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geometry Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is c ...
, curvilinear coordinates are a
coordinate system In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is sig ...
for
Euclidean space Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, that is, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are Euclidea ...
in which the
coordinate line In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is sign ...
s may be curved. These coordinates may be derived from a set of Cartesian coordinates by using a transformation that is locally invertible (a one-to-one map) at each point. This means that one can convert a point given in a Cartesian coordinate system to its curvilinear coordinates and back. The name ''curvilinear coordinates'', coined by the French mathematician Lamé, derives from the fact that the
coordinate surfaces In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is s ...
of the curvilinear systems are curved. Well-known examples of curvilinear coordinate systems in three-dimensional Euclidean space (R3) are
cylindrical A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an in ...
and spherical coordinates. A Cartesian coordinate surface in this space is a
coordinate plane In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is s ...
; for example ''z'' = 0 defines the ''x''-''y'' plane. In the same space, the coordinate surface ''r'' = 1 in spherical coordinates is the surface of a unit
sphere A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the c ...
, which is curved. The formalism of curvilinear coordinates provides a unified and general description of the standard coordinate systems. Curvilinear coordinates are often used to define the location or distribution of physical quantities which may be, for example, scalars, vectors, or
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 ...
s. Mathematical expressions involving these quantities in
vector calculus Vector calculus, or vector analysis, is concerned with differentiation and integration of vector fields, primarily in 3-dimensional Euclidean space \mathbb^3. The term "vector calculus" is sometimes used as a synonym for the broader subjec ...
and tensor analysis (such as the
gradient In vector calculus, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gr ...
,
divergence In vector calculus, divergence is a vector operator that operates on a vector field, producing a scalar field giving the quantity of the vector field's source at each point. More technically, the divergence represents the volume density of ...
, curl, and
Laplacian In mathematics, the Laplace operator or Laplacian is a differential operator given by the divergence of the gradient of a scalar function on Euclidean space. It is usually denoted by the symbols \nabla\cdot\nabla, \nabla^2 (where \nabla is the ...
) can be transformed from one coordinate system to another, according to transformation rules for scalars, vectors, and tensors. Such expressions then become valid for any curvilinear coordinate system. A curvilinear coordinate system may be simpler to use than the Cartesian coordinate system for some applications. The motion of particles under the influence of central forces is usually easier to solve in spherical coordinates than in Cartesian coordinates; this is true of many physical problems with spherical symmetry defined in R3. Equations with
boundary conditions In mathematics, in the field of differential equations, a boundary value problem is a differential equation together with a set of additional constraints, called the boundary conditions. A solution to a boundary value problem is a solution to th ...
that follow coordinate surfaces for a particular curvilinear coordinate system may be easier to solve in that system. While one might describe the motion of a particle in a rectangular box using Cartesian coordinates, it's easier to describe the motion in a sphere with spherical coordinates. Spherical coordinates are the most common curvilinear coordinate systems and are used in Earth sciences,
cartography Cartography (; from grc, χάρτης , "papyrus, sheet of paper, map"; and , "write") is the study and practice of making and using maps. Combining science, aesthetics and technique, cartography builds on the premise that reality (or an i ...
,
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
, relativity, and
engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more speciali ...
.


Orthogonal curvilinear coordinates in 3 dimensions


Coordinates, basis, and vectors

For now, consider 3-D space. A point ''P'' in 3d space (or its
position vector In geometry, a position or position vector, also known as location vector or radius vector, is a Euclidean vector that represents the position of a point ''P'' in space in relation to an arbitrary reference origin ''O''. Usually denoted x, r, or ...
r) can be defined using Cartesian coordinates (''x'', ''y'', ''z'') quivalently written (''x''1, ''x''2, ''x''3) by \mathbf = x \mathbf_x + y\mathbf_y + z\mathbf_z, where e''x'', e''y'', e''z'' are the ''
standard basis In mathematics, the standard basis (also called natural basis or canonical basis) of a coordinate vector space (such as \mathbb^n or \mathbb^n) is the set of vectors whose components are all zero, except one that equals 1. For example, in the ...
vectors''. It can also be defined by its curvilinear coordinates (''q''1, ''q''2, ''q''3) if this triplet of numbers defines a single point in an unambiguous way. The relation between the coordinates is then given by the invertible transformation functions: : x = f^1(q^1, q^2, q^3),\, y = f^2(q^1, q^2, q^3),\, z = f^3(q^1, q^2, q^3) : q^1 = g^1(x,y,z),\, q^2 = g^2(x,y,z),\, q^3 = g^3(x,y,z) The surfaces ''q''1 = constant, ''q''2 = constant, ''q''3 = constant are called the coordinate surfaces; and the space curves formed by their intersection in pairs are called the
coordinate curves In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is s ...
. The coordinate axes are determined by the
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 ...
s to the coordinate curves at the intersection of three surfaces. They are not in general fixed directions in space, which happens to be the case for simple Cartesian coordinates, and thus there is generally no natural global basis for curvilinear coordinates. In the Cartesian system, the standard basis vectors can be derived from the derivative of the location of point ''P'' with respect to the local coordinate :\mathbf_x = \dfrac; \; \mathbf_y = \dfrac; \; \mathbf_z = \dfrac. Applying the same derivatives to the curvilinear system locally at point ''P'' defines the natural basis vectors: :\mathbf_1 = \dfrac; \; \mathbf_2 = \dfrac; \; \mathbf_3 = \dfrac. Such a basis, whose vectors change their direction and/or magnitude from point to point is called a local basis. All bases associated with curvilinear coordinates are necessarily local. Basis vectors that are the same at all points are global bases, and can be associated only with linear or
affine coordinate system In mathematics, an affine space is a geometric structure that generalizes some of the properties of Euclidean spaces in such a way that these are independent of the concepts of distance and measure of angles, keeping only the properties rela ...
s. For this article e is reserved for the
standard basis In mathematics, the standard basis (also called natural basis or canonical basis) of a coordinate vector space (such as \mathbb^n or \mathbb^n) is the set of vectors whose components are all zero, except one that equals 1. For example, in the ...
(Cartesian) and h or b is for the curvilinear basis. These may not have unit length, and may also not be orthogonal. In the case that they ''are'' orthogonal at all points where the derivatives are well-defined, we define the Lamé coefficients (after Gabriel Lamé) by :h_1 = , \mathbf_1, ; \; h_2 = , \mathbf_2, ; \; h_3 = , \mathbf_3, and the curvilinear orthonormal basis vectors by :\mathbf_1 = \dfrac; \; \mathbf_2 = \dfrac; \; \mathbf_3 = \dfrac. These basis vectors may well depend upon the position of ''P''; it is therefore necessary that they are not assumed to be constant over a region. (They technically form a basis for the tangent bundle of \mathbb^3 at ''P'', and so are local to ''P''.) In general, curvilinear coordinates allow the natural basis vectors hi not all mutually perpendicular to each other, and not required to be of unit length: they can be of arbitrary magnitude and direction. The use of an orthogonal basis makes vector manipulations simpler than for non-orthogonal. However, some areas of
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...
and
engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more speciali ...
, particularly
fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics of fluids ( liquids, gases, and plasmas) and the forces on them. It has applications in a wide range of disciplines, including mechanical, aerospace, civil, chemical and ...
and
continuum mechanics Continuum mechanics is a branch of mechanics that deals with the mechanical behavior of materials modeled as a continuous mass rather than as discrete particles. The French mathematician Augustin-Louis Cauchy was the first to formulate such mo ...
, require non-orthogonal bases to describe deformations and fluid transport to account for complicated directional dependences of physical quantities. A discussion of the general case appears later on this page.


Vector calculus


Differential elements

In orthogonal curvilinear coordinates, since the total differential change in r is :d\mathbf=\dfracdq^1 + \dfracdq^2 + \dfracdq^3 = h_1 dq^1 \mathbf_1 + h_2 dq^2 \mathbf_2 + h_3 dq^3 \mathbf_3 so scale factors are h_i = \left, \frac\ In non-orthogonal coordinates the length of d\mathbf= dq^1 \mathbf_1 + dq^2 \mathbf_2 + dq^3 \mathbf_3 is the positive square root of d\mathbf \cdot d\mathbf = dq^i dq^j \mathbf_i \cdot \mathbf_j (with
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 ...
). The six independent scalar products ''gij''=h''i''.h''j'' of the natural basis vectors generalize the three scale factors defined above for orthogonal coordinates. The nine ''gij'' are the components of the
metric tensor In the mathematical field of differential geometry, a metric tensor (or simply metric) is an additional structure on a manifold (such as a surface) that allows defining distances and angles, just as the inner product on a Euclidean space allow ...
, which has only three non zero components in orthogonal coordinates: ''g''11=''h''1''h''1, ''g''22=''h''2''h''2, ''g''33=''h''3''h''3.


Covariant and contravariant bases

Spatial gradients, distances, time derivatives and scale factors are interrelated within a coordinate system by two groups of basis vectors: # basis vectors that are locally tangent to their associated coordinate pathline: \mathbf_i=\dfrac are contravariant vectors (denoted by lowered indices), and # basis vectors that are locally normal to the isosurface created by the other coordinates: \mathbf^i=\nabla q^i are covariant vectors (denoted by raised indices), ∇ is the del operator. Note that, because of Einstein's summation convention, the position of the indices of the vectors is the opposite of that of the coordinates. Consequently, a general curvilinear coordinate system has two sets of basis vectors for every point: is the contravariant basis, and is the covariant (a.k.a. reciprocal) basis. The covariant and contravariant basis vectors types have identical direction for orthogonal curvilinear coordinate systems, but as usual have inverted units with respect to each other. Note the following important equality: \mathbf^i\cdot\mathbf_j = \delta^i_j wherein \delta^i_j denotes the generalized Kronecker delta. A vector v can be specified in terms of either basis, i.e., : \mathbf = v^1\mathbf_1 + v^2\mathbf_2 + v^3\mathbf_3 = v_1\mathbf^1 + v_2\mathbf^2 + v_3\mathbf^3 Using the Einstein summation convention, the basis vectors relate to the components by : \mathbf\cdot\mathbf^i = v^k\mathbf_k\cdot\mathbf^i = v^k\delta^i_k = v^i : \mathbf\cdot\mathbf_i = v_k\mathbf^k\cdot\mathbf_i = v_k\delta_i^k = v_i and : \mathbf\cdot\mathbf_i = v^k\mathbf_k\cdot\mathbf_i = g_v^k : \mathbf\cdot\mathbf^i = v_k\mathbf^k\cdot\mathbf^i = g^v_k where ''g'' is the metric tensor (see below). A vector can be specified with covariant coordinates (lowered indices, written ''vk'') or contravariant coordinates (raised indices, written ''vk''). From the above vector sums, it can be seen that contravariant coordinates are associated with covariant basis vectors, and covariant coordinates are associated with contravariant basis vectors. A key feature of the representation of vectors and tensors in terms of indexed components and basis vectors is ''invariance'' in the sense that vector components which transform in a covariant manner (or contravariant manner) are paired with basis vectors that transform in a contravariant manner (or covariant manner).


Integration


Constructing a covariant basis in one dimension

Consider the one-dimensional curve shown in Fig. 3. At point ''P'', taken as an
origin Origin(s) or The Origin may refer to: Arts, entertainment, and media Comics and manga * Origin (comics), ''Origin'' (comics), a Wolverine comic book mini-series published by Marvel Comics in 2002 * The Origin (Buffy comic), ''The Origin'' (Bu ...
, ''x'' is one of the Cartesian coordinates, and ''q''1 is one of the curvilinear coordinates. The local (non-unit) basis vector is b1 (notated h1 above, with b reserved for unit vectors) and it is built on the ''q''1 axis which is a tangent to that coordinate line at the point ''P''. The axis ''q''1 and thus the vector b1 form an angle \alpha with the Cartesian ''x'' axis and the Cartesian basis vector e1. It can be seen from triangle ''PAB'' that : \cos \alpha = \cfrac \quad \Rightarrow \quad , \mathbf_1, = , \mathbf_1, \cos \alpha where , e1, , , b1, are the magnitudes of the two basis vectors, i.e., the scalar intercepts ''PB'' and ''PA''. ''PA'' is also the projection of b1 on the ''x'' axis. However, this method for basis vector transformations using ''directional cosines'' is inapplicable to curvilinear coordinates for the following reasons: #By increasing the distance from ''P'', the angle between the curved line ''q''1 and Cartesian axis ''x'' increasingly deviates from \alpha. #At the distance ''PB'' the true angle is that which the tangent at point C forms with the ''x'' axis and the latter angle is clearly different from \alpha. The angles that the ''q''1 line and that axis form with the ''x'' axis become closer in value the closer one moves towards point ''P'' and become exactly equal at ''P''. Let point ''E'' be located very close to ''P'', so close that the distance ''PE'' is infinitesimally small. Then ''PE'' measured on the ''q''1 axis almost coincides with ''PE'' measured on the ''q''1 line. At the same time, the ratio ''PD/PE'' (''PD'' being the projection of ''PE'' on the ''x'' axis) becomes almost exactly equal to \cos\alpha. Let the infinitesimally small intercepts ''PD'' and ''PE'' be labelled, respectively, as ''dx'' and d''q''1. Then :\cos \alpha = \cfrac = \frac. Thus, the directional cosines can be substituted in transformations with the more exact ratios between infinitesimally small coordinate intercepts. It follows that the component (projection) of b1 on the ''x'' axis is :p^1 = \mathbf_1\cdot\cfrac = , \mathbf_1, \cfrac\cos\alpha = , \mathbf_1, \cfrac \quad \Rightarrow \quad \cfrac = \cfrac. If ''qi'' = ''qi''(''x''1, ''x''2, ''x''3) and ''xi'' = ''xi''(''q''1, ''q''2, ''q''3) are smooth (continuously differentiable) functions the transformation ratios can be written as \cfrac and \cfrac. That is, those ratios are
partial derivative In mathematics, a partial derivative of a function of several variables is its derivative with respect to one of those variables, with the others held constant (as opposed to the total derivative, in which all variables are allowed to vary). Pa ...
s of coordinates belonging to one system with respect to coordinates belonging to the other system.


Constructing a covariant basis in three dimensions

Doing the same for the coordinates in the other 2 dimensions, b1 can be expressed as: : \mathbf_1 = p^1\mathbf_1 + p^2\mathbf_2 + p^3\mathbf_3 = \cfrac \mathbf_1 + \cfrac \mathbf_2 + \cfrac \mathbf_3 Similar equations hold for b2 and b3 so that the standard basis is transformed to a local (ordered and ''normalised'') basis by the following system of equations: :\begin \mathbf_1 & = \cfrac \mathbf_1 + \cfrac \mathbf_2 + \cfrac \mathbf_3 \\ \mathbf_2 & = \cfrac \mathbf_1 + \cfrac \mathbf_2 + \cfrac \mathbf_3 \\ \mathbf_3 & = \cfrac \mathbf_1 + \cfrac \mathbf_2 + \cfrac \mathbf_3 \end By analogous reasoning, one can obtain the inverse transformation from local basis to standard basis: :\begin \mathbf_1 & = \cfrac \mathbf_1 + \cfrac \mathbf_2 + \cfrac \mathbf_3 \\ \mathbf_2 & = \cfrac \mathbf_1 + \cfrac \mathbf_2 + \cfrac \mathbf_3 \\ \mathbf_3 & = \cfrac \mathbf_1 + \cfrac \mathbf_2 + \cfrac \mathbf_3 \end


Jacobian of the transformation

The above systems of linear equations can be written in matrix form using the Einstein summation convention as :\cfrac \mathbf_i = \mathbf_k, \quad \cfrac \mathbf_i = \mathbf_k. This coefficient matrix of the linear system is the
Jacobian matrix In vector calculus, the Jacobian matrix (, ) of a vector-valued function of several variables is the matrix of all its first-order partial derivatives. When this matrix is square, that is, when the function takes the same number of variable ...
(and its inverse) of the transformation. These are the equations that can be used to transform a Cartesian basis into a curvilinear basis, and vice versa. In three dimensions, the expanded forms of these matrices are : \mathbf = \begin \cfrac & \cfrac & \cfrac \\ \cfrac & \cfrac & \cfrac \\ \cfrac & \cfrac & \cfrac \\ \end,\quad \mathbf^ = \begin \cfrac & \cfrac & \cfrac \\ \cfrac & \cfrac & \cfrac \\ \cfrac & \cfrac & \cfrac \\ \end In the inverse transformation (second equation system), the unknowns are the curvilinear basis vectors. For any specific location there can only exist one and only one set of basis vectors (else the basis is not well defined at that point). This condition is satisfied if and only if the equation system has a single solution. In
linear algebra Linear algebra is the branch of mathematics concerning linear equations such as: :a_1x_1+\cdots +a_nx_n=b, linear maps such as: :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces and through matrice ...
, a linear equation system has a single solution (non-trivial) only if the determinant of its system matrix is non-zero: : \det(\mathbf^) \neq 0 which shows the rationale behind the above requirement concerning the inverse Jacobian determinant.


Generalization to ''n'' dimensions

The formalism extends to any finite dimension as follows. Consider the real Euclidean ''n''-dimensional space, that is R''n'' = R × R × ... × R (''n'' times) where R is the
set Set, The Set, SET or SETS may refer to: Science, technology, and mathematics Mathematics *Set (mathematics), a collection of elements *Category of sets, the category whose objects and morphisms are sets and total functions, respectively Electro ...
of
real numbers In mathematics, a real number is a number that can be used to measure a ''continuous'' one-dimensional quantity such as a distance, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small variations. Every re ...
and × denotes the
Cartesian product In mathematics, specifically set theory, the Cartesian product of two sets ''A'' and ''B'', denoted ''A''×''B'', is the set of all ordered pairs where ''a'' is in ''A'' and ''b'' is in ''B''. In terms of set-builder notation, that is : A\t ...
, which is a
vector space In mathematics and physics, a vector space (also called a linear space) is a set whose elements, often called '' vectors'', may be added together and multiplied ("scaled") by numbers called ''scalars''. Scalars are often real numbers, but can ...
. The
coordinates In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is sig ...
of this space can be denoted by: x = (''x''1, ''x''2,...,''xn''). Since this is a vector (an element of the vector space), it can be written as: : \mathbf = \sum_^n x_i\mathbf^i where e1 = (1,0,0...,0), e2 = (0,1,0...,0), e3 = (0,0,1...,0),...,e''n'' = (0,0,0...,1) is the ''
standard basis In mathematics, the standard basis (also called natural basis or canonical basis) of a coordinate vector space (such as \mathbb^n or \mathbb^n) is the set of vectors whose components are all zero, except one that equals 1. For example, in the ...
set of vectors'' for the space R''n'', and ''i'' = 1, 2,...''n'' is an index labelling components. Each vector has exactly one component in each dimension (or "axis") and they are mutually
orthogonal In mathematics, orthogonality is the generalization of the geometric notion of '' perpendicularity''. By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...
(
perpendicular In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', ⟂. It c ...
) and normalized (has unit magnitude). More generally, we can define basis vectors b''i'' so that they depend on q = (''q''1, ''q''2,...,''qn''), i.e. they change from point to point: b''i'' = b''i''(q). In which case to define the same point x in terms of this alternative basis: the ''
coordinates In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is sig ...
'' with respect to this basis ''vi'' also necessarily depend on x also, that is ''vi'' = ''vi''(x). Then a vector v in this space, with respect to these alternative coordinates and basis vectors, can be expanded as a linear combination in this basis (which simply means to multiply each basis vector e''i'' by a number ''v''''i''scalar multiplication): : \mathbf = \sum_^n \bar^j\mathbf_j = \sum_^n \bar^j(\mathbf)\mathbf_j(\mathbf) The vector sum that describes v in the new basis is composed of different vectors, although the sum itself remains the same.


Transformation of coordinates

From a more general and abstract perspective, a curvilinear coordinate system is simply a coordinate patch on the
differentiable manifold In mathematics, a differentiable manifold (also differential manifold) is a type of manifold that is locally similar enough to a vector space to allow one to apply calculus. Any manifold can be described by a collection of charts (atlas). One ma ...
En (n-dimensional
Euclidean space Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, that is, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are Euclidea ...
) that is diffeomorphic to the Cartesian coordinate patch on the manifold. Two diffeomorphic coordinate patches on a differential manifold need not overlap differentiably. With this simple definition of a curvilinear coordinate system, all the results that follow below are simply applications of standard theorems in
differential topology In mathematics, differential topology is the field dealing with the topological properties and smooth properties of smooth manifolds. In this sense differential topology is distinct from the closely related field of differential geometry, which ...
. The transformation functions are such that there's a one-to-one relationship between points in the "old" and "new" coordinates, that is, those functions are
bijection In mathematics, a bijection, also known as a bijective function, one-to-one correspondence, or invertible function, is a function between the elements of two sets, where each element of one set is paired with exactly one element of the other ...
s, and fulfil the following requirements within their
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined ** Domain of definition of a partial function ** Natural domain of a partial function **Domain of holomorphy of a function * ...
s:


Vector and tensor algebra in three-dimensional curvilinear coordinates

Elementary vector and tensor algebra in curvilinear coordinates is used in some of the older scientific literature in
mechanics Mechanics (from Ancient Greek: μηχανική, ''mēkhanikḗ'', "of machines") is the area of mathematics and physics concerned with the relationships between force, matter, and motion among physical objects. Forces applied to objec ...
and
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...
and can be indispensable to understanding work from the early and mid-1900s, for example the text by Green and Zerna. Some useful relations in the algebra of vectors and second-order tensors in curvilinear coordinates are given in this section. The notation and contents are primarily from Ogden, Naghdi, Simmonds, Green and Zerna, Basar and Weichert, and Ciarlet.


Tensors in curvilinear coordinates

A second-order tensor can be expressed as : \boldsymbol = S^\mathbf_i\otimes\mathbf_j = S^i_j\mathbf_i\otimes\mathbf^j = S_i^j\mathbf^i\otimes\mathbf_j = S_\mathbf^i\otimes\mathbf^j where \scriptstyle\otimes denotes the
tensor product In mathematics, the tensor product V \otimes W of two vector spaces and (over the same field) is a vector space to which is associated a bilinear map V\times W \to V\otimes W that maps a pair (v,w),\ v\in V, w\in W to an element of V \otime ...
. The components ''Sij'' are called the contravariant components, ''Si j'' the mixed right-covariant components, ''Si j'' the mixed left-covariant components, and ''Sij'' the covariant components of the second-order tensor. The components of the second-order tensor are related by : S^ = g^S_k^j = g^S^i_k = g^g^S_


The metric tensor in orthogonal curvilinear coordinates

At each point, one can construct a small line element , so the square of the length of the line element is the scalar product dx • dx and is called the
metric Metric or metrical may refer to: * Metric system, an internationally adopted decimal system of measurement * An adjective indicating relation to measurement in general, or a noun describing a specific type of measurement Mathematics In mathe ...
of the
space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consi ...
, given by: :d\mathbf\cdot d\mathbf = \cfrac\cfracdq^jdq^k . The following portion of the above equation : \cfrac\cfrac = g_(q^i,q^j) = \mathbf_i\cdot\mathbf_j is a ''symmetric'' tensor called the fundamental (or metric) tensor of the
Euclidean space Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, that is, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are Euclidea ...
in curvilinear coordinates. Indices can be raised and lowered by the metric: : v^i = g^v_k


Relation to Lamé coefficients

Defining the scale factors ''hi'' by : h_ih_j = g_ = \mathbf_i\cdot\mathbf_j \quad \Rightarrow \quad h_i =\sqrt= \left, \mathbf_i\=\left, \cfrac\ gives a relation between the metric tensor and the Lamé coefficients, and : g_ = \cfrac\cdot\cfrac = \left( h_\mathbf_k\right)\cdot\left( h_\mathbf_m\right) = h_h_ where ''hij'' are the Lamé coefficients. For an orthogonal basis we also have: : g = g_g_g_ = h_1^2h_2^2h_3^2 \quad \Rightarrow \quad \sqrt = h_1h_2h_3 = J


Example: Polar coordinates

If we consider polar coordinates for R2, : (x, y)=(r \cos \theta, r \sin \theta) (r, θ) are the curvilinear coordinates, and the Jacobian determinant of the transformation (''r'',θ) → (''r'' cos θ, ''r'' sin θ) is ''r''. The
orthogonal In mathematics, orthogonality is the generalization of the geometric notion of '' perpendicularity''. By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...
basis vectors are b''r'' = (cos θ, sin θ), bθ = (−r sin θ, r cos θ). The scale factors are ''h''''r'' = 1 and ''h''θ= ''r''. The fundamental tensor is ''g''11 =1, ''g''22 =''r''2, ''g''12 = ''g''21 =0.


The alternating tensor

In an orthonormal right-handed basis, the third-order alternating tensor is defined as : \boldsymbol = \varepsilon_\mathbf^i\otimes\mathbf^j\otimes\mathbf^k In a general curvilinear basis the same tensor may be expressed as : \boldsymbol = \mathcal_\mathbf^i\otimes\mathbf^j\otimes\mathbf^k = \mathcal^\mathbf_i\otimes\mathbf_j\otimes\mathbf_k It can also be shown that : \mathcal^ = \cfrac\varepsilon_ = \cfrac\varepsilon_


Christoffel symbols

; Christoffel symbols of the first kind \Gamma_: : \mathbf_ = \frac = \mathbf^k \Gamma_ \quad \Rightarrow \quad \mathbf_k \cdot \mathbf_ = \Gamma_ where the comma denotes a
partial derivative In mathematics, a partial derivative of a function of several variables is its derivative with respect to one of those variables, with the others held constant (as opposed to the total derivative, in which all variables are allowed to vary). Pa ...
(see
Ricci calculus In mathematics, Ricci calculus constitutes the rules of index notation and manipulation for tensors and tensor fields on a differentiable manifold, with or without a metric tensor or connection. It is also the modern name for what used to be ...
). To express Γ''kij'' in terms of ''gij'', : \begin g_ & = (\mathbf_i\cdot\mathbf_j)_ = \mathbf_\cdot\mathbf_j + \mathbf_i\cdot\mathbf_ = \Gamma_ + \Gamma_\\ g_ & = (\mathbf_i\cdot\mathbf_k)_ = \mathbf_\cdot\mathbf_k + \mathbf_i\cdot\mathbf_ = \Gamma_ + \Gamma_\\ g_ & = (\mathbf_j\cdot\mathbf_k)_ = \mathbf_\cdot\mathbf_k + \mathbf_j\cdot\mathbf_ = \Gamma_ + \Gamma_ \end Since :\mathbf_ = \mathbf_\quad\Rightarrow\quad\Gamma_ = \Gamma_ using these to rearrange the above relations gives :\Gamma_ = \frac(g_ + g_ - g_) = \frac \mathbf_i\cdot\mathbf_k)_ + (\mathbf_j\cdot\mathbf_k)_ - (\mathbf_i\cdot\mathbf_j)_ ; Christoffel symbols of the second kind \Gamma^k_: :\Gamma^k_ = g^\Gamma_ = \Gamma^k_,\quad \cfrac = \mathbf_k \Gamma^k_ This implies that : \Gamma^k_ = \cfrac\cdot\mathbf^k = -\mathbf_i\cdot\cfrac\quad since \quad\cfrac(\mathbf_i\cdot\mathbf^k)=0. Other relations that follow are : \cfrac = -\Gamma^i_\mathbf^k,\quad \boldsymbol\mathbf_i = \Gamma^k_\mathbf_k\otimes\mathbf^j,\quad \boldsymbol\mathbf^i = -\Gamma^i_\mathbf^k\otimes\mathbf^j


Vector operations


Vector and tensor calculus in three-dimensional curvilinear coordinates

Adjustments need to be made in the calculation of
line Line most often refers to: * Line (geometry), object with zero thickness and curvature that stretches to infinity * Telephone line, a single-user circuit on a telephone communication system Line, lines, The Line, or LINE may also refer to: Art ...
, surface and
volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). ...
integrals In mathematics, an integral assigns numbers to functions in a way that describes displacement, area, volume, and other concepts that arise by combining infinitesimal data. The process of finding integrals is called integration. Along with d ...
. For simplicity, the following restricts to three dimensions and orthogonal curvilinear coordinates. However, the same arguments apply for ''n''-dimensional spaces. When the coordinate system is not orthogonal, there are some additional terms in the expressions. Simmonds, in his book on tensor analysis, quotes
Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theor ...
saying
The magic of this theory will hardly fail to impose itself on anybody who has truly understood it; it represents a genuine triumph of the method of absolute differential calculus, founded by Gauss, Riemann, Ricci, and Levi-Civita.
Vector and tensor calculus in general curvilinear coordinates is used in tensor analysis on four-dimensional curvilinear
manifold In mathematics, a manifold is a topological space that locally resembles Euclidean space near each point. More precisely, an n-dimensional manifold, or ''n-manifold'' for short, is a topological space with the property that each point has a n ...
s in
general relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
, in the
mechanics Mechanics (from Ancient Greek: μηχανική, ''mēkhanikḗ'', "of machines") is the area of mathematics and physics concerned with the relationships between force, matter, and motion among physical objects. Forces applied to objec ...
of curved shells, in examining the invariance properties of
Maxwell's equations Maxwell's equations, or Maxwell–Heaviside equations, are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric circuits ...
which has been of interest in metamaterials and in many other fields. Some useful relations in the calculus of vectors and second-order tensors in curvilinear coordinates are given in this section. The notation and contents are primarily from Ogden, Simmonds, Green and Zerna, Basar and Weichert, and Ciarlet. Let φ = φ(x) be a well defined scalar field and v = v(x) a well-defined vector field, and ''λ''1, ''λ''2... be parameters of the coordinates


Geometric elements


Integration

:


Differentiation

The expressions for the gradient, divergence, and Laplacian can be directly extended to ''n''-dimensions, however the curl is only defined in 3D. The vector field b''i'' is tangent to the ''qi'' coordinate curve and forms a natural basis at each point on the curve. This basis, as discussed at the beginning of this article, is also called the covariant curvilinear basis. We can also define a reciprocal basis, or contravariant curvilinear basis, b''i''. All the algebraic relations between the basis vectors, as discussed in the section on tensor algebra, apply for the natural basis and its reciprocal at each point x. :


Fictitious forces in general curvilinear coordinates

By definition, if a particle with no forces acting on it has its position expressed in an inertial coordinate system, (''x''1, ''x''2, ''x''3, ''t''), then there it will have no acceleration (d2''x''''j''/d''t''2 = 0). In this context, a coordinate system can fail to be “inertial” either due to non-straight time axis or non-straight space axes (or both). In other words, the basis vectors of the coordinates may vary in time at fixed positions, or they may vary with position at fixed times, or both. When equations of motion are expressed in terms of any non-inertial coordinate system (in this sense), extra terms appear, called Christoffel symbols. Strictly speaking, these terms represent components of the absolute acceleration (in classical mechanics), but we may also choose to continue to regard d2''x''''j''/d''t''2 as the acceleration (as if the coordinates were inertial) and treat the extra terms as if they were forces, in which case they are called fictitious forces. The component of any such fictitious force normal to the path of the particle and in the plane of the path's curvature is then called
centrifugal force In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is paralle ...
. This more general context makes clear the correspondence between the concepts of centrifugal force in rotating coordinate systems and in stationary curvilinear coordinate systems. (Both of these concepts appear frequently in the literature.) For a simple example, consider a particle of mass ''m'' moving in a circle of radius ''r'' with angular speed ''w'' relative to a system of polar coordinates rotating with angular speed ''W''. The radial equation of motion is ''mr''” = ''F''''r'' + ''mr''(''w'' + ''W'')2. Thus the centrifugal force is ''mr'' times the square of the absolute rotational speed ''A'' = ''w'' + ''W'' of the particle. If we choose a coordinate system rotating at the speed of the particle, then ''W'' = ''A'' and ''w'' = 0, in which case the centrifugal force is ''mrA''2, whereas if we choose a stationary coordinate system we have ''W'' = 0 and ''w'' = ''A'', in which case the centrifugal force is again ''mrA''2. The reason for this equality of results is that in both cases the basis vectors at the particle's location are changing in time in exactly the same way. Hence these are really just two different ways of describing exactly the same thing, one description being in terms of rotating coordinates and the other being in terms of stationary curvilinear coordinates, both of which are non-inertial according to the more abstract meaning of that term. When describing general motion, the actual forces acting on a particle are often referred to the instantaneous osculating circle tangent to the path of motion, and this circle in the general case is not centered at a fixed location, and so the decomposition into centrifugal and Coriolis components is constantly changing. This is true regardless of whether the motion is described in terms of stationary or rotating coordinates.


See also

* Covariance and contravariance * Introduction to the mathematics of general relativity * Special cases: ** Orthogonal coordinates **
Skew coordinates A system of skew coordinates is a curvilinear coordinate system where the coordinate surfaces are not orthogonal, in contrast to orthogonal coordinates. Skew coordinates tend to be more complicated to work with compared to orthogonal coordinates si ...
*
Tensors in curvilinear coordinates Curvilinear coordinates can be formulated in tensor calculus, with important applications in physics and engineering, particularly for describing transportation of physical quantities and deformation of matter in fluid mechanics and continuum mechan ...
* Frenet–Serret formulas *
Covariant derivative In mathematics, the covariant derivative is a way of specifying a derivative along tangent vectors of a manifold. Alternatively, the covariant derivative is a way of introducing and working with a connection on a manifold by means of a differe ...
* Tensor derivative (continuum mechanics) *
Curvilinear perspective Curvilinear perspective, also five-point perspective, is a graphical projection used to draw 3D objects on 2D surfaces. It was formally codified in 1968 by the artists and art historians André Barre and Albert Flocon in the book ''La Perspective c ...
* Del in cylindrical and spherical coordinates


References


Further reading

* *


External links


Planetmath.org Derivation of Unit vectors in curvilinear coordinates



Prof. R. Brannon's E-Book on Curvilinear Coordinates
* Wikiversity:Introduction to Elasticity/Tensors#The divergence of a tensor field
Wikiversity Wikiversity is a Wikimedia Foundation project that supports learning communities, their learning materials, and resulting activities. It differs from Wikipedia in that it offers tutorials and other materials for the fostering of learning, rather ...
, Introduction to Elasticity/Tensors. {{DEFAULTSORT:Curvilinear Coordinates Coordinate systems *3