# Vector calculus identities

The following identities are important in vector calculus:

## Operator notations

In the three-dimensional Cartesian coordinate system, the gradient of some function is given by:

where i, j, k are the standard unit vectors.

The gradient of a tensor field, , of order n, is generally written as

and is a tensor field of order n + 1. In particular, if the tensor field has order 0 (i.e. a scalar), , the resulting gradient,

is a vector field.

### Divergence

Main article: Divergence

In three-dimensional Cartesian coordinates, the divergence of a continuously differentiable vector field is defined as the scalar-valued function:

The divergence of a tensor field, , of non-zero order n, is generally written as

and is a contraction to a tensor field of order n − 1. Specifically, the divergence of a vector is a scalar. The divergence of a higher order tensor field may be found by decomposing the tensor field into a sum of outer products, thereby allowing the use of the identity,

where is the directional derivative in the direction of multiplied by its magnitude. Specifically, for the outer product of two vectors,

### Curl

Main article: Curl (mathematics)

In Cartesian coordinates, for :

curl() =

where i, j, and k are the unit vectors for the x-, y-, and z-axes, respectively.

For a 3-dimensional vector field , curl is also a 3-dimensional vector field, generally written as:

or in Einstein notation as:

where ε is the Levi-Civita symbol.

### Laplacian

Main article: Laplace operator

In Cartesian coordinates, the Laplacian of a function is

For a tensor field, , the laplacian is generally written as:

and is a tensor field of the same order.

### Special notations

In Feynman subscript notation,

where the notation ∇B means the subscripted gradient operates on only the factor B.[1][2]

A less general but similar idea is used in geometric algebra where the so-called Hestenes overdot notation is employed.[3] The above identity is then expressed as:

where overdots define the scope of the vector derivative. The dotted vector, in this case B, is differentiated, while the (undotted) A is held constant.

For the remainder of this article, Feynman subscript notation will be used where appropriate.

## Properties

### Product rule for the gradient

The gradient of the product of two scalar fields and follows the same form as the product rule in single variable calculus.

### Vector dot product

where JA denotes the Jacobian of A.

Alternatively, using Feynman subscript notation,

As a special case, when A = B,

## Second derivatives

The curl of the gradient of any twice-differentiable scalar field is always the zero vector:

### Divergence of the curl

The divergence of the curl of any vector field A is always zero:

The Laplacian of a scalar field is defined as the divergence of the gradient:

Note that the result is a scalar quantity.

### Curl of the curl

Here,∇2 is the vector Laplacian operating on the vector field A.

## Summary of important identities

### Differentiation

#### Second derivatives

DCG chart: A simple chart depicting all rules pertaining to second derivatives. D, C, G, L and CC stand for divergence, curl, gradient, Laplacian and curl of curl, respectively. Arrows indicate existence of second derivatives. Blue circle in the middle represents curl of curl, whereas the other two red circles(dashed) mean that DD and GG do not exist.

### Integration

Below, the curly symbol ∂ means "boundary of".

#### Surface–volume integrals

In the following surface–volume integral theorems, V denotes a 3d volume with a corresponding 2d boundary S = ∂V (a closed surface):

#### Curve–surface integrals

In the following curve–surface integral theorems, S denotes a 2d open surface with a corresponding 1d boundary C = ∂S (a closed curve):

Integration around a closed curve in the clockwise sense is the negative of the same line integral in the counterclockwise sense (analogous to interchanging the limits in a definite integral):

## References

1. Feynman, R. P.; Leighton, R. B.; Sands, M. (1964). The Feynman Lecture on Physics. Addison-Wesley. Vol II, p. 27–4. ISBN 0-8053-9049-9.
2. Kholmetskii, A. L.; Missevitch, O. V. (2005). "The Faraday induction law in relativity theory". arXiv: [physics.class-ph].
3. Doran, C.; Lasenby, A. (2003). Geometric algebra for physicists. Cambridge University Press. p. 169. ISBN 978-0-521-71595-9.

• Balanis, Constantine A. Advanced Engineering Electromagnetics. ISBN 0-471-62194-3.
• Schey, H. M. (1997). Div Grad Curl and all that: An informal text on vector calculus. W. W. Norton & Company. ISBN 0-393-96997-5.
• Griffiths, David J. (1999). Introduction to Electrodynamics. Prentice Hall. ISBN 0-13-805326-X.