– VoB
Aug 22 '17 at 22:14
$begingroup$
I said that $U, V subset Y$ are open sets just because $f$ is continuous.
$endgroup$
– VoB
Aug 22 '17 at 22:14
$begingroup$
The existence of such neighborhoods have nothing to do with $f$. In my opinion, you should have started with two neighboroods $U^star$ and $V^star$ of $f(x)$ and of $f(y)$ respectively such that $U^starcap V^star=emptyset$ and then say that there are open sets $Uintau_x$ and $Vintau_y$ such that $xin Usubset U^star$ and $yin Vsubset V^star$. Since $U^starcap V^star=emptyset$, $Ucap V=emptyset$.
$endgroup$
– José Carlos Santos
Aug 22 '17 at 22:20
$begingroup$
The existence of such neighborhoods have nothing to do with $f$. In my opinion, you should have started with two neighboroods $U^star$ and $V^star$ of $f(x)$ and of $f(y)$ respectively such that $U^starcap V^star=emptyset$ and then say that there are open sets $Uintau_x$ and $Vintau_y$ such that $xin Usubset U^star$ and $yin Vsubset V^star$. Since $U^starcap V^star=emptyset$, $Ucap V=emptyset$.
$endgroup$
– José Carlos Santos
Aug 22 '17 at 22:20
add a comment |
$begingroup$
Take distinct $x,y in X$ then $f(x), f(y)$ are distinct points in $Y$ (by injectivity) and so we have disjoint open sets $U$ and $V$ containing $f(x)$ and $f(y)$, respectively. Now their preimages $f^{-1}(U)$ and $f^{-1}(V)$ are open (by continuity), disjoint and contain $x$ and $y$.
$endgroup$
add a comment |
$begingroup$
Take distinct $x,y in X$ then $f(x), f(y)$ are distinct points in $Y$ (by injectivity) and so we have disjoint open sets $U$ and $V$ containing $f(x)$ and $f(y)$, respectively. Now their preimages $f^{-1}(U)$ and $f^{-1}(V)$ are open (by continuity), disjoint and contain $x$ and $y$.
$endgroup$
add a comment |
$begingroup$
Take distinct $x,y in X$ then $f(x), f(y)$ are distinct points in $Y$ (by injectivity) and so we have disjoint open sets $U$ and $V$ containing $f(x)$ and $f(y)$, respectively. Now their preimages $f^{-1}(U)$ and $f^{-1}(V)$ are open (by continuity), disjoint and contain $x$ and $y$.
$endgroup$
Take distinct $x,y in X$ then $f(x), f(y)$ are distinct points in $Y$ (by injectivity) and so we have disjoint open sets $U$ and $V$ containing $f(x)$ and $f(y)$, respectively. Now their preimages $f^{-1}(U)$ and $f^{-1}(V)$ are open (by continuity), disjoint and contain $x$ and $y$.
answered Aug 22 '17 at 22:07
Zain PatelZain Patel
15.7k51949
15.7k51949
add a comment |
add a comment |
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