Projection is an open map












32












$begingroup$


Let $X$ and $Y$ be (any) topological spaces. Show that the projection



$pi_1$ : $Xtimes Yto X$



is an open map.










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$endgroup$








  • 3




    $begingroup$
    How do you define the topology on $Xtimes Y$?
    $endgroup$
    – Lior B-S
    Nov 29 '12 at 20:29






  • 4




    $begingroup$
    ProofWiki: Projection from Product Topology is Open
    $endgroup$
    – Martin Sleziak
    Apr 11 '13 at 8:24
















32












$begingroup$


Let $X$ and $Y$ be (any) topological spaces. Show that the projection



$pi_1$ : $Xtimes Yto X$



is an open map.










share|cite|improve this question











$endgroup$








  • 3




    $begingroup$
    How do you define the topology on $Xtimes Y$?
    $endgroup$
    – Lior B-S
    Nov 29 '12 at 20:29






  • 4




    $begingroup$
    ProofWiki: Projection from Product Topology is Open
    $endgroup$
    – Martin Sleziak
    Apr 11 '13 at 8:24














32












32








32


15



$begingroup$


Let $X$ and $Y$ be (any) topological spaces. Show that the projection



$pi_1$ : $Xtimes Yto X$



is an open map.










share|cite|improve this question











$endgroup$




Let $X$ and $Y$ be (any) topological spaces. Show that the projection



$pi_1$ : $Xtimes Yto X$



is an open map.







general-topology






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share|cite|improve this question













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share|cite|improve this question








edited Apr 11 '13 at 8:23









Stefan Hamcke

21.7k42879




21.7k42879










asked Nov 29 '12 at 20:25









AlishaAlisha

170123




170123








  • 3




    $begingroup$
    How do you define the topology on $Xtimes Y$?
    $endgroup$
    – Lior B-S
    Nov 29 '12 at 20:29






  • 4




    $begingroup$
    ProofWiki: Projection from Product Topology is Open
    $endgroup$
    – Martin Sleziak
    Apr 11 '13 at 8:24














  • 3




    $begingroup$
    How do you define the topology on $Xtimes Y$?
    $endgroup$
    – Lior B-S
    Nov 29 '12 at 20:29






  • 4




    $begingroup$
    ProofWiki: Projection from Product Topology is Open
    $endgroup$
    – Martin Sleziak
    Apr 11 '13 at 8:24








3




3




$begingroup$
How do you define the topology on $Xtimes Y$?
$endgroup$
– Lior B-S
Nov 29 '12 at 20:29




$begingroup$
How do you define the topology on $Xtimes Y$?
$endgroup$
– Lior B-S
Nov 29 '12 at 20:29




4




4




$begingroup$
ProofWiki: Projection from Product Topology is Open
$endgroup$
– Martin Sleziak
Apr 11 '13 at 8:24




$begingroup$
ProofWiki: Projection from Product Topology is Open
$endgroup$
– Martin Sleziak
Apr 11 '13 at 8:24










3 Answers
3






active

oldest

votes


















26












$begingroup$

Let $Usubseteq Xtimes Y$ be open. Then, by definition of the product topology, $U$ is a union of finite intersections of sets of the form $pi_X^{-1}(V)=Vtimes Y$ and $pi_Y^{-1}(W)=Xtimes W$ for $Vsubseteq X$ and $Wsubseteq Y$ open. This means (in this case) that we may without loss of generality assume $U=Vtimes W$. Now, clearly, $pi_X(U)=V$ is open.



Edit I will explain why I assume $U=Vtimes W$. In general, we know that $U=bigcup_{iin I} bigcap_{jin J_i} V_{ij}times W_{ij}$ with $I$ possibly infinite, each $J_i$ a finite set and $V_{ij}subseteq X$ as well as $W_{ij}subseteq Y$ open. Note that we have
begin{align*}
(V_1times W_1)cap (V_2times W_2) &= { (v,w) mid vin V_1, vin V_2, win W_1, win W_2 } \&= (V_1cap V_2)times (W_1cap W_2)
end{align*}
and this generalizes to arbitrary finite intersections. Now, we have
begin{align*}
pi_X(U)&=pi_Xleft(bigcup_{iin I}~ bigcap_{jin J_i} V_{ij}times W_{ij}right)
=bigcup_{iin I}~ pi_Xleft(left(bigcap_{jin J_i} V_{ij}right)times left(bigcap_{jin J_i} W_{ij}right)right)
= bigcup_{iin I}~ bigcap_{jin J_i} V_{ij} =: V
end{align*}
and $Vsubseteq X$ is open, because it is a union of finite intersection of open sets. Note for the first equality also that forming the image under any map commutes with unions.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Hi Jesko, nice answer, thanks. It is very helpful for a question I just asked.
    $endgroup$
    – WishingFish
    Jul 5 '13 at 6:05










  • $begingroup$
    But I couldn't follow the very last step - would you please tell me why we can assume $U = V times W$ given the previous construction? Thank you~
    $endgroup$
    – WishingFish
    Jul 5 '13 at 6:08






  • 1




    $begingroup$
    @WishingFish: See my edit. Hope this helps!
    $endgroup$
    – Jesko Hüttenhain
    Jul 5 '13 at 10:58






  • 1




    $begingroup$
    If $V_{ij}$ and $W_{ij}$ are arbitrary open, you don't need any intersections... (Intersection of rectangles is a rectangle.) And forming an image does not commute with intersections.
    $endgroup$
    – tomasz
    Jul 5 '13 at 11:02












  • $begingroup$
    Also, $pi_X(V_{ij} times W_{ij})$ equals $V_{ij}$ (in general) only in the case that $W_{ij}$ is inhabited. Else it is empty.
    $endgroup$
    – Ingo Blechschmidt
    May 22 '14 at 21:03



















14












$begingroup$

Some similar approach is the following: Let $pi_1 :X times Y to X$ be the projection and assume $U subset X times Y$ is open.



We must show that $pi_1(U)$ is open. For this let $x_0 in pi(U)$. Then $x_0 = pi(a_0,b_0)$ for some pair $(a_0,b_0) in U$. Since $(a_0,b_0) in U$ we can find two opens $a_0 in R$ and $b_0 in S$ with $R times S subset U$. That means $R subset pi_1(U)$ and we have $x_0 in R$.



Now, $pi_1(U)$ is a union of opens.






share|cite|improve this answer









$endgroup$





















    3












    $begingroup$

    I was working through this same problem and would like to share my solution since there are some issues with the other answer (and it wasn't accepted). Please feel free to point out any flaws, of course.



    Let $U$ be an open set in $Xtimes Y$. Then $U$ is a union of finite intersections of elements of
    $$ mathcal S = left{pi_1^{-1}(A) : Atext{ open in } Xright} cup left{pi_2^{-1}(B) : B text{ open in } Yright},$$
    that is,
    $$ U = bigcup_{alphain I}bigcap_{iin J_alpha} S_{alpha, i} $$
    where each $J_alpha$ is finite and each $S_{alpha, i}$ is in $mathcal S$. We can write each $S_{alpha,i}=pi_1^{-1}(V_{alpha,i})cappi_2^{-1}(W_{alpha,i})$, where each $V_{alpha, i}$ is open in $X$ and each $W_{alpha,i}$ is open in $Y$ (allowing for the possibility that $V_{alpha,i}=X$ or $W_{alpha,i}=Y$). As $$ pi_1^{-1}(V_{alpha,i})=V_{alpha,i}times Y text{ and } pi_2^{-1}(W_{alpha,i})=Xtimes W_{alpha,i}$$
    it follows that
    $$pi_1^{-1}(V_{alpha,i})cappi_2^{-1}(W_{alpha,i}) = (V_{alpha,i}times Y)cap (Xtimes W_{alpha,i}) = V_{alpha,i}times W_{alpha,i}.$$
    Letting $V_alpha=bigcap_{iin J_i} V_{alpha,i}$ and $W_alpha = bigcap_{iin J_i}W_{alpha,i}$, we have
    $$U = bigcup_{alphain I}bigcap_{iin J_i} V_{alpha,i}cap W_{alpha,i} = bigcup_{alphain I}V_alphatimes W_alpha,$$
    where each $V_alpha$ is open in $X$ and each $W_alpha$ is open in $Y$. It follows that
    $$ pi_1(U) = pi_1left(bigcup_{alphain I}V_alphatimes Walpha right) = bigcup_{alphain I}pi_1(V_alphatimes W_alpha) = bigcup_{alphain I'}V_alpha $$
    (where $I' = {alpha in I : W_alphanevarnothing}$) is open in $X$. We conclude that $pi_1$ is an open map.






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      Do you mean to say $$U=cup_{alpha in I} cap_{i in J_{alpha}}$$ and not $$U=cup_{alpha in I} cap_{i in J_{i}}$$?
      $endgroup$
      – Kam
      Dec 12 '18 at 11:10










    • $begingroup$
      @Kam That appears to have been a typo, thanks.
      $endgroup$
      – Math1000
      Dec 13 '18 at 14:58











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    3 Answers
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    3 Answers
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    26












    $begingroup$

    Let $Usubseteq Xtimes Y$ be open. Then, by definition of the product topology, $U$ is a union of finite intersections of sets of the form $pi_X^{-1}(V)=Vtimes Y$ and $pi_Y^{-1}(W)=Xtimes W$ for $Vsubseteq X$ and $Wsubseteq Y$ open. This means (in this case) that we may without loss of generality assume $U=Vtimes W$. Now, clearly, $pi_X(U)=V$ is open.



    Edit I will explain why I assume $U=Vtimes W$. In general, we know that $U=bigcup_{iin I} bigcap_{jin J_i} V_{ij}times W_{ij}$ with $I$ possibly infinite, each $J_i$ a finite set and $V_{ij}subseteq X$ as well as $W_{ij}subseteq Y$ open. Note that we have
    begin{align*}
    (V_1times W_1)cap (V_2times W_2) &= { (v,w) mid vin V_1, vin V_2, win W_1, win W_2 } \&= (V_1cap V_2)times (W_1cap W_2)
    end{align*}
    and this generalizes to arbitrary finite intersections. Now, we have
    begin{align*}
    pi_X(U)&=pi_Xleft(bigcup_{iin I}~ bigcap_{jin J_i} V_{ij}times W_{ij}right)
    =bigcup_{iin I}~ pi_Xleft(left(bigcap_{jin J_i} V_{ij}right)times left(bigcap_{jin J_i} W_{ij}right)right)
    = bigcup_{iin I}~ bigcap_{jin J_i} V_{ij} =: V
    end{align*}
    and $Vsubseteq X$ is open, because it is a union of finite intersection of open sets. Note for the first equality also that forming the image under any map commutes with unions.






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      Hi Jesko, nice answer, thanks. It is very helpful for a question I just asked.
      $endgroup$
      – WishingFish
      Jul 5 '13 at 6:05










    • $begingroup$
      But I couldn't follow the very last step - would you please tell me why we can assume $U = V times W$ given the previous construction? Thank you~
      $endgroup$
      – WishingFish
      Jul 5 '13 at 6:08






    • 1




      $begingroup$
      @WishingFish: See my edit. Hope this helps!
      $endgroup$
      – Jesko Hüttenhain
      Jul 5 '13 at 10:58






    • 1




      $begingroup$
      If $V_{ij}$ and $W_{ij}$ are arbitrary open, you don't need any intersections... (Intersection of rectangles is a rectangle.) And forming an image does not commute with intersections.
      $endgroup$
      – tomasz
      Jul 5 '13 at 11:02












    • $begingroup$
      Also, $pi_X(V_{ij} times W_{ij})$ equals $V_{ij}$ (in general) only in the case that $W_{ij}$ is inhabited. Else it is empty.
      $endgroup$
      – Ingo Blechschmidt
      May 22 '14 at 21:03
















    26












    $begingroup$

    Let $Usubseteq Xtimes Y$ be open. Then, by definition of the product topology, $U$ is a union of finite intersections of sets of the form $pi_X^{-1}(V)=Vtimes Y$ and $pi_Y^{-1}(W)=Xtimes W$ for $Vsubseteq X$ and $Wsubseteq Y$ open. This means (in this case) that we may without loss of generality assume $U=Vtimes W$. Now, clearly, $pi_X(U)=V$ is open.



    Edit I will explain why I assume $U=Vtimes W$. In general, we know that $U=bigcup_{iin I} bigcap_{jin J_i} V_{ij}times W_{ij}$ with $I$ possibly infinite, each $J_i$ a finite set and $V_{ij}subseteq X$ as well as $W_{ij}subseteq Y$ open. Note that we have
    begin{align*}
    (V_1times W_1)cap (V_2times W_2) &= { (v,w) mid vin V_1, vin V_2, win W_1, win W_2 } \&= (V_1cap V_2)times (W_1cap W_2)
    end{align*}
    and this generalizes to arbitrary finite intersections. Now, we have
    begin{align*}
    pi_X(U)&=pi_Xleft(bigcup_{iin I}~ bigcap_{jin J_i} V_{ij}times W_{ij}right)
    =bigcup_{iin I}~ pi_Xleft(left(bigcap_{jin J_i} V_{ij}right)times left(bigcap_{jin J_i} W_{ij}right)right)
    = bigcup_{iin I}~ bigcap_{jin J_i} V_{ij} =: V
    end{align*}
    and $Vsubseteq X$ is open, because it is a union of finite intersection of open sets. Note for the first equality also that forming the image under any map commutes with unions.






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      Hi Jesko, nice answer, thanks. It is very helpful for a question I just asked.
      $endgroup$
      – WishingFish
      Jul 5 '13 at 6:05










    • $begingroup$
      But I couldn't follow the very last step - would you please tell me why we can assume $U = V times W$ given the previous construction? Thank you~
      $endgroup$
      – WishingFish
      Jul 5 '13 at 6:08






    • 1




      $begingroup$
      @WishingFish: See my edit. Hope this helps!
      $endgroup$
      – Jesko Hüttenhain
      Jul 5 '13 at 10:58






    • 1




      $begingroup$
      If $V_{ij}$ and $W_{ij}$ are arbitrary open, you don't need any intersections... (Intersection of rectangles is a rectangle.) And forming an image does not commute with intersections.
      $endgroup$
      – tomasz
      Jul 5 '13 at 11:02












    • $begingroup$
      Also, $pi_X(V_{ij} times W_{ij})$ equals $V_{ij}$ (in general) only in the case that $W_{ij}$ is inhabited. Else it is empty.
      $endgroup$
      – Ingo Blechschmidt
      May 22 '14 at 21:03














    26












    26








    26





    $begingroup$

    Let $Usubseteq Xtimes Y$ be open. Then, by definition of the product topology, $U$ is a union of finite intersections of sets of the form $pi_X^{-1}(V)=Vtimes Y$ and $pi_Y^{-1}(W)=Xtimes W$ for $Vsubseteq X$ and $Wsubseteq Y$ open. This means (in this case) that we may without loss of generality assume $U=Vtimes W$. Now, clearly, $pi_X(U)=V$ is open.



    Edit I will explain why I assume $U=Vtimes W$. In general, we know that $U=bigcup_{iin I} bigcap_{jin J_i} V_{ij}times W_{ij}$ with $I$ possibly infinite, each $J_i$ a finite set and $V_{ij}subseteq X$ as well as $W_{ij}subseteq Y$ open. Note that we have
    begin{align*}
    (V_1times W_1)cap (V_2times W_2) &= { (v,w) mid vin V_1, vin V_2, win W_1, win W_2 } \&= (V_1cap V_2)times (W_1cap W_2)
    end{align*}
    and this generalizes to arbitrary finite intersections. Now, we have
    begin{align*}
    pi_X(U)&=pi_Xleft(bigcup_{iin I}~ bigcap_{jin J_i} V_{ij}times W_{ij}right)
    =bigcup_{iin I}~ pi_Xleft(left(bigcap_{jin J_i} V_{ij}right)times left(bigcap_{jin J_i} W_{ij}right)right)
    = bigcup_{iin I}~ bigcap_{jin J_i} V_{ij} =: V
    end{align*}
    and $Vsubseteq X$ is open, because it is a union of finite intersection of open sets. Note for the first equality also that forming the image under any map commutes with unions.






    share|cite|improve this answer











    $endgroup$



    Let $Usubseteq Xtimes Y$ be open. Then, by definition of the product topology, $U$ is a union of finite intersections of sets of the form $pi_X^{-1}(V)=Vtimes Y$ and $pi_Y^{-1}(W)=Xtimes W$ for $Vsubseteq X$ and $Wsubseteq Y$ open. This means (in this case) that we may without loss of generality assume $U=Vtimes W$. Now, clearly, $pi_X(U)=V$ is open.



    Edit I will explain why I assume $U=Vtimes W$. In general, we know that $U=bigcup_{iin I} bigcap_{jin J_i} V_{ij}times W_{ij}$ with $I$ possibly infinite, each $J_i$ a finite set and $V_{ij}subseteq X$ as well as $W_{ij}subseteq Y$ open. Note that we have
    begin{align*}
    (V_1times W_1)cap (V_2times W_2) &= { (v,w) mid vin V_1, vin V_2, win W_1, win W_2 } \&= (V_1cap V_2)times (W_1cap W_2)
    end{align*}
    and this generalizes to arbitrary finite intersections. Now, we have
    begin{align*}
    pi_X(U)&=pi_Xleft(bigcup_{iin I}~ bigcap_{jin J_i} V_{ij}times W_{ij}right)
    =bigcup_{iin I}~ pi_Xleft(left(bigcap_{jin J_i} V_{ij}right)times left(bigcap_{jin J_i} W_{ij}right)right)
    = bigcup_{iin I}~ bigcap_{jin J_i} V_{ij} =: V
    end{align*}
    and $Vsubseteq X$ is open, because it is a union of finite intersection of open sets. Note for the first equality also that forming the image under any map commutes with unions.







    share|cite|improve this answer














    share|cite|improve this answer



    share|cite|improve this answer








    edited Jul 5 '15 at 20:28

























    answered Nov 29 '12 at 20:39









    Jesko HüttenhainJesko Hüttenhain

    10.5k12356




    10.5k12356












    • $begingroup$
      Hi Jesko, nice answer, thanks. It is very helpful for a question I just asked.
      $endgroup$
      – WishingFish
      Jul 5 '13 at 6:05










    • $begingroup$
      But I couldn't follow the very last step - would you please tell me why we can assume $U = V times W$ given the previous construction? Thank you~
      $endgroup$
      – WishingFish
      Jul 5 '13 at 6:08






    • 1




      $begingroup$
      @WishingFish: See my edit. Hope this helps!
      $endgroup$
      – Jesko Hüttenhain
      Jul 5 '13 at 10:58






    • 1




      $begingroup$
      If $V_{ij}$ and $W_{ij}$ are arbitrary open, you don't need any intersections... (Intersection of rectangles is a rectangle.) And forming an image does not commute with intersections.
      $endgroup$
      – tomasz
      Jul 5 '13 at 11:02












    • $begingroup$
      Also, $pi_X(V_{ij} times W_{ij})$ equals $V_{ij}$ (in general) only in the case that $W_{ij}$ is inhabited. Else it is empty.
      $endgroup$
      – Ingo Blechschmidt
      May 22 '14 at 21:03


















    • $begingroup$
      Hi Jesko, nice answer, thanks. It is very helpful for a question I just asked.
      $endgroup$
      – WishingFish
      Jul 5 '13 at 6:05










    • $begingroup$
      But I couldn't follow the very last step - would you please tell me why we can assume $U = V times W$ given the previous construction? Thank you~
      $endgroup$
      – WishingFish
      Jul 5 '13 at 6:08






    • 1




      $begingroup$
      @WishingFish: See my edit. Hope this helps!
      $endgroup$
      – Jesko Hüttenhain
      Jul 5 '13 at 10:58






    • 1




      $begingroup$
      If $V_{ij}$ and $W_{ij}$ are arbitrary open, you don't need any intersections... (Intersection of rectangles is a rectangle.) And forming an image does not commute with intersections.
      $endgroup$
      – tomasz
      Jul 5 '13 at 11:02












    • $begingroup$
      Also, $pi_X(V_{ij} times W_{ij})$ equals $V_{ij}$ (in general) only in the case that $W_{ij}$ is inhabited. Else it is empty.
      $endgroup$
      – Ingo Blechschmidt
      May 22 '14 at 21:03
















    $begingroup$
    Hi Jesko, nice answer, thanks. It is very helpful for a question I just asked.
    $endgroup$
    – WishingFish
    Jul 5 '13 at 6:05




    $begingroup$
    Hi Jesko, nice answer, thanks. It is very helpful for a question I just asked.
    $endgroup$
    – WishingFish
    Jul 5 '13 at 6:05












    $begingroup$
    But I couldn't follow the very last step - would you please tell me why we can assume $U = V times W$ given the previous construction? Thank you~
    $endgroup$
    – WishingFish
    Jul 5 '13 at 6:08




    $begingroup$
    But I couldn't follow the very last step - would you please tell me why we can assume $U = V times W$ given the previous construction? Thank you~
    $endgroup$
    – WishingFish
    Jul 5 '13 at 6:08




    1




    1




    $begingroup$
    @WishingFish: See my edit. Hope this helps!
    $endgroup$
    – Jesko Hüttenhain
    Jul 5 '13 at 10:58




    $begingroup$
    @WishingFish: See my edit. Hope this helps!
    $endgroup$
    – Jesko Hüttenhain
    Jul 5 '13 at 10:58




    1




    1




    $begingroup$
    If $V_{ij}$ and $W_{ij}$ are arbitrary open, you don't need any intersections... (Intersection of rectangles is a rectangle.) And forming an image does not commute with intersections.
    $endgroup$
    – tomasz
    Jul 5 '13 at 11:02






    $begingroup$
    If $V_{ij}$ and $W_{ij}$ are arbitrary open, you don't need any intersections... (Intersection of rectangles is a rectangle.) And forming an image does not commute with intersections.
    $endgroup$
    – tomasz
    Jul 5 '13 at 11:02














    $begingroup$
    Also, $pi_X(V_{ij} times W_{ij})$ equals $V_{ij}$ (in general) only in the case that $W_{ij}$ is inhabited. Else it is empty.
    $endgroup$
    – Ingo Blechschmidt
    May 22 '14 at 21:03




    $begingroup$
    Also, $pi_X(V_{ij} times W_{ij})$ equals $V_{ij}$ (in general) only in the case that $W_{ij}$ is inhabited. Else it is empty.
    $endgroup$
    – Ingo Blechschmidt
    May 22 '14 at 21:03











    14












    $begingroup$

    Some similar approach is the following: Let $pi_1 :X times Y to X$ be the projection and assume $U subset X times Y$ is open.



    We must show that $pi_1(U)$ is open. For this let $x_0 in pi(U)$. Then $x_0 = pi(a_0,b_0)$ for some pair $(a_0,b_0) in U$. Since $(a_0,b_0) in U$ we can find two opens $a_0 in R$ and $b_0 in S$ with $R times S subset U$. That means $R subset pi_1(U)$ and we have $x_0 in R$.



    Now, $pi_1(U)$ is a union of opens.






    share|cite|improve this answer









    $endgroup$


















      14












      $begingroup$

      Some similar approach is the following: Let $pi_1 :X times Y to X$ be the projection and assume $U subset X times Y$ is open.



      We must show that $pi_1(U)$ is open. For this let $x_0 in pi(U)$. Then $x_0 = pi(a_0,b_0)$ for some pair $(a_0,b_0) in U$. Since $(a_0,b_0) in U$ we can find two opens $a_0 in R$ and $b_0 in S$ with $R times S subset U$. That means $R subset pi_1(U)$ and we have $x_0 in R$.



      Now, $pi_1(U)$ is a union of opens.






      share|cite|improve this answer









      $endgroup$
















        14












        14








        14





        $begingroup$

        Some similar approach is the following: Let $pi_1 :X times Y to X$ be the projection and assume $U subset X times Y$ is open.



        We must show that $pi_1(U)$ is open. For this let $x_0 in pi(U)$. Then $x_0 = pi(a_0,b_0)$ for some pair $(a_0,b_0) in U$. Since $(a_0,b_0) in U$ we can find two opens $a_0 in R$ and $b_0 in S$ with $R times S subset U$. That means $R subset pi_1(U)$ and we have $x_0 in R$.



        Now, $pi_1(U)$ is a union of opens.






        share|cite|improve this answer









        $endgroup$



        Some similar approach is the following: Let $pi_1 :X times Y to X$ be the projection and assume $U subset X times Y$ is open.



        We must show that $pi_1(U)$ is open. For this let $x_0 in pi(U)$. Then $x_0 = pi(a_0,b_0)$ for some pair $(a_0,b_0) in U$. Since $(a_0,b_0) in U$ we can find two opens $a_0 in R$ and $b_0 in S$ with $R times S subset U$. That means $R subset pi_1(U)$ and we have $x_0 in R$.



        Now, $pi_1(U)$ is a union of opens.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Oct 25 '14 at 17:37







        user42761






























            3












            $begingroup$

            I was working through this same problem and would like to share my solution since there are some issues with the other answer (and it wasn't accepted). Please feel free to point out any flaws, of course.



            Let $U$ be an open set in $Xtimes Y$. Then $U$ is a union of finite intersections of elements of
            $$ mathcal S = left{pi_1^{-1}(A) : Atext{ open in } Xright} cup left{pi_2^{-1}(B) : B text{ open in } Yright},$$
            that is,
            $$ U = bigcup_{alphain I}bigcap_{iin J_alpha} S_{alpha, i} $$
            where each $J_alpha$ is finite and each $S_{alpha, i}$ is in $mathcal S$. We can write each $S_{alpha,i}=pi_1^{-1}(V_{alpha,i})cappi_2^{-1}(W_{alpha,i})$, where each $V_{alpha, i}$ is open in $X$ and each $W_{alpha,i}$ is open in $Y$ (allowing for the possibility that $V_{alpha,i}=X$ or $W_{alpha,i}=Y$). As $$ pi_1^{-1}(V_{alpha,i})=V_{alpha,i}times Y text{ and } pi_2^{-1}(W_{alpha,i})=Xtimes W_{alpha,i}$$
            it follows that
            $$pi_1^{-1}(V_{alpha,i})cappi_2^{-1}(W_{alpha,i}) = (V_{alpha,i}times Y)cap (Xtimes W_{alpha,i}) = V_{alpha,i}times W_{alpha,i}.$$
            Letting $V_alpha=bigcap_{iin J_i} V_{alpha,i}$ and $W_alpha = bigcap_{iin J_i}W_{alpha,i}$, we have
            $$U = bigcup_{alphain I}bigcap_{iin J_i} V_{alpha,i}cap W_{alpha,i} = bigcup_{alphain I}V_alphatimes W_alpha,$$
            where each $V_alpha$ is open in $X$ and each $W_alpha$ is open in $Y$. It follows that
            $$ pi_1(U) = pi_1left(bigcup_{alphain I}V_alphatimes Walpha right) = bigcup_{alphain I}pi_1(V_alphatimes W_alpha) = bigcup_{alphain I'}V_alpha $$
            (where $I' = {alpha in I : W_alphanevarnothing}$) is open in $X$. We conclude that $pi_1$ is an open map.






            share|cite|improve this answer











            $endgroup$













            • $begingroup$
              Do you mean to say $$U=cup_{alpha in I} cap_{i in J_{alpha}}$$ and not $$U=cup_{alpha in I} cap_{i in J_{i}}$$?
              $endgroup$
              – Kam
              Dec 12 '18 at 11:10










            • $begingroup$
              @Kam That appears to have been a typo, thanks.
              $endgroup$
              – Math1000
              Dec 13 '18 at 14:58
















            3












            $begingroup$

            I was working through this same problem and would like to share my solution since there are some issues with the other answer (and it wasn't accepted). Please feel free to point out any flaws, of course.



            Let $U$ be an open set in $Xtimes Y$. Then $U$ is a union of finite intersections of elements of
            $$ mathcal S = left{pi_1^{-1}(A) : Atext{ open in } Xright} cup left{pi_2^{-1}(B) : B text{ open in } Yright},$$
            that is,
            $$ U = bigcup_{alphain I}bigcap_{iin J_alpha} S_{alpha, i} $$
            where each $J_alpha$ is finite and each $S_{alpha, i}$ is in $mathcal S$. We can write each $S_{alpha,i}=pi_1^{-1}(V_{alpha,i})cappi_2^{-1}(W_{alpha,i})$, where each $V_{alpha, i}$ is open in $X$ and each $W_{alpha,i}$ is open in $Y$ (allowing for the possibility that $V_{alpha,i}=X$ or $W_{alpha,i}=Y$). As $$ pi_1^{-1}(V_{alpha,i})=V_{alpha,i}times Y text{ and } pi_2^{-1}(W_{alpha,i})=Xtimes W_{alpha,i}$$
            it follows that
            $$pi_1^{-1}(V_{alpha,i})cappi_2^{-1}(W_{alpha,i}) = (V_{alpha,i}times Y)cap (Xtimes W_{alpha,i}) = V_{alpha,i}times W_{alpha,i}.$$
            Letting $V_alpha=bigcap_{iin J_i} V_{alpha,i}$ and $W_alpha = bigcap_{iin J_i}W_{alpha,i}$, we have
            $$U = bigcup_{alphain I}bigcap_{iin J_i} V_{alpha,i}cap W_{alpha,i} = bigcup_{alphain I}V_alphatimes W_alpha,$$
            where each $V_alpha$ is open in $X$ and each $W_alpha$ is open in $Y$. It follows that
            $$ pi_1(U) = pi_1left(bigcup_{alphain I}V_alphatimes Walpha right) = bigcup_{alphain I}pi_1(V_alphatimes W_alpha) = bigcup_{alphain I'}V_alpha $$
            (where $I' = {alpha in I : W_alphanevarnothing}$) is open in $X$. We conclude that $pi_1$ is an open map.






            share|cite|improve this answer











            $endgroup$













            • $begingroup$
              Do you mean to say $$U=cup_{alpha in I} cap_{i in J_{alpha}}$$ and not $$U=cup_{alpha in I} cap_{i in J_{i}}$$?
              $endgroup$
              – Kam
              Dec 12 '18 at 11:10










            • $begingroup$
              @Kam That appears to have been a typo, thanks.
              $endgroup$
              – Math1000
              Dec 13 '18 at 14:58














            3












            3








            3





            $begingroup$

            I was working through this same problem and would like to share my solution since there are some issues with the other answer (and it wasn't accepted). Please feel free to point out any flaws, of course.



            Let $U$ be an open set in $Xtimes Y$. Then $U$ is a union of finite intersections of elements of
            $$ mathcal S = left{pi_1^{-1}(A) : Atext{ open in } Xright} cup left{pi_2^{-1}(B) : B text{ open in } Yright},$$
            that is,
            $$ U = bigcup_{alphain I}bigcap_{iin J_alpha} S_{alpha, i} $$
            where each $J_alpha$ is finite and each $S_{alpha, i}$ is in $mathcal S$. We can write each $S_{alpha,i}=pi_1^{-1}(V_{alpha,i})cappi_2^{-1}(W_{alpha,i})$, where each $V_{alpha, i}$ is open in $X$ and each $W_{alpha,i}$ is open in $Y$ (allowing for the possibility that $V_{alpha,i}=X$ or $W_{alpha,i}=Y$). As $$ pi_1^{-1}(V_{alpha,i})=V_{alpha,i}times Y text{ and } pi_2^{-1}(W_{alpha,i})=Xtimes W_{alpha,i}$$
            it follows that
            $$pi_1^{-1}(V_{alpha,i})cappi_2^{-1}(W_{alpha,i}) = (V_{alpha,i}times Y)cap (Xtimes W_{alpha,i}) = V_{alpha,i}times W_{alpha,i}.$$
            Letting $V_alpha=bigcap_{iin J_i} V_{alpha,i}$ and $W_alpha = bigcap_{iin J_i}W_{alpha,i}$, we have
            $$U = bigcup_{alphain I}bigcap_{iin J_i} V_{alpha,i}cap W_{alpha,i} = bigcup_{alphain I}V_alphatimes W_alpha,$$
            where each $V_alpha$ is open in $X$ and each $W_alpha$ is open in $Y$. It follows that
            $$ pi_1(U) = pi_1left(bigcup_{alphain I}V_alphatimes Walpha right) = bigcup_{alphain I}pi_1(V_alphatimes W_alpha) = bigcup_{alphain I'}V_alpha $$
            (where $I' = {alpha in I : W_alphanevarnothing}$) is open in $X$. We conclude that $pi_1$ is an open map.






            share|cite|improve this answer











            $endgroup$



            I was working through this same problem and would like to share my solution since there are some issues with the other answer (and it wasn't accepted). Please feel free to point out any flaws, of course.



            Let $U$ be an open set in $Xtimes Y$. Then $U$ is a union of finite intersections of elements of
            $$ mathcal S = left{pi_1^{-1}(A) : Atext{ open in } Xright} cup left{pi_2^{-1}(B) : B text{ open in } Yright},$$
            that is,
            $$ U = bigcup_{alphain I}bigcap_{iin J_alpha} S_{alpha, i} $$
            where each $J_alpha$ is finite and each $S_{alpha, i}$ is in $mathcal S$. We can write each $S_{alpha,i}=pi_1^{-1}(V_{alpha,i})cappi_2^{-1}(W_{alpha,i})$, where each $V_{alpha, i}$ is open in $X$ and each $W_{alpha,i}$ is open in $Y$ (allowing for the possibility that $V_{alpha,i}=X$ or $W_{alpha,i}=Y$). As $$ pi_1^{-1}(V_{alpha,i})=V_{alpha,i}times Y text{ and } pi_2^{-1}(W_{alpha,i})=Xtimes W_{alpha,i}$$
            it follows that
            $$pi_1^{-1}(V_{alpha,i})cappi_2^{-1}(W_{alpha,i}) = (V_{alpha,i}times Y)cap (Xtimes W_{alpha,i}) = V_{alpha,i}times W_{alpha,i}.$$
            Letting $V_alpha=bigcap_{iin J_i} V_{alpha,i}$ and $W_alpha = bigcap_{iin J_i}W_{alpha,i}$, we have
            $$U = bigcup_{alphain I}bigcap_{iin J_i} V_{alpha,i}cap W_{alpha,i} = bigcup_{alphain I}V_alphatimes W_alpha,$$
            where each $V_alpha$ is open in $X$ and each $W_alpha$ is open in $Y$. It follows that
            $$ pi_1(U) = pi_1left(bigcup_{alphain I}V_alphatimes Walpha right) = bigcup_{alphain I}pi_1(V_alphatimes W_alpha) = bigcup_{alphain I'}V_alpha $$
            (where $I' = {alpha in I : W_alphanevarnothing}$) is open in $X$. We conclude that $pi_1$ is an open map.







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited Dec 13 '18 at 14:58

























            answered Oct 17 '14 at 23:53









            Math1000Math1000

            19.1k31745




            19.1k31745












            • $begingroup$
              Do you mean to say $$U=cup_{alpha in I} cap_{i in J_{alpha}}$$ and not $$U=cup_{alpha in I} cap_{i in J_{i}}$$?
              $endgroup$
              – Kam
              Dec 12 '18 at 11:10










            • $begingroup$
              @Kam That appears to have been a typo, thanks.
              $endgroup$
              – Math1000
              Dec 13 '18 at 14:58


















            • $begingroup$
              Do you mean to say $$U=cup_{alpha in I} cap_{i in J_{alpha}}$$ and not $$U=cup_{alpha in I} cap_{i in J_{i}}$$?
              $endgroup$
              – Kam
              Dec 12 '18 at 11:10










            • $begingroup$
              @Kam That appears to have been a typo, thanks.
              $endgroup$
              – Math1000
              Dec 13 '18 at 14:58
















            $begingroup$
            Do you mean to say $$U=cup_{alpha in I} cap_{i in J_{alpha}}$$ and not $$U=cup_{alpha in I} cap_{i in J_{i}}$$?
            $endgroup$
            – Kam
            Dec 12 '18 at 11:10




            $begingroup$
            Do you mean to say $$U=cup_{alpha in I} cap_{i in J_{alpha}}$$ and not $$U=cup_{alpha in I} cap_{i in J_{i}}$$?
            $endgroup$
            – Kam
            Dec 12 '18 at 11:10












            $begingroup$
            @Kam That appears to have been a typo, thanks.
            $endgroup$
            – Math1000
            Dec 13 '18 at 14:58




            $begingroup$
            @Kam That appears to have been a typo, thanks.
            $endgroup$
            – Math1000
            Dec 13 '18 at 14:58


















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