“List of formulas in Riemannian geometry”的意思、由来-开放百科全书

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List of formulas in Riemannian geometry

释义

Christoffel symbols, covariant derivative
Curvature tensors
Riemann curvature tensor Ricci and scalar curvatures Einstein tensor Weyl tensor
Gradient, divergence, Laplace–Beltrami operator
Kulkarni–Nomizu product
In an inertial frame
Under a conformal change
See also

This is a list of formulas encountered in Riemannian geometry.

Christoffel symbols, covariant derivative

In a smooth coordinate chart, the Christoffel symbols of the first kind are given by

and the Christoffel symbols of the second kind by

Here is the inverse matrix to the metric tensor . In other words,

and thus

is the dimension of the manifold.

Christoffel symbols satisfy the symmetry relations

or, respectively, ,

the second of which is equivalent to the torsion-freeness of the Levi-Civita connection.

The contracting relations on the Christoffel symbols are given by

and

where |g| is the absolute value of the determinant of the metric tensor . These are useful when dealing with divergences and Laplacians (see below).

The covariant derivative of a vector field with components is given by:

and similarly the covariant derivative of a -tensor field with components is given by:

For a -tensor field with components this becomes

and likewise for tensors with more indices.

The covariant derivative of a function (scalar) is just its usual differential:

Because the Levi-Civita connection is metric-compatible, the covariant derivatives of metrics vanish,

as well as the covariant derivatives of the metric's determinant (and volume element)

The geodesic starting at the origin with initial speed has Taylor expansion in the chart:

Curvature tensors

Riemann curvature tensor

If one defines the curvature operator as

and the coordinate components of the -Riemann curvature tensor by , then these components are given by:

Lowering indices with one gets

The symmetries of the tensor are

and

That is, it is symmetric in the exchange of the first and last pair of indices, and antisymmetric in the flipping of a pair.

The cyclic permutation sum (sometimes called first Bianchi identity) is

The (second) Bianchi identity is

that is,

which amounts to a cyclic permutation sum of the last three indices, leaving the first two fixed.

Ricci and scalar curvatures

Ricci and scalar curvatures are contractions of the Riemann tensor. They simplify the Riemann tensor, but contain less information.

The Ricci curvature tensor is essentially the unique (up to sign) nontrivial way of contracting the Riemann tensor:

Due to the symmetries of the Riemann tensor, contracting on the 4th instead of the 3rd index yields the same tensor, but with the sign reversed - see sign convention (contracting on the 1st lower index results in an array of zeros). The Ricci tensor is symmetric.

By the contracting relations on the Christoffel symbols, we have

The scalar curvature is the trace of the Ricci curvature,

.

The "gradient" of the scalar curvature follows from the Bianchi identity (proof):

that is,

Einstein tensor

The Einstein tensor G^ab is defined in terms of the Ricci tensor R^ab and the Ricci scalar R,

where g is the metric tensor.

The Einstein tensor is symmetric, with a vanishing divergence (proof) which is due to the Bianchi identity:

Weyl tensor

The Weyl tensor is given by

where denotes the dimension of the Riemannian manifold.

The Weyl tensor satisfies the first (algebraic) Bianchi identity:

The Weyl tensor is a symmetric product of alternating 2-forms,

just like the Riemann tensor. Moreover, taking the trace over any two indices gives zero,

The Weyl tensor vanishes () if and only if a manifold of dimension is locally conformally flat. In other words, can be covered by coordinate systems in which the metric satisfies

This is essentially because is invariant under conformal changes.

Gradient, divergence, Laplace–Beltrami operator

The gradient of a function is obtained by raising the index of the differential , whose components are given by:

The divergence of a vector field with components is

The Laplace–Beltrami operator acting on a function is given by the divergence of the gradient:

The divergence of an antisymmetric tensor field of type simplifies to

The Hessian of a map is given by

Kulkarni–Nomizu product

The Kulkarni–Nomizu product is an important tool for constructing new tensors from existing tensors on a Riemannian manifold. Let and be symmetric covariant 2-tensors. In coordinates,

Then we can multiply these in a sense to get a new covariant 4-tensor, which is often denoted . The defining formula is

Clearly, the product satisfies

In an inertial frame

An orthonormal inertial frame is a coordinate chart such that, at the origin, one has the relations and (but these may not hold at other points in the frame). These coordinates are also called normal coordinates.

In such a frame, the expression for several operators is simpler. Note that the formulae given below are valid at the origin of the frame only.

Under a conformal change

Let be a Riemannian metric on a smooth manifold , and a smooth real-valued function on . Then

is also a Riemannian metric on . We say that is conformal to . Evidently, conformality of metrics is an equivalence relation. Here are some formulas for conformal changes in tensors associated with the metric. (Quantities marked with a tilde will be associated with , while those unmarked with such will be associated with .)

Note that the difference between the Christoffel symbols of two different metrics always form the components of a tensor.

We can also write this in a coordinate-free manner as

whenever is locally a conformal diffeomorphism, i.e. ( in our case), and are vector fields.

Here is the Riemannian volume element.

Here is the Kulkarni–Nomizu product defined earlier in this article. The symbol denotes partial derivative, while denotes covariant derivative. From this formula and the orthogonal decomposition of the Riemann tensor, it follows that