Skip to main content
changed traceless to symmetric, a correction
Source Link
Ben McKay
  • 26.3k
  • 7
  • 67
  • 102

The space of affine connections is an affine space, since any two affine connections, say with Christoffel symbols $\Gamma^i_{jk}$ and $\bar\Gamma^i_{jk}$ differ by the difference of their Christoffel symbols $\Gamma^i_{jk}-\bar\Gamma^i_{jk}=a^i_{jk}$, so the difference determines a tensor $a^i_{jk}dx^j dx^k\partial_i$. If they are torsion free this tensor will be tracelesssymmetric. So both the moduli space of affine connections and of affine torsion free connections are affine spaces, modelled on $T^* \otimes T^* \otimes T$ and $S^2T^* \otimes T$ respectively.

Every affine connection has the same unparameterized geodesics as its associated torsion free connection with symmetrized Christoffel symbols $\bar\Gamma^i_{jk}=(1/2)(\Gamma^i_{jk}+\Gamma^i_{kj})$ (proof: write out the geodesic equations). Every torsion free connection with Christoffel symbols $\Gamma^i_{jk}$ has the same unparameterized geodesics as another torsion free connection just when the second has Christoffel symbols $\Gamma^i_{jk}+a_j\delta^i_k+a_k\delta^i_j$ for any $1$-form $a_idx^i$, in any local coordinates. So the space of normal projective connections is the quotient space of the space of affine connections, which we can assume to be torsion free, by the space of $1$-forms. If they are torsion free with the same geodesics, it will be of this form $a^i_{jk}=a_j\delta^i_k+a_k\delta^i_j$. So the space of normal projective connections is therefore an affine space modelled on the quotient space, i.e. on the affine space of traceless symmetric tensors $a^i_{jk}dx^j dx^k\partial_i$, $a^i_{jk}=a^i_{kj}$, $a^i_{ik}=0$, which we can denote $T \otimes_0 S^2 T^*$, or something like that. This discussion works the same for the real normal projective connections and for the holomorphic ones, so we see that the moduli space of holomorphic normal projective connections on any compact complex manifold is finite dimensional, modelled on the vector space $H^0(M,T \otimes_0 S^2 T^*)$.

On a complex manifold, a holomorphic affine connection exists just when the Atiyah class of the tangent bundle vanishes. Less obviously: a holomorphic projective connection exists just when a normal holomorphic projective connection exists, both just when the tracefree part of the Atiyah class of the tangent bundle vanishes.

The space of affine connections is an affine space, since any two affine connections, say with Christoffel symbols $\Gamma^i_{jk}$ and $\bar\Gamma^i_{jk}$ differ by the difference of their Christoffel symbols $\Gamma^i_{jk}-\bar\Gamma^i_{jk}=a^i_{jk}$, so the difference determines a tensor $a^i_{jk}dx^j dx^k\partial_i$. If they are torsion free this tensor will be traceless. So both the moduli space of affine connections and of affine torsion free connections are affine spaces, modelled on $T^* \otimes T^* \otimes T$ and $S^2T^* \otimes T$ respectively.

Every affine connection has the same unparameterized geodesics as its associated torsion free connection with symmetrized Christoffel symbols $\bar\Gamma^i_{jk}=(1/2)(\Gamma^i_{jk}+\Gamma^i_{kj})$ (proof: write out the geodesic equations). Every torsion free connection with Christoffel symbols $\Gamma^i_{jk}$ has the same unparameterized geodesics as another torsion free connection just when the second has Christoffel symbols $\Gamma^i_{jk}+a_j\delta^i_k+a_k\delta^i_j$ for any $1$-form $a_idx^i$, in any local coordinates. So the space of normal projective connections is the quotient space of the space of affine connections, which we can assume to be torsion free, by the space of $1$-forms. If they are torsion free with the same geodesics, it will be of this form $a^i_{jk}=a_j\delta^i_k+a_k\delta^i_j$. So the space of normal projective connections is therefore an affine space modelled on the quotient space, i.e. on the affine space of traceless symmetric tensors $a^i_{jk}dx^j dx^k\partial_i$, $a^i_{jk}=a^i_{kj}$, $a^i_{ik}=0$, which we can denote $T \otimes_0 S^2 T^*$, or something like that. This discussion works the same for the real normal projective connections and for the holomorphic ones, so we see that the moduli space of holomorphic normal projective connections on any compact complex manifold is finite dimensional, modelled on the vector space $H^0(M,T \otimes_0 S^2 T^*)$.

On a complex manifold, a holomorphic affine connection exists just when the Atiyah class of the tangent bundle vanishes. Less obviously: a holomorphic projective connection exists just when a normal holomorphic projective connection exists, both just when the tracefree part of the Atiyah class of the tangent bundle vanishes.

The space of affine connections is an affine space, since any two affine connections, say with Christoffel symbols $\Gamma^i_{jk}$ and $\bar\Gamma^i_{jk}$ differ by the difference of their Christoffel symbols $\Gamma^i_{jk}-\bar\Gamma^i_{jk}=a^i_{jk}$, so the difference determines a tensor $a^i_{jk}dx^j dx^k\partial_i$. If they are torsion free this tensor will be symmetric. So both the moduli space of affine connections and of affine torsion free connections are affine spaces, modelled on $T^* \otimes T^* \otimes T$ and $S^2T^* \otimes T$ respectively.

Every affine connection has the same unparameterized geodesics as its associated torsion free connection with symmetrized Christoffel symbols $\bar\Gamma^i_{jk}=(1/2)(\Gamma^i_{jk}+\Gamma^i_{kj})$ (proof: write out the geodesic equations). Every torsion free connection with Christoffel symbols $\Gamma^i_{jk}$ has the same unparameterized geodesics as another torsion free connection just when the second has Christoffel symbols $\Gamma^i_{jk}+a_j\delta^i_k+a_k\delta^i_j$ for any $1$-form $a_idx^i$, in any local coordinates. So the space of normal projective connections is the quotient space of the space of affine connections, which we can assume to be torsion free, by the space of $1$-forms. If they are torsion free with the same geodesics, it will be of this form $a^i_{jk}=a_j\delta^i_k+a_k\delta^i_j$. So the space of normal projective connections is therefore an affine space modelled on the quotient space, i.e. on the affine space of traceless symmetric tensors $a^i_{jk}dx^j dx^k\partial_i$, $a^i_{jk}=a^i_{kj}$, $a^i_{ik}=0$, which we can denote $T \otimes_0 S^2 T^*$, or something like that. This discussion works the same for the real normal projective connections and for the holomorphic ones, so we see that the moduli space of holomorphic normal projective connections on any compact complex manifold is finite dimensional, modelled on the vector space $H^0(M,T \otimes_0 S^2 T^*)$.

On a complex manifold, a holomorphic affine connection exists just when the Atiyah class of the tangent bundle vanishes. Less obviously: a holomorphic projective connection exists just when a normal holomorphic projective connection exists, both just when the tracefree part of the Atiyah class of the tangent bundle vanishes.

discussed the existence and Atiyah class
Source Link
Ben McKay
  • 26.3k
  • 7
  • 67
  • 102

The space of affine connections is an affine space, since any two affine connections, say with Christoffel symbols $\Gamma^i_{jk}$ and $\bar\Gamma^i_{jk}$ differ by the difference of their Christoffel symbols $\Gamma^i_{jk}-\bar\Gamma^i_{jk}=a^i_{jk}$, so the difference determines a tensor $a^i_{jk}dx^j dx^k\partial_i$. If they are torsion free this tensor will be traceless. So both the moduli space of affine connections and of affine torsion free connections are affine spaces, modelled on $T^* \otimes T^* \otimes T$ and $S^2T^* \otimes T$ respectively.

Every affine connection has the same unparameterized geodesics as its associated torsion free connection with symmetrized Christoffel symbols $\bar\Gamma^i_{jk}=(1/2)(\Gamma^i_{jk}+\Gamma^i_{kj})$ (proof: write out the geodesic equations). Every torsion free connection with Christoffel symbols $\Gamma^i_{jk}$ has the same unparameterized geodesics as another torsion free connection just when the second has Christoffel symbols $\Gamma^i_{jk}+a_j\delta^i_k+a_k\delta^i_j$ for any $1$-form $a_idx^i$, in any local coordinates. So the space of normal projective connections is the quotient space of the space of affine connections, which we can assume to be torsion free, by the space of $1$-forms. If they are torsion free with the same geodesics, it will be of this form $a^i_{jk}=a_j\delta^i_k+a_k\delta^i_j$. So the space of normal projective connections is therefore an affine space modelled on the quotient space, i.e. on the affine space of traceless symmetric tensors $a^i_{jk}dx^j dx^k\partial_i$, $a^i_{jk}=a^i_{kj}$, $a^i_{ik}=0$, which we can denote $T \otimes_0 S^2 T^*$, or something like that. This discussion works the same for the real normal projective connections and for the holomorphic ones, so we see that the moduli space of holomorphic normal projective connections on any compact complex manifold is finite dimensional, modelled on the vector space $H^0(M,T \otimes_0 S^2 T^*)$.

On a complex manifold, a holomorphic affine connection exists just when the Atiyah class of the tangent bundle vanishes. Less obviously: a holomorphic projective connection exists just when a normal holomorphic projective connection exists, both just when the tracefree part of the Atiyah class of the tangent bundle vanishes.

The space of affine connections is an affine space, since any two affine connections, say with Christoffel symbols $\Gamma^i_{jk}$ and $\bar\Gamma^i_{jk}$ differ by the difference of their Christoffel symbols $\Gamma^i_{jk}-\bar\Gamma^i_{jk}=a^i_{jk}$, so the difference determines a tensor $a^i_{jk}dx^j dx^k\partial_i$. If they are torsion free this tensor will be traceless. So both the moduli space of affine connections and of affine torsion free connections are affine spaces, modelled on $T^* \otimes T^* \otimes T$ and $S^2T^* \otimes T$ respectively.

Every affine connection has the same unparameterized geodesics as its associated torsion free connection with symmetrized Christoffel symbols $\bar\Gamma^i_{jk}=(1/2)(\Gamma^i_{jk}+\Gamma^i_{kj})$. Every torsion free connection with Christoffel symbols $\Gamma^i_{jk}$ has the same unparameterized geodesics as another torsion free connection just when the second has Christoffel symbols $\Gamma^i_{jk}+a_j\delta^i_k+a_k\delta^i_j$ for any $1$-form $a_idx^i$, in any local coordinates. So the space of normal projective connections is the quotient space of the space of affine connections, which we can assume to be torsion free, by the space of $1$-forms. If they are torsion free with the same geodesics, it will be of this form $a^i_{jk}=a_j\delta^i_k+a_k\delta^i_j$. So the space of normal projective connections is therefore an affine space modelled on the quotient space, i.e. on the affine space of traceless symmetric tensors $a^i_{jk}dx^j dx^k\partial_i$, $a^i_{jk}=a^i_{kj}$, $a^i_{ik}=0$, which we can denote $T \otimes_0 S^2 T^*$, or something like that. This discussion works the same for the real normal projective connections and for the holomorphic ones, so we see that the moduli space of holomorphic normal projective connections on any compact complex manifold is finite dimensional, modelled on the vector space $H^0(M,T \otimes_0 S^2 T^*)$.

The space of affine connections is an affine space, since any two affine connections, say with Christoffel symbols $\Gamma^i_{jk}$ and $\bar\Gamma^i_{jk}$ differ by the difference of their Christoffel symbols $\Gamma^i_{jk}-\bar\Gamma^i_{jk}=a^i_{jk}$, so the difference determines a tensor $a^i_{jk}dx^j dx^k\partial_i$. If they are torsion free this tensor will be traceless. So both the moduli space of affine connections and of affine torsion free connections are affine spaces, modelled on $T^* \otimes T^* \otimes T$ and $S^2T^* \otimes T$ respectively.

Every affine connection has the same unparameterized geodesics as its associated torsion free connection with symmetrized Christoffel symbols $\bar\Gamma^i_{jk}=(1/2)(\Gamma^i_{jk}+\Gamma^i_{kj})$ (proof: write out the geodesic equations). Every torsion free connection with Christoffel symbols $\Gamma^i_{jk}$ has the same unparameterized geodesics as another torsion free connection just when the second has Christoffel symbols $\Gamma^i_{jk}+a_j\delta^i_k+a_k\delta^i_j$ for any $1$-form $a_idx^i$, in any local coordinates. So the space of normal projective connections is the quotient space of the space of affine connections, which we can assume to be torsion free, by the space of $1$-forms. If they are torsion free with the same geodesics, it will be of this form $a^i_{jk}=a_j\delta^i_k+a_k\delta^i_j$. So the space of normal projective connections is therefore an affine space modelled on the quotient space, i.e. on the affine space of traceless symmetric tensors $a^i_{jk}dx^j dx^k\partial_i$, $a^i_{jk}=a^i_{kj}$, $a^i_{ik}=0$, which we can denote $T \otimes_0 S^2 T^*$, or something like that. This discussion works the same for the real normal projective connections and for the holomorphic ones, so we see that the moduli space of holomorphic normal projective connections on any compact complex manifold is finite dimensional, modelled on the vector space $H^0(M,T \otimes_0 S^2 T^*)$.

On a complex manifold, a holomorphic affine connection exists just when the Atiyah class of the tangent bundle vanishes. Less obviously: a holomorphic projective connection exists just when a normal holomorphic projective connection exists, both just when the tracefree part of the Atiyah class of the tangent bundle vanishes.

Source Link
Ben McKay
  • 26.3k
  • 7
  • 67
  • 102

The space of affine connections is an affine space, since any two affine connections, say with Christoffel symbols $\Gamma^i_{jk}$ and $\bar\Gamma^i_{jk}$ differ by the difference of their Christoffel symbols $\Gamma^i_{jk}-\bar\Gamma^i_{jk}=a^i_{jk}$, so the difference determines a tensor $a^i_{jk}dx^j dx^k\partial_i$. If they are torsion free this tensor will be traceless. So both the moduli space of affine connections and of affine torsion free connections are affine spaces, modelled on $T^* \otimes T^* \otimes T$ and $S^2T^* \otimes T$ respectively.

Every affine connection has the same unparameterized geodesics as its associated torsion free connection with symmetrized Christoffel symbols $\bar\Gamma^i_{jk}=(1/2)(\Gamma^i_{jk}+\Gamma^i_{kj})$. Every torsion free connection with Christoffel symbols $\Gamma^i_{jk}$ has the same unparameterized geodesics as another torsion free connection just when the second has Christoffel symbols $\Gamma^i_{jk}+a_j\delta^i_k+a_k\delta^i_j$ for any $1$-form $a_idx^i$, in any local coordinates. So the space of normal projective connections is the quotient space of the space of affine connections, which we can assume to be torsion free, by the space of $1$-forms. If they are torsion free with the same geodesics, it will be of this form $a^i_{jk}=a_j\delta^i_k+a_k\delta^i_j$. So the space of normal projective connections is therefore an affine space modelled on the quotient space, i.e. on the affine space of traceless symmetric tensors $a^i_{jk}dx^j dx^k\partial_i$, $a^i_{jk}=a^i_{kj}$, $a^i_{ik}=0$, which we can denote $T \otimes_0 S^2 T^*$, or something like that. This discussion works the same for the real normal projective connections and for the holomorphic ones, so we see that the moduli space of holomorphic normal projective connections on any compact complex manifold is finite dimensional, modelled on the vector space $H^0(M,T \otimes_0 S^2 T^*)$.