In
mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, a tangent vector is a
vector that is
tangent
In geometry, the tangent line (or simply tangent) to a plane curve at a given point is the straight line that "just touches" the curve at that point. Leibniz defined it as the line through a pair of infinitely close points on the curve. Mo ...
to a
curve or
surface at a given point. Tangent vectors are described in the
differential geometry of curves in the context of curves in R
''n''. More generally, tangent vectors are elements of a
tangent space of a
differentiable manifold. Tangent vectors can also be described in terms of
germs. Formally, a tangent vector at the point
is a linear
derivation of the algebra defined by the set of germs at
.
Motivation
Before proceeding to a general definition of the tangent vector, we discuss its use in
calculus
Calculus, originally called infinitesimal calculus or "the calculus of infinitesimals", is the mathematics, mathematical study of continuous change, in the same way that geometry is the study of shape, and algebra is the study of generalizati ...
and its
tensor
In mathematics, a tensor is an algebraic object that describes a multilinear relationship between sets of algebraic objects related to a vector space. Tensors may map between different objects such as vectors, scalars, and even other tensor ...
properties.
Calculus
Let
be a parametric
smooth curve. The tangent vector is given by
, where we have used a prime instead of the usual dot to indicate differentiation with respect to parameter . The unit tangent vector is given by
Example
Given the curve
in
, the unit tangent vector at
is given by
Contravariance
If
is given parametrically in the
''n''-dimensional coordinate system (here we have used superscripts as an index instead of the usual subscript) by
or
then the tangent vector field
is given by
Under a change of coordinates
the tangent vector
in the -coordinate system is given by
where we have used the
Einstein summation convention
In mathematics, especially the usage of linear algebra in Mathematical physics, Einstein notation (also known as the Einstein summation convention or Einstein summation notation) is a notational convention that implies summation over a set of ...
. Therefore, a tangent vector of a smooth curve will transform as a
contravariant tensor of order one under a change of coordinates.
Definition
Let
be a differentiable function and let
be a vector in
. We define the directional derivative in the
direction at a point
by
The tangent vector at the point
may then be defined
[A. Gray (1993)] as
Properties
Let
be differentiable functions, let
be tangent vectors in
at
, and let
. Then
#
#
#
Tangent vector on manifolds
Let
be a differentiable manifold and let
be the algebra of real-valued differentiable functions on
. Then the tangent vector to
at a point
in the manifold is given by the
derivation which shall be linear — i.e., for any
and
we have
:
Note that the derivation will by definition have the Leibniz property
:
See also
*
*
References
Bibliography
* .
* .
* {{citation, first=David, last=Kay, title=Schaums Outline of Theory and Problems of Tensor Calculus, publisher=McGraw-Hill, publication-place=New York, year=1988.
Vectors (mathematics and physics)