This article is about a generalized derivative of a multivariate function. For another use in mathematics, see
Slope. For a similarly spelled unit of angle, see
Gradian. For other uses, see
The gradient, represented by the blue arrows, denotes the direction of greatest change of a scalar function. The values of the function are represented in greyscale and increase in value from white (low) to dark (high).
where is the total infinitesimal change in for an infinitesimal displacement , and is seen to be maximal when is in the direction of the gradient . The
nabla symbol, written as an upside-down triangle and pronounced "del", denotes the
vector differential operator.
When a coordinate system is used in which the basis vectors are not functions of position, the gradient is given by the
vector[a] whose components are the
partial derivatives of at . That is, for , its gradient is defined at the point in n-dimensional space as the vector[b]
Gradient of the 2D function f(x, y) = xe−(x2 + y2) is plotted as arrows over the pseudocolor plot of the function.
Consider a room where the temperature is given by a
scalar field, T, so at each point (x, y, z) the temperature is T(x, y, z), independent of time. At each point in the room, the gradient of T at that point will show the direction in which the temperature rises most quickly, moving away from (x, y, z). The magnitude of the gradient will determine how fast the temperature rises in that direction.
Consider a surface whose height above sea level at point (x, y) is H(x, y). The gradient of H at a point is a plane vector pointing in the direction of the steepest slope or
grade at that point. The steepness of the slope at that point is given by the magnitude of the gradient vector.
The gradient can also be used to measure how a scalar field changes in other directions, rather than just the direction of greatest change, by taking a
dot product. Suppose that the steepest slope on a hill is 40%. A road going directly uphill has slope 40%, but a road going around the hill at an angle will have a shallower slope. For example, if the road is at a 60° angle from the uphill direction (when both directions are projected onto the horizontal plane), then the slope along the road will be the dot product between the gradient vector and a
unit vector along the road, namely 40% times the
cosine of 60°, or 20%.
The gradient of the function f(x,y) = −(cos2x + cos2y)2 depicted as a projected
vector field on the bottom plane.
The gradient (or gradient vector field) of a scalar function f(x1, x2, x3, …, xn) is denoted ∇f or ∇→f where ∇ (
nabla) denotes the vector
del. The notation grad f is also commonly used to represent the gradient. The gradient of f is defined as the unique vector field whose dot product with any
vectorv at each point x is the directional derivative of f along v. That is,
where i, j, k are the
standard unit vectors in the directions of the x, y and z coordinates, respectively. For example, the gradient of the function
In some applications it is customary to represent the gradient as a
row vector or
column vector of its components in a rectangular coordinate system; this article follows the convention of the gradient being a column vector, while the derivative is a row vector.
where r is the radial distance, φ is the azimuthal angle and θ is the polar angle, and er, eθ and eφ are again local unit vectors pointing in the coordinate directions (that is, the normalized
general coordinates, which we write as x1, …, xi, …, xn, where n is the number of dimensions of the domain. Here, the upper index refers to the position in the list of the coordinate or component, so x2 refers to the second component—not the quantity x squared. The index variable i refers to an arbitrary element xi. Using
Einstein notation, the gradient can then be written as:
If the coordinates are orthogonal we can easily express the gradient (and the
differential) in terms of the normalized bases, which we refer to as and , using the scale factors (also known as
Lamé coefficients) :
where we cannot use Einstein notation, since it is impossible to avoid the repetition of more than two indices. Despite the use of upper and lower indices, , , and are neither contravariant nor covariant.
The latter expression evaluates to the expressions given above for cylindrical and spherical coordinates.
The gradient is closely related to the
total derivative (
total differential) : they are
dual) to each other. Using the convention that vectors in are represented by
column vectors, and that covectors (linear maps ) are represented by
row vectors,[a] the gradient and the derivative are expressed as a column and row vector, respectively, with the same components, but transpose of each other:
While these both have the same components, they differ in what kind of mathematical object they represent: at each point, the derivative is a
cotangent vector, a
linear form (
covector) which expresses how much the (scalar) output changes for a given infinitesimal change in (vector) input, while at each point, the gradient is a
tangent vector, which represents an infinitesimal change in (vector) input. In symbols, the gradient is an element of the tangent space at a point, , while the derivative is a map from the tangent space to the real numbers, . The tangent spaces at each point of can be "naturally" identified[d] with the vector space itself, and similarly the cotangent space at each point can be naturally identified with the
dual vector space of covectors; thus the value of the gradient at a point can be thought of a vector in the original , not just as a tangent vector.
Computationally, given a tangent vector, the vector can be multiplied by the derivative (as matrices), which is equal to taking the
dot product with the gradient:
Differential or (exterior) derivative
The best linear approximation to a differentiable function
Much as the derivative of a function of a single variable represents the
slope of the
tangent to the
graph of the function, the directional derivative of a function in several variables represents the slope of the tangent
hyperplane in the direction of the vector.
The gradient is related to the differential by the formula
for any , where is the
dot product: taking the dot product of a vector with the gradient is the same as taking the directional derivative along the vector.
If is viewed as the space of (dimension ) column vectors (of real numbers), then one can regard as the row vector with components
so that is given by
matrix multiplication. Assuming the standard Euclidean metric on , the gradient is then the corresponding column vector, that is,
Linear approximation to a function
linear approximation to a function can be expressed in terms of the gradient, rather than the derivative. The gradient of a
function from the Euclidean space to at any particular point in characterizes the best
linear approximation to at . The approximation is as follows:
for close to , where is the gradient of computed at , and the dot denotes the dot product on . This equation is equivalent to the first two terms in the
multivariable Taylor series expansion of at .
Relationship with Fréchet derivative
Let U be an
open set in Rn. If the function f : U → R is differentiable, then the differential of f is the
Fréchet derivative of f. Thus ∇f is a function from U to the space Rn such that
where · is the dot product.
As a consequence, the usual properties of the derivative hold for the gradient, though the gradient is not a derivative itself, but rather dual to the derivative:
Suppose that f : A → R is a real-valued function defined on a subset A of Rn, and that f is differentiable at a point a. There are two forms of the chain rule applying to the gradient. First, suppose that the function g is a
parametric curve; that is, a function g : I → Rn maps a subset I ⊂ R into Rn. If g is differentiable at a point c ∈ I such that g(c) = a, then
A level surface, or
isosurface, is the set of all points where some function has a given value.
If f is differentiable, then the dot product (∇f )x ⋅ v of the gradient at a point x with a vector v gives the directional derivative of f at x in the direction v. It follows that in this case the gradient of f is
orthogonal to the
level sets of f. For example, a level surface in three-dimensional space is defined by an equation of the form F(x, y, z) = c. The gradient of F is then normal to the surface.
More generally, any
embeddedhypersurface in a Riemannian manifold can be cut out by an equation of the form F(P) = 0 such that dF is nowhere zero. The gradient of F is then normal to the hypersurface.
affine algebraic hypersurface may be defined by an equation F(x1, ..., xn) = 0, where F is a polynomial. The gradient of F is zero at a singular point of the hypersurface (this is the definition of a singular point). At a non-singular point, it is a nonzero normal vector.
Conservative vector fields and the gradient theorem
The gradient of a function is called a gradient field. A (continuous) gradient field is always a
conservative vector field: its
line integral along any path depends only on the endpoints of the path, and can be evaluated by the gradient theorem (the fundamental theorem of calculus for line integrals). Conversely, a (continuous) conservative vector field is always the gradient of a function.
Suppose f : Rn → Rm is a function such that each of its first-order partial derivatives exist on ℝn. Then the Jacobian matrix of f is defined to be an m×n matrix, denoted by or simply . The (i,j)th entry is . Explicitly
where gjk are the components of the inverse
metric tensor and the ei are the coordinate basis vectors.
Expressed more invariantly, the gradient of a vector field f can be defined by the
Levi-Civita connection and metric tensor:
where ∇c is the connection.
smooth functionf on a Riemannian manifold (M, g), the gradient of f is the vector field ∇f such that for any vector field X,
where gx( , ) denotes the
inner product of tangent vectors at x defined by the metric g and ∂Xf is the function that takes any point x ∈ M to the directional derivative of f in the direction X, evaluated at x. In other words, in a
coordinate chartφ from an open subset of M to an open subset of Rn, (∂Xf )(x) is given by:
where Xj denotes the jth component of X in this coordinate chart.
So, the local form of the gradient takes the form:
Generalizing the case M = Rn, the gradient of a function is related to its exterior derivative, since
More precisely, the gradient ∇f is the vector field associated to the differential 1-form df using the
(called "sharp") defined by the metric g. The relation between the exterior derivative and the gradient of a function on Rn is a special case of this in which the metric is the flat metric given by the dot product.
^The value of the gradient at a point can be thought of as a vector in the original space , while the value of the derivative at a point can be thought of as a covector on the original space: a linear map .
^Informally, "naturally" identified means that this can be done without making any arbitrary choices. This can be formalized with a
Korn, Theresa M.; Korn, Granino Arthur (2000). Mathematical Handbook for Scientists and Engineers: Definitions, Theorems, and Formulas for Reference and Review. Dover Publications. pp. 157–160.
Look up gradient in Wiktionary, the free dictionary.