Definitions

# mathematics

[math-uh-mat-iks]
mathematics, deductive study of numbers, geometry, and various abstract constructs, or structures; the latter often "abstract" the features common to several models derived from the empirical, or applied, sciences, although many emerge from purely mathematical or logical considerations. Mathematics is very broadly divided into foundations, algebra, analysis, geometry, and applied mathematics, which includes theoretical computer science.

## Branches of Mathematics

Foundations

The term foundations is used to refer to the formulation and analysis of the language, axioms, and logical methods on which all of mathematics rests (see logic; symbolic logic). The scope and complexity of modern mathematics requires a very fine analysis of the formal language in which meaningful mathematical statements may be formulated and perhaps be proved true or false. Most apparent mathematical contradictions have been shown to derive from an imprecise and inconsistent use of language. A basic task is to furnish a set of axioms effectively free of contradictions and at the same time rich enough to constitute a deductive source for all of modern mathematics. The modern axiom schemes proposed for this purpose are all couched within the theory of sets, originated by Georg Cantor, which now constitutes a universal mathematical language.

Algebra

Historically, algebra is the study of solutions of one or several algebraic equations, involving the polynomial functions of one or several variables. The case where all the polynomials have degree one (systems of linear equations) leads to linear algebra. The case of a single equation, in which one studies the roots of one polynomial, leads to field theory and to the so-called Galois theory. The general case of several equations of high degree leads to algebraic geometry, so named because the sets of solutions of such systems are often studied by geometric methods.

Modern algebraists have increasingly abstracted and axiomatized the structures and patterns of argument encountered not only in the theory of equations, but in mathematics generally. Examples of these structures include groups (first witnessed in relation to symmetry properties of the roots of a polynomial and now ubiquitous throughout mathematics), rings (of which the integers, or whole numbers, constitute a basic example), and fields (of which the rational, real, and complex numbers are examples). Some of the concepts of modern algebra have found their way into elementary mathematics education in the so-called new mathematics.

Some important abstractions recently introduced in algebra are the notions of category and functor, which grew out of so-called homological algebra. Arithmetic and number theory, which are concerned with special properties of the integers—e.g., unique factorization, primes, equations with integer coefficients (Diophantine equations), and congruences—are also a part of algebra. Analytic number theory, however, also applies the nonalgebraic methods of analysis to such problems.

Analysis

The essential ingredient of analysis is the use of infinite processes, involving passage to a limit. For example, the area of a circle may be computed as the limiting value of the areas of inscribed regular polygons as the number of sides of the polygons increases indefinitely. The basic branch of analysis is the calculus. The general problem of measuring lengths, areas, volumes, and other quantities as limits by means of approximating polygonal figures leads to the integral calculus. The differential calculus arises similarly from the problem of finding the tangent line to a curve at a point. Other branches of analysis result from the application of the concepts and methods of the calculus to various mathematical entities. For example, vector analysis is the calculus of functions whose variables are vectors. Here various types of derivatives and integrals may be introduced. They lead, among other things, to the theory of differential and integral equations, in which the unknowns are functions rather than numbers, as in algebraic equations. Differential equations are often the most natural way in which to express the laws governing the behavior of various physical systems. Calculus is one of the most powerful and supple tools of mathematics. Its applications, both in pure mathematics and in virtually every scientific domain, are manifold.

Geometry

The shape, size, and other properties of figures and the nature of space are in the province of geometry. Euclidean geometry is concerned with the axiomatic study of polygons, conic sections, spheres, polyhedra, and related geometric objects in two and three dimensions—in particular, with the relations of congruence and of similarity between such objects. The unsuccessful attempt to prove the "parallel postulate" from the other axioms of Euclid led in the 19th cent. to the discovery of two different types of non-Euclidean geometry.

The 20th cent. has seen an enormous development of topology, which is the study of very general geometric objects, called topological spaces, with respect to relations that are much weaker than congruence and similarity. Other branches of geometry include algebraic geometry and differential geometry, in which the methods of analysis are brought to bear on geometric problems. These fields are now in a vigorous state of development.

Applied Mathematics

The term applied mathematics loosely designates a wide range of studies with significant current use in the empirical sciences. It includes numerical methods and computer science, which seeks concrete solutions, sometimes approximate, to explicit mathematical problems (e.g., differential equations, large systems of linear equations). It has a major use in technology for modeling and simulation. For example, the huge wind tunnels, formerly used to test expensive prototypes of airplanes, have all but disappeared. The entire design and testing process is now largely carried out by computer simulation, using mathematically tailored software. It also includes mathematical physics, which now strongly interacts with all of the central areas of mathematics. In addition, probability theory and mathematical statistics are often considered parts of applied mathematics. The distinction between pure and applied mathematics is now becoming less significant.

## Development of Mathematics

The earliest records of mathematics show it arising in response to practical needs in agriculture, business, and industry. In Egypt and Mesopotamia, where evidence dates from the 2d and 3d millennia B.C., it was used for surveying and mensuration; estimates of the value of π (pi) are found in both locations. There is some evidence of similar developments in India and China during this same period, but few records have survived. This early mathematics is generally empirical, arrived at by trial and error as the best available means for obtaining results, with no proofs given. However, it is now known that the Babylonians were aware of the necessity of proofs prior to the Greeks, who had been presumed the originators of this important step.

Greek Contributions

A profound change occurred in the nature and approach to mathematics with the contributions of the Greeks. The earlier (Hellenic) period is represented by Thales (6th cent. B.C.), Pythagoras, Plato, and Aristotle, and by the schools associated with them. The Pythagorean theorem, known earlier in Mesopotamia, was discovered by the Greeks during this period.

During the Golden Age (5th cent. B.C.), Hippocrates of Chios made the beginnings of an axiomatic approach to geometry and Zeno of Elea proposed his famous paradoxes concerning the infinite and the infinitesimal, raising questions about the nature of and relationships among points, lines, and numbers. The discovery through geometry of irrational numbers, such as 2, also dates from this period. Eudoxus of Cnidus (4th cent. B.C.) resolved certain of the problems by proposing alternative methods to those involving infinitesimals; he is known for his work on geometric proportions and for his exhaustion theory for determining areas and volumes.

The later (Hellenistic) period of Greek science is associated with the school of Alexandria. The greatest work of Greek mathematics, Euclid's Elements (c.300 B.C.), appeared at the beginning of this period. Elementary geometry as taught in high school is still largely based on Euclid's presentation, which has served as a model for deductive systems in other parts of mathematics and in other sciences. In this method primitive terms, such as point and line, are first defined, then certain axioms and postulates relating to them and seeming to follow directly from them are stated without proof; a number of statements are then derived by deduction from the definitions, axioms, and postulates. Euclid also contributed to the development of arithmetic and presented a geometric theory of quadratic equations.

In the 3d cent. B.C., Archimedes, in addition to his work in mechanics, made an estimate of π and used the exhaustion theory of Eudoxus to obtain results that foreshadowed those much later of the integral calculus, and Apollonius of Perga named the conic sections and gave the first theory for them. A second Alexandrian school of the Roman period included contributions by Menelaus (c.A.D. 100, spherical triangles), Heron of Alexandria (geometry), Ptolemy (A.D. 150, astronomy, geometry, cartography), Pappus (3d cent., geometry), and Diophantus (3d cent., arithmetic).

Following the decline of learning in the West after the 3d cent., the development of mathematics continued in the East. In China, Tsu Ch'ung-Chih estimated π by inscribed and circumscribed polygons, as Archimedes had done, and in India the numerals now used throughout the civilized world were invented and contributions to geometry were made by Aryabhata and Brahmagupta (5th and 6th cent. A.D.). The Arabs were responsible for preserving the work of the Greeks, which they translated, commented upon, and augmented. In Baghdad, Al-Khowarizmi (9th cent.) wrote an important work on algebra and introduced the Hindu numerals for the first time to the West, and Al-Battani worked on trigonometry. In Egypt, Ibn al-Haytham was concerned with the solids of revolution and geometrical optics. The Persian poet Omar Khayyam wrote on algebra.

Western Developments from the Twelfth to Eighteenth Centuries

Word of the Chinese and Middle Eastern works began to reach the West in the 12th and 13th cent. One of the first important European mathematicians was Leonardo da Pisa (Leonardo Fibonacci), who wrote on arithmetic and algebra (Liber abaci, 1202) and on geometry (Practica geometriae, 1220). With the Renaissance came a great revival of interest in learning, and the invention of printing made many of the earlier books widely available. By the end of the 16th cent. advances had been made in algebra by Niccolò Tartaglia and Geronimo Cardano, in trigonometry by François Viète, and in such areas of applied mathematics as mapmaking by Mercator and others.

The 17th cent., however, saw the greatest revolution in mathematics, as the scientific revolution spread to all fields. Decimal fractions were invented by Simon Stevin and logarithms by John Napier and Henry Briggs; the beginnings of projective geometry were made by Gérard Desargues and Blaise Pascal; number theory was greatly extended by Pierre de Fermat; and the theory of probability was founded by Pascal, Fermat, and others. In the application of mathematics to mechanics and astronomy, Galileo and Johannes Kepler made fundamental contributions.

The greatest mathematical advances of the 17th cent., however, were the invention of analytic geometry by René Descartes and that of the calculus by Isaac Newton and, independently, by G. W. Leibniz. Descartes's invention (anticipated by Fermat, whose work was not published until later) made possible the expression of geometric problems in algebraic form and vice versa. It was indispensable in creating the calculus, which built upon and superseded earlier special methods for finding areas, volumes, and tangents to curves, developed by F. B. Cavalieri, Fermat, and others. The calculus is probably the greatest tool ever invented for the mathematical formulation and solution of physical problems.

The history of mathematics in the 18th cent. is dominated by the development of the methods of the calculus and their application to such problems, both terrestrial and celestial, with leading roles being played by the Bernoulli family (especially Jakob, Johann, and Daniel), Leonhard Euler, Guillaume de L'Hôpital, and J. L. Lagrange. Important advances in geometry began toward the end of the century with the work of Gaspard Monge in descriptive geometry and in differential geometry and continued through his influence on others, e.g., his pupil J. V. Poncelet, who founded projective geometry (1822).

In the Nineteenth Century

The modern period of mathematics dates from the beginning of the 19th cent., and its dominant figure is C. F. Gauss. In the area of geometry Gauss made fundamental contributions to differential geometry, did much to found what was first called analysis situs but is now called topology, and anticipated (although he did not publish his results) the great breakthrough of non-Euclidean geometry. This breakthrough was made by N. I. Lobachevsky (1826) and independently by János Bolyai (1832), the son of a close friend of Gauss, whom each proceeded by establishing the independence of Euclid's fifth (parallel) postulate and showing that a different, self-consistent geometry could be derived by substituting another postulate in its place. Still another non-Euclidean geometry was invented by Bernhard Riemann (1854), whose work also laid the foundations for the modern tensor calculus description of space, so important in the general theory of relativity.

In the area of arithmetic, number theory, and algebra, Gauss again led the way. He established the modern theory of numbers, gave the first clear exposition of complex numbers, and investigated the functions of complex variables. The concept of number was further extended by W. R. Hamilton, whose theory of quaternions (1843) provided the first example of a noncommutative algebra (i.e., one in which ab ≠ ba). This work was generalized the following year by H. G. Grassmann, who showed that several different consistent algebras may be derived by choosing different sets of axioms governing the operations on the elements of the algebra.

These developments continued with the group theory of M. S. Lie in the late 19th cent. and reached full expression in the wide scope of modern abstract algebra. Number theory received significant contributions in the latter half of the 19th cent. through the work of Georg Cantor, J. W. R. Dedekind, and K. W. Weierstrass. Still another influence of Gauss was his insistence on rigorous proof in all areas of mathematics. In analysis this close examination of the foundations of the calculus resulted in A. L. Cauchy's theory of limits (1821), which in turn yielded new and clearer definitions of continuity, the derivative, and the definite integral. A further important step toward rigor was taken by Weierstrass, who raised new questions about these concepts and showed that ultimately the foundations of analysis rest on the properties of the real number system.

In the Twentieth Century

In the 20th cent. the trend has been toward increasing generalization and abstraction, with the elements and operations of systems being defined so broadly that their interpretations connect such areas as algebra, geometry, and topology. The key to this approach has been the use of formal axiomatics, in which the notion of axioms as "self-evident truths" has been discarded. Instead the emphasis is on such logical concepts as consistency and completeness. The roots of formal axiomatics lie in the discoveries of alternative systems of geometry and algebra in the 19th cent.; the approach was first systematically undertaken by David Hilbert in his work on the foundations of geometry (1899).

The emphasis on deductive logic inherent in this view of mathematics and the discovery of the interconnections between the various branches of mathematics and their ultimate basis in number theory led to intense activity in the field of mathematical logic after the turn of the century. Rival schools of thought grew up under the leadership of Hilbert, Bertrand Russell and A. N. Whitehead, and L. E. J. Brouwer. Important contributions in the investigation of the logical foundations of mathematics were made by Kurt Gödel and A. Church.

## Bibliography

See R. Courant and H. Robbins, What Is Mathematics? (1941); E. T. Bell, The Development of Mathematics (2d ed. 1945) and Men of Mathematics (1937, repr. 1961); J. R. Newman, ed., The World of Mathematics (4 vol., 1956); E. E. Kramer, The Nature and Growth of Mathematics (1970); M. Kline, Mathematical Thought from Ancient to Modern Times (1973); D. J. Albers and G. L. Alexanderson, ed., Mathematical People (1985).

Branch of philosophy concerned with the epistemology and ontology of mathematics. Early in the 20th century, three main schools of thought—called logicism, formalism, and intuitionism—arose to account for and resolve the crisis in the foundations of mathematics. Logicism argues that all mathematical notions are reducible to laws of pure thought, or logical principles; a variant known as mathematical Platonism holds that mathematical notions are transcendent Ideals, or Forms, independent of human consciousness. Formalism holds that mathematics consists simply of the manipulation of finite configurations of symbols according to prescribed rules; a “game” independent of any physical interpretation of the symbols. Intuitionism is characterized by its rejection of any knowledge- or evidence-transcendent notion of truth. Hence, only objects that can be constructed (see constructivism) in a finite number of steps are admitted, while actual infinities and the law of the excluded middle (see laws of thought) are rejected. These three schools of thought were principally led, respectively, by Bertrand Russell, David Hilbert, and the Dutch mathematician Luitzen Egbertus Jan Brouwer (1881–1966).

Scientific inquiry into the nature of mathematical theories and the scope of mathematical methods. It began with Euclid's Elements as an inquiry into the logical and philosophical basis of mathematics—in essence, whether the axioms of any system (be it Euclidean geometry or calculus) can ensure its completeness and consistency. In the modern era, this debate for a time divided into three schools of thought: logicism, formalism, and intuitionism. Logicists supposed that abstract mathematical objects can be entirely developed starting from basic ideas of sets and rational, or logical, thought; a variant of logicism, known as mathematical Platonism, views these objects as existing external to and independent of an observer. Formalists believed mathematics to be the manipulation of configurations of symbols according to prescribed rules, a “game” independent of any physical interpretation of the symbols. Intuitionists rejected certain concepts of logic and the notion that the axiomatic method would suffice to explain all of mathematics, instead seeing mathematics as an intellectual activity dealing with mental constructions (see constructivism) independent of language and any external reality. In the 20th century, Gödel's theorem ended any hope of finding an axiomatic basis of mathematics that was both complete and free from contradictions.

Science of structure, order, and relation that has evolved from counting, measuring, and describing the shapes of objects. It deals with logical reasoning and quantitative calculation. Since the 17th century it has been an indispensable adjunct to the physical sciences and technology, to the extent that it is considered the underlying language of science. Among the principal branches of mathematics are algebra, analysis, arithmetic, combinatorics, Euclidean and non-Euclidean geometries, game theory, number theory, numerical analysis, optimization, probability, set theory, statistics, topology, and trigonometry.

Branch of mathematics concerned with the selection, arrangement, and combination of objects chosen from a finite set. The number of possible bridge hands is a simple example; more complex problems include scheduling classes in classrooms at a large university and designing a routing system for telephone signals. No standard algebraic procedures apply to all combinatorial problems; a separate logical analysis may be required for each problem. Combinatorics has its roots in antiquity, but new uses in computer science and systems management have increased its importance in recent years. Seealso permutations and combinations.

In vector calculus, curl (also named: rotor) is a vector operator that shows a vector field's "rotation"; that is, the direction of the axis of rotation and the magnitude of the rotation. It can also be described as the circulation density.

"Rotation" and "circulation" are used here for properties of a vector function of position, regardless of their possible change in time.

A vector field which has zero curl everywhere is called irrotational.

The alternative terminology rotor and alternative notation $operatorname\left\{rot\right\}\left(mathbf\left\{F\right\}\right)$ are often used (especially in many European countries) for curl and $operatorname\left\{curl\right\}\left(mathbf\left\{F\right\}\right)$.

## Definition

The curl of a vector field, denoted $operatorname\left\{curl\right\}\left(mathbf\left\{F\right\}\right)$ or $vec\left\{nabla\right\} times vec\left\{F\right\}$, at a point is defined to be the limiting value of a closed line integral in a plane as the path the integral uses becomes infinitesimally close to the point, divided by the area enclosed. As such, the curl operator maps C1 functions from R3 to R3 to C0 functions from R3 to R3.

Explicitly, curl is defined by:

$left\left(vec\left\{nabla\right\} times vec\left\{F\right\}right\right) cdot hat\left\{n\right\} overset\left\{underset\left\{mathrm\left\{def\right\}\right\}\left\{\right\}\right\}\left\{=\right\} lim_\left\{A to 0\right\} frac\left\{oint_\left\{C\right\} vec\left\{F\right\} cdot vec\left\{ds\right\}\right\}\left\{A\right\}$

where $hat\left\{n\right\}$ is a unit vector normal to the plane, $oint_\left\{C\right\} vec\left\{F\right\} cdot vec\left\{ds\right\}$ is a line integral around the area in question, and A is the magnitude of the area. If $hat\left\{nu\right\}$ is an outward pointing normal to A (see caption at right) in the plane normal to $hat\left\{n\right\}$, then the orientation of C is chosen so that a vector $hat\left\{omega\right\}$ tangent to C is positively oriented if and only if $\left\{hat\left\{n\right\},hat\left\{nu\right\},hat\left\{omega\right\}\right\}$ forms a positively oriented basis for R3. This is essentially the right hand rule. If you point your right thumb in the direction of $hat\left\{n\right\}$ then C is traversed in the direction your fingers curl.

## Usage

In practice, this definition is rarely used because in virtually all cases, the curl operator can be applied using some set of curvilinear coordinates, for which simpler representations have been derived.

Although the usage of $vec\left\{nabla\right\} times vec\left\{F\right\}$ is strictly an abuse of notation, it is still useful as a mnemonic in Cartesian coordinates if we take $nabla$ as a vector differential operator del or nabla. Such notation involving operators is common in physics and algebra.

Expanded in Cartesian coordinates (see: Del in cylindrical and spherical coordinates for spherical and cylindrical coordinate representations), $vec\left\{nabla\right\} times vec\left\{F\right\}$ is, for F composed of [Fx, Fy, Fz]:

$begin\left\{vmatrix\right\} mathbf\left\{i\right\} & mathbf\left\{j\right\} & mathbf\left\{k\right\}$
{frac{partial}{partial x}} & {frac{partial}{partial y}} & {frac{partial}{partial z}} F_x & F_y & F_z end{vmatrix}

where i, j, and k are the unit vectors for the x-, y-, and z-axes, respectively. This expands as follows:

$left\left(frac\left\{partial F_z\right\}\left\{partial y\right\} - frac\left\{partial F_y\right\}\left\{partial z\right\}right\right) mathbf\left\{i\right\} + left\left(frac\left\{partial F_x\right\}\left\{partial z\right\} - frac\left\{partial F_z\right\}\left\{partial x\right\}right\right) mathbf\left\{j\right\} + left\left(frac\left\{partial F_y\right\}\left\{partial x\right\} - frac\left\{partial F_x\right\}\left\{partial y\right\}right\right) mathbf\left\{k\right\}$

Although expressed in terms of coordinates, the result is invariant under proper rotations of the coordinate axes but the result inverts under reflection.

In Einstein notation, with the Levi-Civita symbol it is written as:

$\left(vec\left\{nabla\right\} times vec\left\{F\right\} \right)_k = epsilon_\left\{kell m\right\} partial_ell F_m$

or as:

$\left(vec\left\{nabla\right\} times vec\left\{F\right\} \right) = boldsymbol\left\{hat\left\{e\right\}\right\}_kepsilon_\left\{kell m\right\} partial_ell F_m$

for unit vectors:$boldsymbol\left\{hat\left\{e\right\}\right\}_k$, k=1,2,3 corresponding to $boldsymbol\left\{hat\left\{x\right\}\right\}, boldsymbol\left\{hat\left\{y\right\}\right\}$, and $boldsymbol\left\{hat\left\{z\right\}\right\}$ respectively.

Using the exterior derivative, the curl can be expressed as:

$vec\left\{nabla\right\} times vec\left\{F\right\} = left\left[star left\left(\left\{mathbf d\right\} F^flat right\right) right\right]^sharp$

Here $flat$ and $sharp$ are the musical isomorphisms, and $star$ is the Hodge dual. This formula shows how to calculate the curl of F in any coordinate system, and how to extend the curl to any oriented three dimensional Riemmannian manifold. Since this depends on a choice of orientation, curl is a chiral operation. In other words, if the orientation is reversed, then the direction of the curl is also reversed.

## Interpreting the curl

The curl of vector field tells us about the rotation the field has at any point. The magnitude of the curl tells us how much rotation there is. The direction tells us, by the right-hand rule (four fingers of the right hand are curled in the direction of the motion and the thumb points in the direction of the rotation) about which axis the field is rotating.

A commonly used device for thinking about curl is the paddle wheel. If we were to place a very small paddle wheel at a point in the vector field in question and treat the drawn vectors and their lengths as currents in a river with magnitude and direction, whichever way the paddle wheel would tend to turn is the direction of the curl at that point. For example, if two currents are trying to rotate the wheel in opposite directions, the stronger one (the longer vector) will win.

## Examples

### A simple vector field

Take the vector field constructed using unit vectors

$vec\left\{F\right\}\left(x,y,z\right)=yboldsymbol\left\{hat\left\{x\right\}\right\}-xboldsymbol\left\{hat\left\{y\right\}\right\}.$

Its plot looks like this:

Simply by visual inspection, we can see that the field is rotating. If we stick a paddle wheel anywhere, we see immediately its tendency to rotate clockwise. Using the right-hand rule, we expect the curl to be into the page. If we are to keep a right-handed coordinate system, into the page will be in the negative z direction. The lack of x and y directions is analogous to the cross product operation.

If we do the math and find the curl:

$vec\left\{nabla\right\} times vec\left\{F\right\} =0boldsymbol\left\{hat\left\{x\right\}\right\}+0boldsymbol\left\{hat\left\{y\right\}\right\}+ \left[\left\{frac\left\{partial\right\}\left\{partial x\right\}\right\}\left(-x\right) -\left\{frac\left\{partial\right\}\left\{partial y\right\}\right\} y\right]boldsymbol\left\{hat\left\{z\right\}\right\}=-2boldsymbol\left\{hat\left\{z\right\}\right\}$

Which is indeed in the negative z direction, as expected. In this case, the curl is actually a constant, irrespective of position. The "amount" of rotation in the above vector field is the same at any point (x,y). Plotting the curl of F isn't very interesting:

### A more involved example

Suppose we now consider a slightly more complicated vector field:

$vec\left\{F\right\}\left(x,y,z\right)=-x^2boldsymbol\left\{hat\left\{y\right\}\right\}.$

Its plot:

We might not see any rotation initially, but if we closely look at the right, we see a larger field at, say, x=4 than at x=3. Intuitively, if we placed a small paddle wheel there, the larger "current" on its right side would cause the paddlewheel to rotate clockwise, which corresponds to a curl in the negative z direction. By contrast, if we look at a point on the left and placed a small paddle wheel there, the larger "current" on its left side would cause the paddlewheel to rotate counterclockwise, which corresponds to a curl in the positive z direction. Let's check out our guess by doing the math:

$vec\left\{nabla\right\} times vec\left\{F\right\} =0boldsymbol\left\{hat\left\{x\right\}\right\}+0boldsymbol\left\{hat\left\{y\right\}\right\}+ \left\{frac\left\{partial\right\}\left\{partial x\right\}\right\}\left(-x^2\right) boldsymbol\left\{hat\left\{z\right\}\right\}=-2xboldsymbol\left\{hat\left\{z\right\}\right\}.$

Indeed the curl is in the positive z direction for negative x and in the negative z direction for positive x, as expected. Since this curl is not the same at every point, its plot is a bit more interesting:

We note that the plot of this curl has no dependence on y or z (as it shouldn't) and is in the negative z direction for positive x and in the positive z direction for negative x.

### Three common identities

Consider the example × [v × F ]. Using Cartesian coordinates, it can be shown that
$mathbf\left\{ nabla times\right\} left\left(mathbf\left\{v times F\right\} right\right) = left\left[left\left(mathbf\left\{ nabla cdot F \right\} right\right) + mathbf\left\{F cdot nabla\right\} right\right] mathbf\left\{v\right\}- left\left[left\left(mathbf\left\{ nabla cdot v \right\} right\right) + mathbf\left\{v cdot nabla\right\} right\right] mathbf\left\{F\right\} .$

In the case where the vector field v and are interchanged:

$mathbf\left\{v times \right\} left\left(mathbf\left\{ nabla times F\right\} right\right) =nabla_F left\left(mathbf\left\{v cdot F \right\} right\right) - left\left(mathbf\left\{v cdot nabla \right\} right\right) mathbf\left\{ F\right\} ,$

which introduces the Feynman subscript notation F, which means the subscripted gradient operates on only the factor F.

Another example is × [ × F ]. Using Cartesian coordinates, it can be shown that:

$nabla times left\left(mathbf\left\{nabla times F\right\} right\right) = mathbf\left\{nabla\right\} \left(mathbf\left\{nabla cdot F\right\}\right) - nabla^2 mathbf\left\{F\right\} ,$

which can be construed as a special case of the first example with the substitution v.

### Descriptive examples

• In a tornado the winds are rotating about the eye, and a vector field showing wind velocities would have a non-zero curl at the eye, and possibly elsewhere (see vorticity).
• In a vector field that describes the linear velocities of each individual part of a rotating disk, the curl will have a constant value on all parts of the disk.
• If velocities of cars on a freeway were described with a vector field, and the lanes had different speed limits, the curl on the borders between lanes would be non-zero.
• Faraday's law of induction, one of Maxwell's equations, can be expressed very simply using curl. It states that the curl of an electric field is equal to the opposite of the time rate of change of the magnetic field.