Does every Riemannian manifold has a local orthonormal divergence free frame of vector fields?











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Let $(M,g)$ be a smooth Riemannian manifold, and let $p in M$.




Is there an open neighbourhood $U$ of $p$ that admits an orthonormal frame of divergence-free vector fields?




Edit: At least for dimension $2$, $M$ admits such a local frame if and only if it's flat (See a proof below). I am not sure about higher dimensions.



Edit 2:



It seems this question is addressed here. However, I don't understand the "counting" of the number of equations and variables:




Robert Bryant writes that this is a system of $n$ first-order PDE for $tfrac12n(n{-}1)$ unknowns. I think that the following argument explains this:




The orthonormality conditions $langle X_i,X_j rangle=delta_{ij}$ form $tfrac12n(n{+}1)$ equations, and $text{div}(X_i)=0$ form additional $n$ equations. So, together we have $tfrac12n(n{+}1)+n$ equations in $n^2$ variables, when we express each $X_i$ in terms of some fixed arbitrary frame.



However, we can reduce the system by restricting the discussion in advance to orthonormal frames:



Indeed, given some fixe orthonormal frame $E_i$, we can represent any other orthonormal frame on $U$ as $QE_i$ where $Q:U to text{O}(n)$ is a smooth map. So, we have an "$ text{O}(n)$" freedom to choose orthonormal frames; More explicitly, since $ dim(text{O}(n))=tfrac12n(n{-}1)$, we can take $Q(p)=text{Id}$ and for $q in U$, $Q(q)$ can be expressed in terms of $tfrac12n(n{-}1)$ functions (If $U$ is small enough so $Q(U)$ is contained in a coordinate chart around $text{Id}$).





A proof for the $2$D case:



Suppose that $X,Y$ are orthonormal and divergence-free. Then
$$ langle X,X rangle=1 Rightarrow langle nabla_YX,X rangle=langle nabla_XX,X rangle=0. tag{1}$$



The divergence-free condition means that
$$ 0=text{div}X=text{trace}(nabla X)=langle nabla_XX,X rangle+langle nabla_YX,Y rangle=langle nabla_YX,Y rangle. tag{2}$$



Combining equations $(1),(2)$ we see that
$$ langle nabla_YX,X rangle=langle nabla_YX,Y rangle=0$$



so $nabla_YX=0$. By symmetry, we also have $nabla_XY=0$, so the symmetry of the connection implies $[X,Y]=0$. This in turn implies $X,Y$ can be realized as coordinate vector fields, but since they are orthonormal this means the metric is flat.



Alternatively, we can proceed from $nabla_YX=nabla_XY=0$ as follows:



Differentiating $langle X,Y rangle=0$ via $X$, we get



$$ 0=langle nabla_XX,Y rangle+langle X,nabla_XY rangle=langle nabla_XX,Y rangle. tag{3}$$



Combining this with $langle nabla_XX,X rangle=0$ (see equation $(1)$ again) we deduce $nabla_XX=0$, which together with $nabla_YX=0$ implies $X$ is parallel. By symmetry, $Y$ is also parallel, so we have a parallel frame for $(TM,nabla)$ which implies $g$ is flat.





Comment: We always have a local orthonormal frame;



Furthermore, there are always divergence-free frames: Indeed, every volume form can be locally written as $dx^1 wedge dots wedge dx^n$, for some coordinates $x_i$. The divergence w.r.t this form is the standard one, i.e. if $V=v^ipartial_i$, then $text{div}V=partial_i v^i$, so in particular the coordinate frame $partial_i$ form a divergence-free frame.



We can apply the Gram-Schmidt process on $partial_i$, but I see now reason why the "divergence-free" property should be preserved.





By divergence of a vector field $X$, I refer to the Riemannian notion:



$text{div} X= text{trace}(nabla X)$, where $nabla$ is the Levi-Civita connection of $(M,g)$. Alternatively, $text{div} X=0 iff L_Xtext{Vol}_g=0$ where $text{Vol}_g$ is the Riemannian volume form of $(M,g)$.










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    Let $(M,g)$ be a smooth Riemannian manifold, and let $p in M$.




    Is there an open neighbourhood $U$ of $p$ that admits an orthonormal frame of divergence-free vector fields?




    Edit: At least for dimension $2$, $M$ admits such a local frame if and only if it's flat (See a proof below). I am not sure about higher dimensions.



    Edit 2:



    It seems this question is addressed here. However, I don't understand the "counting" of the number of equations and variables:




    Robert Bryant writes that this is a system of $n$ first-order PDE for $tfrac12n(n{-}1)$ unknowns. I think that the following argument explains this:




    The orthonormality conditions $langle X_i,X_j rangle=delta_{ij}$ form $tfrac12n(n{+}1)$ equations, and $text{div}(X_i)=0$ form additional $n$ equations. So, together we have $tfrac12n(n{+}1)+n$ equations in $n^2$ variables, when we express each $X_i$ in terms of some fixed arbitrary frame.



    However, we can reduce the system by restricting the discussion in advance to orthonormal frames:



    Indeed, given some fixe orthonormal frame $E_i$, we can represent any other orthonormal frame on $U$ as $QE_i$ where $Q:U to text{O}(n)$ is a smooth map. So, we have an "$ text{O}(n)$" freedom to choose orthonormal frames; More explicitly, since $ dim(text{O}(n))=tfrac12n(n{-}1)$, we can take $Q(p)=text{Id}$ and for $q in U$, $Q(q)$ can be expressed in terms of $tfrac12n(n{-}1)$ functions (If $U$ is small enough so $Q(U)$ is contained in a coordinate chart around $text{Id}$).





    A proof for the $2$D case:



    Suppose that $X,Y$ are orthonormal and divergence-free. Then
    $$ langle X,X rangle=1 Rightarrow langle nabla_YX,X rangle=langle nabla_XX,X rangle=0. tag{1}$$



    The divergence-free condition means that
    $$ 0=text{div}X=text{trace}(nabla X)=langle nabla_XX,X rangle+langle nabla_YX,Y rangle=langle nabla_YX,Y rangle. tag{2}$$



    Combining equations $(1),(2)$ we see that
    $$ langle nabla_YX,X rangle=langle nabla_YX,Y rangle=0$$



    so $nabla_YX=0$. By symmetry, we also have $nabla_XY=0$, so the symmetry of the connection implies $[X,Y]=0$. This in turn implies $X,Y$ can be realized as coordinate vector fields, but since they are orthonormal this means the metric is flat.



    Alternatively, we can proceed from $nabla_YX=nabla_XY=0$ as follows:



    Differentiating $langle X,Y rangle=0$ via $X$, we get



    $$ 0=langle nabla_XX,Y rangle+langle X,nabla_XY rangle=langle nabla_XX,Y rangle. tag{3}$$



    Combining this with $langle nabla_XX,X rangle=0$ (see equation $(1)$ again) we deduce $nabla_XX=0$, which together with $nabla_YX=0$ implies $X$ is parallel. By symmetry, $Y$ is also parallel, so we have a parallel frame for $(TM,nabla)$ which implies $g$ is flat.





    Comment: We always have a local orthonormal frame;



    Furthermore, there are always divergence-free frames: Indeed, every volume form can be locally written as $dx^1 wedge dots wedge dx^n$, for some coordinates $x_i$. The divergence w.r.t this form is the standard one, i.e. if $V=v^ipartial_i$, then $text{div}V=partial_i v^i$, so in particular the coordinate frame $partial_i$ form a divergence-free frame.



    We can apply the Gram-Schmidt process on $partial_i$, but I see now reason why the "divergence-free" property should be preserved.





    By divergence of a vector field $X$, I refer to the Riemannian notion:



    $text{div} X= text{trace}(nabla X)$, where $nabla$ is the Levi-Civita connection of $(M,g)$. Alternatively, $text{div} X=0 iff L_Xtext{Vol}_g=0$ where $text{Vol}_g$ is the Riemannian volume form of $(M,g)$.










    share|cite|improve this question


























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      Let $(M,g)$ be a smooth Riemannian manifold, and let $p in M$.




      Is there an open neighbourhood $U$ of $p$ that admits an orthonormal frame of divergence-free vector fields?




      Edit: At least for dimension $2$, $M$ admits such a local frame if and only if it's flat (See a proof below). I am not sure about higher dimensions.



      Edit 2:



      It seems this question is addressed here. However, I don't understand the "counting" of the number of equations and variables:




      Robert Bryant writes that this is a system of $n$ first-order PDE for $tfrac12n(n{-}1)$ unknowns. I think that the following argument explains this:




      The orthonormality conditions $langle X_i,X_j rangle=delta_{ij}$ form $tfrac12n(n{+}1)$ equations, and $text{div}(X_i)=0$ form additional $n$ equations. So, together we have $tfrac12n(n{+}1)+n$ equations in $n^2$ variables, when we express each $X_i$ in terms of some fixed arbitrary frame.



      However, we can reduce the system by restricting the discussion in advance to orthonormal frames:



      Indeed, given some fixe orthonormal frame $E_i$, we can represent any other orthonormal frame on $U$ as $QE_i$ where $Q:U to text{O}(n)$ is a smooth map. So, we have an "$ text{O}(n)$" freedom to choose orthonormal frames; More explicitly, since $ dim(text{O}(n))=tfrac12n(n{-}1)$, we can take $Q(p)=text{Id}$ and for $q in U$, $Q(q)$ can be expressed in terms of $tfrac12n(n{-}1)$ functions (If $U$ is small enough so $Q(U)$ is contained in a coordinate chart around $text{Id}$).





      A proof for the $2$D case:



      Suppose that $X,Y$ are orthonormal and divergence-free. Then
      $$ langle X,X rangle=1 Rightarrow langle nabla_YX,X rangle=langle nabla_XX,X rangle=0. tag{1}$$



      The divergence-free condition means that
      $$ 0=text{div}X=text{trace}(nabla X)=langle nabla_XX,X rangle+langle nabla_YX,Y rangle=langle nabla_YX,Y rangle. tag{2}$$



      Combining equations $(1),(2)$ we see that
      $$ langle nabla_YX,X rangle=langle nabla_YX,Y rangle=0$$



      so $nabla_YX=0$. By symmetry, we also have $nabla_XY=0$, so the symmetry of the connection implies $[X,Y]=0$. This in turn implies $X,Y$ can be realized as coordinate vector fields, but since they are orthonormal this means the metric is flat.



      Alternatively, we can proceed from $nabla_YX=nabla_XY=0$ as follows:



      Differentiating $langle X,Y rangle=0$ via $X$, we get



      $$ 0=langle nabla_XX,Y rangle+langle X,nabla_XY rangle=langle nabla_XX,Y rangle. tag{3}$$



      Combining this with $langle nabla_XX,X rangle=0$ (see equation $(1)$ again) we deduce $nabla_XX=0$, which together with $nabla_YX=0$ implies $X$ is parallel. By symmetry, $Y$ is also parallel, so we have a parallel frame for $(TM,nabla)$ which implies $g$ is flat.





      Comment: We always have a local orthonormal frame;



      Furthermore, there are always divergence-free frames: Indeed, every volume form can be locally written as $dx^1 wedge dots wedge dx^n$, for some coordinates $x_i$. The divergence w.r.t this form is the standard one, i.e. if $V=v^ipartial_i$, then $text{div}V=partial_i v^i$, so in particular the coordinate frame $partial_i$ form a divergence-free frame.



      We can apply the Gram-Schmidt process on $partial_i$, but I see now reason why the "divergence-free" property should be preserved.





      By divergence of a vector field $X$, I refer to the Riemannian notion:



      $text{div} X= text{trace}(nabla X)$, where $nabla$ is the Levi-Civita connection of $(M,g)$. Alternatively, $text{div} X=0 iff L_Xtext{Vol}_g=0$ where $text{Vol}_g$ is the Riemannian volume form of $(M,g)$.










      share|cite|improve this question















      Let $(M,g)$ be a smooth Riemannian manifold, and let $p in M$.




      Is there an open neighbourhood $U$ of $p$ that admits an orthonormal frame of divergence-free vector fields?




      Edit: At least for dimension $2$, $M$ admits such a local frame if and only if it's flat (See a proof below). I am not sure about higher dimensions.



      Edit 2:



      It seems this question is addressed here. However, I don't understand the "counting" of the number of equations and variables:




      Robert Bryant writes that this is a system of $n$ first-order PDE for $tfrac12n(n{-}1)$ unknowns. I think that the following argument explains this:




      The orthonormality conditions $langle X_i,X_j rangle=delta_{ij}$ form $tfrac12n(n{+}1)$ equations, and $text{div}(X_i)=0$ form additional $n$ equations. So, together we have $tfrac12n(n{+}1)+n$ equations in $n^2$ variables, when we express each $X_i$ in terms of some fixed arbitrary frame.



      However, we can reduce the system by restricting the discussion in advance to orthonormal frames:



      Indeed, given some fixe orthonormal frame $E_i$, we can represent any other orthonormal frame on $U$ as $QE_i$ where $Q:U to text{O}(n)$ is a smooth map. So, we have an "$ text{O}(n)$" freedom to choose orthonormal frames; More explicitly, since $ dim(text{O}(n))=tfrac12n(n{-}1)$, we can take $Q(p)=text{Id}$ and for $q in U$, $Q(q)$ can be expressed in terms of $tfrac12n(n{-}1)$ functions (If $U$ is small enough so $Q(U)$ is contained in a coordinate chart around $text{Id}$).





      A proof for the $2$D case:



      Suppose that $X,Y$ are orthonormal and divergence-free. Then
      $$ langle X,X rangle=1 Rightarrow langle nabla_YX,X rangle=langle nabla_XX,X rangle=0. tag{1}$$



      The divergence-free condition means that
      $$ 0=text{div}X=text{trace}(nabla X)=langle nabla_XX,X rangle+langle nabla_YX,Y rangle=langle nabla_YX,Y rangle. tag{2}$$



      Combining equations $(1),(2)$ we see that
      $$ langle nabla_YX,X rangle=langle nabla_YX,Y rangle=0$$



      so $nabla_YX=0$. By symmetry, we also have $nabla_XY=0$, so the symmetry of the connection implies $[X,Y]=0$. This in turn implies $X,Y$ can be realized as coordinate vector fields, but since they are orthonormal this means the metric is flat.



      Alternatively, we can proceed from $nabla_YX=nabla_XY=0$ as follows:



      Differentiating $langle X,Y rangle=0$ via $X$, we get



      $$ 0=langle nabla_XX,Y rangle+langle X,nabla_XY rangle=langle nabla_XX,Y rangle. tag{3}$$



      Combining this with $langle nabla_XX,X rangle=0$ (see equation $(1)$ again) we deduce $nabla_XX=0$, which together with $nabla_YX=0$ implies $X$ is parallel. By symmetry, $Y$ is also parallel, so we have a parallel frame for $(TM,nabla)$ which implies $g$ is flat.





      Comment: We always have a local orthonormal frame;



      Furthermore, there are always divergence-free frames: Indeed, every volume form can be locally written as $dx^1 wedge dots wedge dx^n$, for some coordinates $x_i$. The divergence w.r.t this form is the standard one, i.e. if $V=v^ipartial_i$, then $text{div}V=partial_i v^i$, so in particular the coordinate frame $partial_i$ form a divergence-free frame.



      We can apply the Gram-Schmidt process on $partial_i$, but I see now reason why the "divergence-free" property should be preserved.





      By divergence of a vector field $X$, I refer to the Riemannian notion:



      $text{div} X= text{trace}(nabla X)$, where $nabla$ is the Levi-Civita connection of $(M,g)$. Alternatively, $text{div} X=0 iff L_Xtext{Vol}_g=0$ where $text{Vol}_g$ is the Riemannian volume form of $(M,g)$.







      differential-geometry riemannian-geometry divergence






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