 Del

For other uses, see Del (disambiguation).∇
In vector calculus, del is a vector differential operator, usually represented by the nabla symbol . When applied to a function defined on a onedimensional domain, it denotes its standard derivative as defined in calculus. When applied to a field (a function defined on a multidimensional domain), del may denote the gradient (locally steepest slope) of a scalar field, the divergence of a vector field, or the curl (rotation) of a vector field, depending on the way it is applied.
Strictly speaking, del is not a specific operator, but rather a convenient mathematical notation for those three operators, that makes many equations easier to write and remember. The del symbol can be interpreted as a vector of partial derivative operators, and its three possible meanings—gradient, divergence, and curl—can be formally viewed as the product of scalars, dot product, and cross product, respectively, of the del "operator" with the field. These formal products do not necessarily commute with other operators or products.
Contents
Definition
In the threedimensional Cartesian coordinate system R^{3} with coordinates (x, y, z), del is defined in terms of partial derivative operators as
where are the unit vectors in their respective directions. Though this page chiefly treats del in three dimensions, this definition can be generalized to the ndimensional Euclidean space R^{n}. In the Cartesian coordinate system with coordinates (x_{1}, x_{2}, ..., x_{n}), del is:
where is the standard basis in this space.
More compactly, using the Einstein summation notation, del is written as
Del can also be expressed in other coordinate systems, see for example del in cylindrical and spherical coordinates.
Notational uses
Del is used as a shorthand form to simplify many long mathematical expressions. It is most commonly used to simplify expressions for the gradient, divergence, curl, directional derivative, and Laplacian.
Gradient
The vector derivative of a scalar field f is called the gradient, and it can be represented as:
It always points in the direction of greatest increase of f, and it has a magnitude equal to the maximum rate of increase at the point—just like a standard derivative. In particular, if a hill is defined as a height function over a plane h(x,y), the 2d projection of the gradient at a given location will be a vector in the xyplane (sort of like an arrow on a map) pointing along the steepest direction. The magnitude of the gradient is the value of this steepest slope.
In particular, this notation is powerful because the gradient product rule looks very similar to the 1dderivative case:
However, the rules for dot products do not turn out to be simple, as illustrated by:
Divergence
The divergence of a vector field is a scalar function that can be represented as:
The divergence is roughly a measure of a vector field's increase in the direction it points; but more accurately, it is a measure of that field's tendency to converge toward or repel from a point.
The power of the del notation is shown by the following product rule:
The formula for the vector product is slightly less intuitive, because this product is not commutative:
Curl
The curl of a vector field is a vector function that can be represented as:
The curl at a point is proportional to the onaxis torque to which a tiny pinwheel would be subjected if it were centered at that point.
The vector product operation can be visualized as a pseudodeterminant:
Again the power of the notation is shown by the product rule:
Unfortunately the rule for the vector product does not turn out to be simple:
Directional derivative
The directional derivative of a scalar field f(x,y,z) in the direction is defined as:
This gives the change of a field f in the direction of a. In operator notation, the element in parentheses can be considered a single coherent unit; fluid dynamics uses this convention extensively, terming it the convective derivative—the "moving" derivative of the fluid.
Laplacian
The Laplace operator is a scalar operator that can be applied to either vector or scalar fields; it is defined as:
The Laplacian is ubiquitous throughout modern mathematical physics, appearing in Laplace's equation, Poisson's equation, the heat equation, the wave equation, and the Schrödinger equation—to name a few.
Tensor derivative
Del can also be applied to a vector field with the result being a tensor. The tensor derivative of a vector field is a 9term secondrank tensor, but can be denoted simply as , where represents the dyadic product. This quantity is equivalent to the transpose of the Jacobian matrix of the vector field with respect to space.
For a small displacement , the change in the vector field is given by:
Second derivatives
When del operates on a scalar or vector, generally a scalar or vector is returned. Because of the diversity of vector products (scalar, dot, cross) one application of del already gives rise to three major derivatives: the gradient (scalar product), divergence (dot product), and curl (cross product). Applying these three sorts of derivatives again to each other gives five possible second derivatives, for a scalar field f or a vector field v; the use of the scalar Laplacian and vector Laplacian gives two more:
These are of interest principally because they are not always unique or independent of each other. As long as the functions are wellbehaved, two of them are always zero:
Two of them are always equal:
The 3 remaining vector derivatives are related by the equation:
And one of them can even be expressed with the tensor product, if the functions are wellbehaved:
Precautions
Most of the above vector properties (except for those that rely explicitly on del's differential properties—for example, the product rule) rely only on symbol rearrangement, and must necessarily hold if del is replaced by any other vector. This is part of the tremendous value gained in representing this operator as a vector in its own right.
Though you can often replace del with a vector and obtain a vector identity, making those identities intuitive, the reverse is not necessarily reliable, because del does not often commute.
A counterexample that relies on del's failure to commute:
A counterexample that relies on del's differential properties:
Central to these distinctions is the fact that del is not simply a vector; it is a vector operator. Whereas a vector is an object with both a precise numerical magnitude and direction, del does not have a precise value for either until it is allowed to operate on something.
For that reason, identities involving del must be derived with care, using both vector identities and differentiation identities such as the product rule.
See also
 Del in cylindrical and spherical coordinates
 Maxwell's equations
 NavierStokes equations
 Table of mathematical symbols
 Vector calculus identities
References
 Div, Grad, Curl, and All That, H. M. Schey, ISBN 0393969975
 Jeff Miller, Earliest Uses of Symbols of Calculus
 "History of Nabla". netlib.org. January 26, 1998. http://www.netlib.org/nadigesthtml/98/v98n03.html#2.
External links
 A survey of the improper use of ∇ in vector analysis (1994) Tai, Chen
Categories: Calculus
 Vector calculus
 Mathematical notation
 Differential operators

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