Effect of a Lie group morphism associated to a Lie algebra morphism to wedge product












0












$begingroup$


I currently struggeling with the last exercise on my assignment:



Fix $omegainbigwedge^3(mathbb R^n)^*$. Let $G$ be a Lie group, $rhocolon Gto GL(n,mathbb R)$ a Lie group morphism such that
$$
omega(rho(g)w_1, rho(g)w_2, rho(g)w_3) = omega(w_1,w_2,w_3)
$$

for all $gin G$ and $w_iinmathbb R^n$. Now we take the associated Lie algebra morphism $R = (rho)_{*e} colon mathfrak{g}to Mat(n,mathbb R)$.




Show that
$$
omega(R(v)w_1,w_2,w_3)+omega(w_1,R(v)w_2,w_3)+omega(w_1,w_2,R(v)w_3) = 0
$$

for all $vinmathfrak g$ and $w_iinmathbb R^n$.






It looks to me like somehow differentiating the first equation and ending up with the second but to apply the exterior derivative of a k-form we need another representation.
Another thought was using the relation
$$
omega(rho(g)w_1, rho(g)w_2, rho(g)w_3) = det(rho(g))omega(w_1,w_2,w_3)
$$

thus we see that $det(rho(g))=1$ by the first equation and we get $R(v)$ has trace zero.



Any help would be very much appreciated. :-)










share|cite|improve this question









$endgroup$

















    0












    $begingroup$


    I currently struggeling with the last exercise on my assignment:



    Fix $omegainbigwedge^3(mathbb R^n)^*$. Let $G$ be a Lie group, $rhocolon Gto GL(n,mathbb R)$ a Lie group morphism such that
    $$
    omega(rho(g)w_1, rho(g)w_2, rho(g)w_3) = omega(w_1,w_2,w_3)
    $$

    for all $gin G$ and $w_iinmathbb R^n$. Now we take the associated Lie algebra morphism $R = (rho)_{*e} colon mathfrak{g}to Mat(n,mathbb R)$.




    Show that
    $$
    omega(R(v)w_1,w_2,w_3)+omega(w_1,R(v)w_2,w_3)+omega(w_1,w_2,R(v)w_3) = 0
    $$

    for all $vinmathfrak g$ and $w_iinmathbb R^n$.






    It looks to me like somehow differentiating the first equation and ending up with the second but to apply the exterior derivative of a k-form we need another representation.
    Another thought was using the relation
    $$
    omega(rho(g)w_1, rho(g)w_2, rho(g)w_3) = det(rho(g))omega(w_1,w_2,w_3)
    $$

    thus we see that $det(rho(g))=1$ by the first equation and we get $R(v)$ has trace zero.



    Any help would be very much appreciated. :-)










    share|cite|improve this question









    $endgroup$















      0












      0








      0





      $begingroup$


      I currently struggeling with the last exercise on my assignment:



      Fix $omegainbigwedge^3(mathbb R^n)^*$. Let $G$ be a Lie group, $rhocolon Gto GL(n,mathbb R)$ a Lie group morphism such that
      $$
      omega(rho(g)w_1, rho(g)w_2, rho(g)w_3) = omega(w_1,w_2,w_3)
      $$

      for all $gin G$ and $w_iinmathbb R^n$. Now we take the associated Lie algebra morphism $R = (rho)_{*e} colon mathfrak{g}to Mat(n,mathbb R)$.




      Show that
      $$
      omega(R(v)w_1,w_2,w_3)+omega(w_1,R(v)w_2,w_3)+omega(w_1,w_2,R(v)w_3) = 0
      $$

      for all $vinmathfrak g$ and $w_iinmathbb R^n$.






      It looks to me like somehow differentiating the first equation and ending up with the second but to apply the exterior derivative of a k-form we need another representation.
      Another thought was using the relation
      $$
      omega(rho(g)w_1, rho(g)w_2, rho(g)w_3) = det(rho(g))omega(w_1,w_2,w_3)
      $$

      thus we see that $det(rho(g))=1$ by the first equation and we get $R(v)$ has trace zero.



      Any help would be very much appreciated. :-)










      share|cite|improve this question









      $endgroup$




      I currently struggeling with the last exercise on my assignment:



      Fix $omegainbigwedge^3(mathbb R^n)^*$. Let $G$ be a Lie group, $rhocolon Gto GL(n,mathbb R)$ a Lie group morphism such that
      $$
      omega(rho(g)w_1, rho(g)w_2, rho(g)w_3) = omega(w_1,w_2,w_3)
      $$

      for all $gin G$ and $w_iinmathbb R^n$. Now we take the associated Lie algebra morphism $R = (rho)_{*e} colon mathfrak{g}to Mat(n,mathbb R)$.




      Show that
      $$
      omega(R(v)w_1,w_2,w_3)+omega(w_1,R(v)w_2,w_3)+omega(w_1,w_2,R(v)w_3) = 0
      $$

      for all $vinmathfrak g$ and $w_iinmathbb R^n$.






      It looks to me like somehow differentiating the first equation and ending up with the second but to apply the exterior derivative of a k-form we need another representation.
      Another thought was using the relation
      $$
      omega(rho(g)w_1, rho(g)w_2, rho(g)w_3) = det(rho(g))omega(w_1,w_2,w_3)
      $$

      thus we see that $det(rho(g))=1$ by the first equation and we get $R(v)$ has trace zero.



      Any help would be very much appreciated. :-)







      differential-geometry lie-groups lie-algebras






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      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked Dec 26 '18 at 15:41









      flaxflax

      32




      32






















          1 Answer
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          $begingroup$

          If $vinmathfrak g$ and $tinmathbb R$, then$$omegabigl(rho(e^{tv})w_1,rho(e^{tv})w_2,rho(e^{tv})w_3bigr)=omega(w_1,w_2,w_3).$$Differentiating both sides at $t=0$, you get$$omegabigl(R(v)w_1,w_2,w_3bigr)+omegabigl(w_1,R(v)w_2,w_3bigr)+omegabigl(w_1,w_2,R(v)w_3bigr)=0.$$






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            This already helped a lot, thanks! But I have two little questions: For your first equation there has to be a $vinmathfrak g$ for every $gin G$ such that $$ rho(g) = R(e^{tv}). $$ I could see why there exist a matrix $Ainmathbb R^{ntimes n}$ for every $Bin GL(n,mathbb R)$ such that $B=e^{A}$ but why does it still hold for the morphisms? For the differentiation I need $$ frac{mathrm d}{mathrm dt}|_{t=0} R(mathrm e^{tv}) = R(frac{mathrm d}{mathrm dt}|_{t=0} mathrm e^{tv}). $$ I think there was a property which said that morphisms and the differentiation commute?
            $endgroup$
            – flax
            Dec 26 '18 at 17:39












          • $begingroup$
            Concerning your first question, all that I need (and all that I used) was that$$(forall vinmathfrak{g})(forall tinmathbb{R}):e^{iv}in G.$$Now use the fact that$$left.frac{mathrm d}{mathrm dt}right|_{t=0}e^{tv}=v$$together with the chain rule.
            $endgroup$
            – José Carlos Santos
            Dec 26 '18 at 19:21










          • $begingroup$
            I think my main problem was the $e^{tv}$-notion because I have just a theorem which says that there exists a curve with the same properties but everything works now. Thank you very much! :-)
            $endgroup$
            – flax
            Dec 27 '18 at 10:18










          • $begingroup$
            Unless you think that I am asking too much, perhaps that you could upvote my answer.
            $endgroup$
            – José Carlos Santos
            Dec 27 '18 at 10:26










          • $begingroup$
            Hehe, I already did but apparently I don't have enough reputation for it to take effect. But it said as soon as I have enough the well-earned upvote will be counted. :-)
            $endgroup$
            – flax
            Dec 27 '18 at 10:30












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          $begingroup$

          If $vinmathfrak g$ and $tinmathbb R$, then$$omegabigl(rho(e^{tv})w_1,rho(e^{tv})w_2,rho(e^{tv})w_3bigr)=omega(w_1,w_2,w_3).$$Differentiating both sides at $t=0$, you get$$omegabigl(R(v)w_1,w_2,w_3bigr)+omegabigl(w_1,R(v)w_2,w_3bigr)+omegabigl(w_1,w_2,R(v)w_3bigr)=0.$$






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            This already helped a lot, thanks! But I have two little questions: For your first equation there has to be a $vinmathfrak g$ for every $gin G$ such that $$ rho(g) = R(e^{tv}). $$ I could see why there exist a matrix $Ainmathbb R^{ntimes n}$ for every $Bin GL(n,mathbb R)$ such that $B=e^{A}$ but why does it still hold for the morphisms? For the differentiation I need $$ frac{mathrm d}{mathrm dt}|_{t=0} R(mathrm e^{tv}) = R(frac{mathrm d}{mathrm dt}|_{t=0} mathrm e^{tv}). $$ I think there was a property which said that morphisms and the differentiation commute?
            $endgroup$
            – flax
            Dec 26 '18 at 17:39












          • $begingroup$
            Concerning your first question, all that I need (and all that I used) was that$$(forall vinmathfrak{g})(forall tinmathbb{R}):e^{iv}in G.$$Now use the fact that$$left.frac{mathrm d}{mathrm dt}right|_{t=0}e^{tv}=v$$together with the chain rule.
            $endgroup$
            – José Carlos Santos
            Dec 26 '18 at 19:21










          • $begingroup$
            I think my main problem was the $e^{tv}$-notion because I have just a theorem which says that there exists a curve with the same properties but everything works now. Thank you very much! :-)
            $endgroup$
            – flax
            Dec 27 '18 at 10:18










          • $begingroup$
            Unless you think that I am asking too much, perhaps that you could upvote my answer.
            $endgroup$
            – José Carlos Santos
            Dec 27 '18 at 10:26










          • $begingroup$
            Hehe, I already did but apparently I don't have enough reputation for it to take effect. But it said as soon as I have enough the well-earned upvote will be counted. :-)
            $endgroup$
            – flax
            Dec 27 '18 at 10:30
















          2












          $begingroup$

          If $vinmathfrak g$ and $tinmathbb R$, then$$omegabigl(rho(e^{tv})w_1,rho(e^{tv})w_2,rho(e^{tv})w_3bigr)=omega(w_1,w_2,w_3).$$Differentiating both sides at $t=0$, you get$$omegabigl(R(v)w_1,w_2,w_3bigr)+omegabigl(w_1,R(v)w_2,w_3bigr)+omegabigl(w_1,w_2,R(v)w_3bigr)=0.$$






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            This already helped a lot, thanks! But I have two little questions: For your first equation there has to be a $vinmathfrak g$ for every $gin G$ such that $$ rho(g) = R(e^{tv}). $$ I could see why there exist a matrix $Ainmathbb R^{ntimes n}$ for every $Bin GL(n,mathbb R)$ such that $B=e^{A}$ but why does it still hold for the morphisms? For the differentiation I need $$ frac{mathrm d}{mathrm dt}|_{t=0} R(mathrm e^{tv}) = R(frac{mathrm d}{mathrm dt}|_{t=0} mathrm e^{tv}). $$ I think there was a property which said that morphisms and the differentiation commute?
            $endgroup$
            – flax
            Dec 26 '18 at 17:39












          • $begingroup$
            Concerning your first question, all that I need (and all that I used) was that$$(forall vinmathfrak{g})(forall tinmathbb{R}):e^{iv}in G.$$Now use the fact that$$left.frac{mathrm d}{mathrm dt}right|_{t=0}e^{tv}=v$$together with the chain rule.
            $endgroup$
            – José Carlos Santos
            Dec 26 '18 at 19:21










          • $begingroup$
            I think my main problem was the $e^{tv}$-notion because I have just a theorem which says that there exists a curve with the same properties but everything works now. Thank you very much! :-)
            $endgroup$
            – flax
            Dec 27 '18 at 10:18










          • $begingroup$
            Unless you think that I am asking too much, perhaps that you could upvote my answer.
            $endgroup$
            – José Carlos Santos
            Dec 27 '18 at 10:26










          • $begingroup$
            Hehe, I already did but apparently I don't have enough reputation for it to take effect. But it said as soon as I have enough the well-earned upvote will be counted. :-)
            $endgroup$
            – flax
            Dec 27 '18 at 10:30














          2












          2








          2





          $begingroup$

          If $vinmathfrak g$ and $tinmathbb R$, then$$omegabigl(rho(e^{tv})w_1,rho(e^{tv})w_2,rho(e^{tv})w_3bigr)=omega(w_1,w_2,w_3).$$Differentiating both sides at $t=0$, you get$$omegabigl(R(v)w_1,w_2,w_3bigr)+omegabigl(w_1,R(v)w_2,w_3bigr)+omegabigl(w_1,w_2,R(v)w_3bigr)=0.$$






          share|cite|improve this answer











          $endgroup$



          If $vinmathfrak g$ and $tinmathbb R$, then$$omegabigl(rho(e^{tv})w_1,rho(e^{tv})w_2,rho(e^{tv})w_3bigr)=omega(w_1,w_2,w_3).$$Differentiating both sides at $t=0$, you get$$omegabigl(R(v)w_1,w_2,w_3bigr)+omegabigl(w_1,R(v)w_2,w_3bigr)+omegabigl(w_1,w_2,R(v)w_3bigr)=0.$$







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Jan 4 at 14:30

























          answered Dec 26 '18 at 15:51









          José Carlos SantosJosé Carlos Santos

          174k23133242




          174k23133242












          • $begingroup$
            This already helped a lot, thanks! But I have two little questions: For your first equation there has to be a $vinmathfrak g$ for every $gin G$ such that $$ rho(g) = R(e^{tv}). $$ I could see why there exist a matrix $Ainmathbb R^{ntimes n}$ for every $Bin GL(n,mathbb R)$ such that $B=e^{A}$ but why does it still hold for the morphisms? For the differentiation I need $$ frac{mathrm d}{mathrm dt}|_{t=0} R(mathrm e^{tv}) = R(frac{mathrm d}{mathrm dt}|_{t=0} mathrm e^{tv}). $$ I think there was a property which said that morphisms and the differentiation commute?
            $endgroup$
            – flax
            Dec 26 '18 at 17:39












          • $begingroup$
            Concerning your first question, all that I need (and all that I used) was that$$(forall vinmathfrak{g})(forall tinmathbb{R}):e^{iv}in G.$$Now use the fact that$$left.frac{mathrm d}{mathrm dt}right|_{t=0}e^{tv}=v$$together with the chain rule.
            $endgroup$
            – José Carlos Santos
            Dec 26 '18 at 19:21










          • $begingroup$
            I think my main problem was the $e^{tv}$-notion because I have just a theorem which says that there exists a curve with the same properties but everything works now. Thank you very much! :-)
            $endgroup$
            – flax
            Dec 27 '18 at 10:18










          • $begingroup$
            Unless you think that I am asking too much, perhaps that you could upvote my answer.
            $endgroup$
            – José Carlos Santos
            Dec 27 '18 at 10:26










          • $begingroup$
            Hehe, I already did but apparently I don't have enough reputation for it to take effect. But it said as soon as I have enough the well-earned upvote will be counted. :-)
            $endgroup$
            – flax
            Dec 27 '18 at 10:30


















          • $begingroup$
            This already helped a lot, thanks! But I have two little questions: For your first equation there has to be a $vinmathfrak g$ for every $gin G$ such that $$ rho(g) = R(e^{tv}). $$ I could see why there exist a matrix $Ainmathbb R^{ntimes n}$ for every $Bin GL(n,mathbb R)$ such that $B=e^{A}$ but why does it still hold for the morphisms? For the differentiation I need $$ frac{mathrm d}{mathrm dt}|_{t=0} R(mathrm e^{tv}) = R(frac{mathrm d}{mathrm dt}|_{t=0} mathrm e^{tv}). $$ I think there was a property which said that morphisms and the differentiation commute?
            $endgroup$
            – flax
            Dec 26 '18 at 17:39












          • $begingroup$
            Concerning your first question, all that I need (and all that I used) was that$$(forall vinmathfrak{g})(forall tinmathbb{R}):e^{iv}in G.$$Now use the fact that$$left.frac{mathrm d}{mathrm dt}right|_{t=0}e^{tv}=v$$together with the chain rule.
            $endgroup$
            – José Carlos Santos
            Dec 26 '18 at 19:21










          • $begingroup$
            I think my main problem was the $e^{tv}$-notion because I have just a theorem which says that there exists a curve with the same properties but everything works now. Thank you very much! :-)
            $endgroup$
            – flax
            Dec 27 '18 at 10:18










          • $begingroup$
            Unless you think that I am asking too much, perhaps that you could upvote my answer.
            $endgroup$
            – José Carlos Santos
            Dec 27 '18 at 10:26










          • $begingroup$
            Hehe, I already did but apparently I don't have enough reputation for it to take effect. But it said as soon as I have enough the well-earned upvote will be counted. :-)
            $endgroup$
            – flax
            Dec 27 '18 at 10:30
















          $begingroup$
          This already helped a lot, thanks! But I have two little questions: For your first equation there has to be a $vinmathfrak g$ for every $gin G$ such that $$ rho(g) = R(e^{tv}). $$ I could see why there exist a matrix $Ainmathbb R^{ntimes n}$ for every $Bin GL(n,mathbb R)$ such that $B=e^{A}$ but why does it still hold for the morphisms? For the differentiation I need $$ frac{mathrm d}{mathrm dt}|_{t=0} R(mathrm e^{tv}) = R(frac{mathrm d}{mathrm dt}|_{t=0} mathrm e^{tv}). $$ I think there was a property which said that morphisms and the differentiation commute?
          $endgroup$
          – flax
          Dec 26 '18 at 17:39






          $begingroup$
          This already helped a lot, thanks! But I have two little questions: For your first equation there has to be a $vinmathfrak g$ for every $gin G$ such that $$ rho(g) = R(e^{tv}). $$ I could see why there exist a matrix $Ainmathbb R^{ntimes n}$ for every $Bin GL(n,mathbb R)$ such that $B=e^{A}$ but why does it still hold for the morphisms? For the differentiation I need $$ frac{mathrm d}{mathrm dt}|_{t=0} R(mathrm e^{tv}) = R(frac{mathrm d}{mathrm dt}|_{t=0} mathrm e^{tv}). $$ I think there was a property which said that morphisms and the differentiation commute?
          $endgroup$
          – flax
          Dec 26 '18 at 17:39














          $begingroup$
          Concerning your first question, all that I need (and all that I used) was that$$(forall vinmathfrak{g})(forall tinmathbb{R}):e^{iv}in G.$$Now use the fact that$$left.frac{mathrm d}{mathrm dt}right|_{t=0}e^{tv}=v$$together with the chain rule.
          $endgroup$
          – José Carlos Santos
          Dec 26 '18 at 19:21




          $begingroup$
          Concerning your first question, all that I need (and all that I used) was that$$(forall vinmathfrak{g})(forall tinmathbb{R}):e^{iv}in G.$$Now use the fact that$$left.frac{mathrm d}{mathrm dt}right|_{t=0}e^{tv}=v$$together with the chain rule.
          $endgroup$
          – José Carlos Santos
          Dec 26 '18 at 19:21












          $begingroup$
          I think my main problem was the $e^{tv}$-notion because I have just a theorem which says that there exists a curve with the same properties but everything works now. Thank you very much! :-)
          $endgroup$
          – flax
          Dec 27 '18 at 10:18




          $begingroup$
          I think my main problem was the $e^{tv}$-notion because I have just a theorem which says that there exists a curve with the same properties but everything works now. Thank you very much! :-)
          $endgroup$
          – flax
          Dec 27 '18 at 10:18












          $begingroup$
          Unless you think that I am asking too much, perhaps that you could upvote my answer.
          $endgroup$
          – José Carlos Santos
          Dec 27 '18 at 10:26




          $begingroup$
          Unless you think that I am asking too much, perhaps that you could upvote my answer.
          $endgroup$
          – José Carlos Santos
          Dec 27 '18 at 10:26












          $begingroup$
          Hehe, I already did but apparently I don't have enough reputation for it to take effect. But it said as soon as I have enough the well-earned upvote will be counted. :-)
          $endgroup$
          – flax
          Dec 27 '18 at 10:30




          $begingroup$
          Hehe, I already did but apparently I don't have enough reputation for it to take effect. But it said as soon as I have enough the well-earned upvote will be counted. :-)
          $endgroup$
          – flax
          Dec 27 '18 at 10:30


















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