Vector calculus identities

The following are important identities involving derivatives and integrals in vector calculus. Gradient For a function in three-dimensional Cartesian coordinate variables, the gradient is the vector field: where i, j, k are the standard unit vectors for the x, y, z-axes. More generally, for a function of n variables , also called a scalar field, the gradient is the vector field: where are orthogonal unit vectors in arbitrary directions. As the name implies, the gradient is proportional to and points in the direction of the function's most rapid (positive) change. For a vector field , also called a tensor field of order 1, the gradient or total derivative is the n × n Jacobian matrix: For a tensor field of any order k, the gradient is a tensor field of order k + 1. For a tensor field of order k > 0, the tensor field of order k + 1 is defined by the recursive relation where is an arbitrary constant vector. Divergence In Cartesian coordinates, the divergence of a continuously differentiable vector field is the scalar-valued function: As the name implies the divergence is a measure of how much vectors are diverging. The divergence of a tensor field of non-zero order k is written as , a contraction to a tensor field of order k − 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 and using the identity, where is the directional derivative in the direction of multiplied by its magnitude. Specifically, for the outer product of two vectors, For a tensor field of order k > 1, the tensor field of order k − 1 is defined by the recursive relation where is an arbitrary constant vector. Curl (mathematics) In Cartesian coordinates, for the curl is the vector field: where i, j, and k are the unit vectors for the x-, y-, and z-axes, respectively. As the name implies the curl is a measure of how much nearby vectors tend in a circular direction. In Einstein notation, the vector field has curl given by: where = ±1 or 0 is the Levi-Civita parity symbol.