Chern connection

The Chern connection is the unique connection on a Hermitian holomorphic bundle compatible with both its metric and holomorphic structure. Its curvature has type $(1,1)$.

For a Hermitian metric on a complex manifold, the Chern connection on its holomorphic tangent bundle corresponds to a real metric-compatible connection preserving the almost complex structure. Its mixed torsion vanishes, and in holomorphic coordinates its remaining coefficients are ${\Gamma }_{ij}^k=h^{k\bar{l}}{\partial }_i{h}_{j\bar{l}}$. Symmetry in $i,j$ is equivalent to $\partial \omega =0$; its conjugate gives $d\omega =0$. Thus zero torsion is exactly the Kähler metric condition, and then uniqueness identifies this connection with the Levi-Civita connection.

The vector-bundle curvature of the unique metric-compatible holomorphic Chern connection is an endomorphism-valued form of type $(1,1)$. It is Dolbeault closed and represents the curvature Dolbeault class. Its trace gives normalized first-Chern forms, while its full endomorphism action enters the Lefschetz-Dolbeault curvature commutator.

For the $(1,0)$ component of a Chern connection, $(D_E^{\prime }{)}^{\ast }=-{\ast }_{E^{\ast }}{D}_{E^{\ast }}^{\prime }{\ast }_E$, where the middle operator is the corresponding component of the dual Chern connection. This is the holomorphic-degree counterpart of the bundle-valued Dolbeault adjoint.

In a holomorphic local frame the Chern matrix is $A=H^{-1}\partial H$. The identity $\partial A+A\wedge A=0$ gives vector-bundle curvature $\Theta =\bar{\partial }A$ of type $(1,1)$. Therefore ${\bar{\partial }}_{\mathrm{End}E}\Theta =0$, allowing the vector-bundle curvature to represent a bundle-valued Dolbeault class.

In a holomorphic local frame, a Hermitian metric on a holomorphic vector bundle has matrix $h$. The trace of its Chern connection curvature is $\bar{\partial }\partial \log \det h$. A frame change multiplies $\det h$ by the squared modulus of a holomorphic unit, whose logarithm has vanishing mixed derivative. For two metrics, the quotient of their determinants is a global positive function, and the trace curvatures differ by the exterior derivative of its logarithmic $\partial$-derivative.

If $S$ is a holomorphic subbundle of a Hermitian vector bundle $E$, its induced Chern connection is $D_S={\pi }_S{D}_E$ on sections of $S$. Orthogonal projection preserves metric compatibility, and holomorphicity of $S$ preserves the required $(0,1)$ operator. Uniqueness of the Chern connection therefore identifies the projected connection.

For a holomorphic subbundle $S\subset E$ and quotient $Q=E/S$, the second fundamental form is $A(s)=(I-{\pi }_S)D_{E}s$. The Leibniz rule gives $A(fs)=fA(s)$, and holomorphicity of the subbundle makes its $(0,1)$ part vanish. Thus $A\in {\mathcal{A}}^{1,0}(\mathrm{Hom}(S,Q))$: the form factor is a cotangent bundle, not a tangent bundle.

In an adapted smooth unitary frame with input-index connection convention, the Chern connection matrix has blocks $({\theta }_F,B;-{\overline{B}}^t,{\theta }_Q)$, where the second fundamental form of a holomorphic subbundle $B$ has type $(1,0)$. The Cartan curvature equation with input indices gives the displayed formula. If the ambient curvature is zero, ${\Theta }_F(X,\overline{X})=-B(X)B(X{)}^{†}$ is negative semidefinite. Dualizing the determinant line bundle therefore gives semipositive curvature.

For a holomorphic local frame $e$ with $h_e=h(e,e)>0$, this form isIt is a closed real (1, 1)-form representing the First Chern class. A positive Hermitian metric makes it a positive real (1, 1)-form. Replacing $h$ by $e^{-u}h$ adds $(i/(2\pi ))\partial \bar{\partial }u$ to the form.

In a holomorphic local frame of a Hermitian holomorphic vector bundle, let $H$ be the matrix of its Hermitian metric on a holomorphic vector bundle. With the convention $\nabla e=eA$, the Chern connection hasThe condition ${\nabla }^{0,1}={\bar{\partial }}_E$ forces $A$ to have type $(1,0)$, while metric compatibility forces this formula, proving both existence and uniqueness.

At any point of a Hermitian holomorphic vector bundle one may choose a holomorphic local frame with $H=I$ and $\partial H=0$. First normalize $H$ by a constant frame change, then prescribe a holomorphic matrix's first jet to cancel $H^{-1}\partial H$. The Chern matrix vanishes there. This frame reduces first-order bundle identities at that point to scalar identities.

Let $D_1$ and $D_2$ be Chern connections on the same holomorphic vector bundle, possibly for different Hermitian metrics, and put $a=D_1-D_2$. Then $a$ has type $(1,0)$ andIndeed, the curvature difference formula has only types $(2,0)$ and $(1,1)$; both Chern curvatures have type $(1,1)$, so the $(2,0)$ part vanishes and the remaining part is $D_2^{0,1}a={\bar{\partial }}_{\mathrm{End}E}a$.

Let $e$ be a holomorphic local frame of a Hermitian holomorphic line bundle and put $h=h(e,e)$. The Chern connection and its curvature are locallyThus $F_{\nabla }$ has type $(1,1)$. Because the connection is unitary, ${\bar{F}}_{\nabla }=-F_{\nabla }$, so $i{F}_{\nabla }$ is a real (1, 1)-form.

The Ricci form is the real curvature form $i{F}_{\nabla }$ of the Chern connection on $T^{1,0}X$. In complex dimension one, $\rho =K\omega =\frac{1}{2}\mathrm{Scal}\omega$.