In mathematics, a **curve** (also called a **curved line** in older texts) is an object similar to a line, but that does not have to be straight.

- History
- Topological curve
- Differentiable curve
- Differentiable arc
- Length of a curve
- Differential geometry
- Algebraic curve
- See also
- Notes
- References
- External links

Intuitively, a curve may be thought of as the trace left by a moving point. This is the definition that appeared more than 2000 years ago in Euclid's *Elements*: "The [curved] line^{ [lower-alpha 1] } is […] the first species of quantity, which has only one dimension, namely length, without any width nor depth, and is nothing else than the flow or run of the point which […] will leave from its imaginary moving some vestige in length, exempt of any width."^{ [1] }

This definition of a curve has been formalized in modern mathematics as: *A curve is the image of an interval to a topological space by a continuous function *. In some contexts, the function that defines the curve is called a *parametrization*, and the curve is a parametric curve. In this article, these curves are sometimes called *topological curves* to distinguish them from more constrained curves such as differentiable curves. This definition encompasses most curves that are studied in mathematics; notable exceptions are level curves (which are unions of curves and isolated points), and algebraic curves (see below). Level curves and algebraic curves are sometimes called implicit curves, since they are generally defined by implicit equations.

Nevertheless, the class of topological curves is very broad, and contains some curves that do not look as one may expect for a curve, or even cannot be drawn. This is the case of space-filling curves and fractal curves. For ensuring more regularity, the function that defines a curve is often supposed to be differentiable, and the curve is then said to be a differentiable curve.

A plane algebraic curve is the zero set of a polynomial in two indeterminates. More generally, an algebraic curve is the zero set of a finite set of polynomials, which satisfies the further condition of being an algebraic variety of dimension one. If the coefficients of the polynomials belong to a field k, the curve is said to be *defined over*k. In the common case of a real algebraic curve, where k is the field of real numbers, an algebraic curve is a finite union of topological curves. When complex zeros are considered, one has a *complex algebraic curve*, which, from the topological point of view, is not a curve, but a surface, and is often called a Riemann surface. Although not being curves in the common sense, algebraic curves defined over other fields have been widely studied. In particular, algebraic curves over a finite field are widely used in modern cryptography.

Interest in curves began long before they were the subject of mathematical study. This can be seen in numerous examples of their decorative use in art and on everyday objects dating back to prehistoric times.^{ [2] } Curves, or at least their graphical representations, are simple to create, for example with a stick on the sand on a beach.

Historically, the term *line* was used in place of the more modern term *curve*. Hence the terms *straight line* and *right line* were used to distinguish what are today called lines from curved lines. For example, in Book I of Euclid's Elements, a line is defined as a "breadthless length" (Def. 2), while a *straight* line is defined as "a line that lies evenly with the points on itself" (Def. 4). Euclid's idea of a line is perhaps clarified by the statement "The extremities of a line are points," (Def. 3).^{ [3] } Later commentators further classified lines according to various schemes. For example:^{ [4] }

- Composite lines (lines forming an angle)
- Incomposite lines
- Determinate (lines that do not extend indefinitely, such as the circle)
- Indeterminate (lines that extend indefinitely, such as the straight line and the parabola)

The Greek geometers had studied many other kinds of curves. One reason was their interest in solving geometrical problems that could not be solved using standard compass and straightedge construction. These curves include:

- The conic sections, studied in depth by Apollonius of Perga
- The cissoid of Diocles, studied by Diocles and used as a method to double the cube.
^{ [5] } - The conchoid of Nicomedes, studied by Nicomedes as a method to both double the cube and to trisect an angle.
^{ [6] } - The Archimedean spiral, studied by Archimedes as a method to trisect an angle and square the circle.
^{ [7] } - The spiric sections, sections of tori studied by Perseus as sections of cones had been studied by Apollonius.

A fundamental advance in the theory of curves was the introduction of analytic geometry by René Descartes in the seventeenth century. This enabled a curve to be described using an equation rather than an elaborate geometrical construction. This not only allowed new curves to be defined and studied, but it enabled a formal distinction to be made between algebraic curves that can be defined using polynomial equations, and transcendental curves that cannot. Previously, curves had been described as "geometrical" or "mechanical" according to how they were, or supposedly could be, generated.^{ [2] }

Conic sections were applied in astronomy by Kepler. Newton also worked on an early example in the calculus of variations. Solutions to variational problems, such as the brachistochrone and tautochrone questions, introduced properties of curves in new ways (in this case, the cycloid). The catenary gets its name as the solution to the problem of a hanging chain, the sort of question that became routinely accessible by means of differential calculus.

In the eighteenth century came the beginnings of the theory of plane algebraic curves, in general. Newton had studied the cubic curves, in the general description of the real points into 'ovals'. The statement of Bézout's theorem showed a number of aspects which were not directly accessible to the geometry of the time, to do with singular points and complex solutions.

Since the nineteenth century, curve theory is viewed as the special case of dimension one of the theory of manifolds and algebraic varieties. Nevertheless, many questions remain specific to curves, such as space-filling curves, Jordan curve theorem and Hilbert's sixteenth problem.

A **topological curve** can be specified by a continuous function from an interval I of the real numbers into a topological space X. Properly speaking, the *curve* is the image of However, in some contexts, itself is called a curve, especially when the image does not look like what is generally called a curve and does not characterize sufficiently

For example, the image of the Peano curve or, more generally, a space-filling curve completely fills a square, and therefore does not give any information on how is defined.

A curve is **closed**^{ [8] } or is a loop if and . A closed curve is thus the image of a continuous mapping of a circle.

If the domain of a topological curve is a closed and bounded interval , it is called a * path *, also known as *topological arc* (or just **arc**).

A curve is **simple** if it is the image of an interval or a circle by an injective continuous function. In other words, if a curve is defined by a continuous function with an interval as a domain, the curve is simple if and only if two different points of the interval have different images, except, possibly, if the points are the end points of the interval. Intuitively, a simple curve is a curve that "does not cross itself and has no missing points".^{ [9] }

A simple closed curve is also called a Jordan curve. The Jordan curve theorem states that the set complement in a plane of a Jordan curve consists of two connected components (that is the curve divides the plane in two non-intersecting regions that are both connected).

A * plane curve * is a curve for which is the Euclidean plane —these are the examples first encountered—or in some cases the projective plane. A *space curve* is a curve for which is at least three-dimensional; a *skew curve* is a space curve which lies in no plane. These definitions of plane, space and skew curves apply also to real algebraic curves, although the above definition of a curve does not apply (a real algebraic curve may be disconnected).

The definition of a curve includes figures that can hardly be called curves in common usage. For example, the image of a simple curve can cover a square in the plane (space-filling curve) and thus have a positive area.^{ [10] } Fractal curves can have properties that are strange for the common sense. For example, a fractal curve can have a Hausdorff dimension bigger than one (see Koch snowflake) and even a positive area. An example is the dragon curve, which has many other unusual properties.

Roughly speaking a *differentiable curve* is a curve that is defined as being locally the image of an injective differentiable function from an interval I of the real numbers into a differentiable manifold X, often

More precisely, a differentiable curve is a subset C of X where every point of C has a neighborhood U such that is diffeomorphic to an interval of the real numbers.^{[ clarification needed ]} In other words, a differentiable curve is a differentiable manifold of dimension one.

In Euclidean geometry, an **arc** (symbol: **⌒**) is a connected subset of a differentiable curve.

Arcs of lines are called segments or rays, depending whether they are bounded or not.

A common curved example is an arc of a circle, called a circular arc.

In a sphere (or a spheroid), an arc of a great circle (or a great ellipse) is called a **great arc**.

If is the -dimensional Euclidean space, and if is an injective and continuously differentiable function, then the length of is defined as the quantity

The length of a curve is independent of the parametrization .

In particular, the length of the graph of a continuously differentiable function defined on a closed interval is

More generally, if is a metric space with metric , then we can define the length of a curve by

where the supremum is taken over all and all partitions of .

A rectifiable curve is a curve with finite length. A curve is called *natural* (or unit-speed or parametrized by arc length) if for any such that , we have

If is a Lipschitz-continuous function, then it is automatically rectifiable. Moreover, in this case, one can define the speed (or metric derivative) of at as

and then show that

While the first examples of curves that are met are mostly plane curves (that is, in everyday words, *curved lines* in *two-dimensional space*), there are obvious examples such as the helix which exist naturally in three dimensions. The needs of geometry, and also for example classical mechanics are to have a notion of curve in space of any number of dimensions. In general relativity, a world line is a curve in spacetime.

If is a differentiable manifold, then we can define the notion of *differentiable curve* in . This general idea is enough to cover many of the applications of curves in mathematics. From a local point of view one can take to be Euclidean space. On the other hand, it is useful to be more general, in that (for example) it is possible to define the tangent vectors to by means of this notion of curve.

If is a smooth manifold, a *smooth curve* in is a smooth map

- .

This is a basic notion. There are less and more restricted ideas, too. If is a manifold (i.e., a manifold whose charts are times continuously differentiable), then a curve in is such a curve which is only assumed to be (i.e. times continuously differentiable). If is an analytic manifold (i.e. infinitely differentiable and charts are expressible as power series), and is an analytic map, then is said to be an *analytic curve*.

A differentiable curve is said to be **regular** if its derivative never vanishes. (In words, a regular curve never slows to a stop or backtracks on itself.) Two differentiable curves

- and

are said to be *equivalent* if there is a bijective map

such that the inverse map

is also , and

for all . The map is called a *reparametrization* of ; and this makes an equivalence relation on the set of all differentiable curves in . A *arc* is an equivalence class of curves under the relation of reparametrization.

Algebraic curves are the curves considered in algebraic geometry. A plane algebraic curve is the set of the points of coordinates *x*, *y* such that *f*(*x*, *y*) = 0, where *f* is a polynomial in two variables defined over some field *F*. One says that the curve is *defined over**F*. Algebraic geometry normally considers not only points with coordinates in *F* but all the points with coordinates in an algebraically closed field *K*.

If *C* is a curve defined by a polynomial *f* with coefficients in *F*, the curve is said to be defined over *F*.

In the case of a curve defined over the real numbers, one normally considers points with complex coordinates. In this case, a point with real coordinates is a *real point*, and the set of all real points is the *real part* of the curve. It is therefore only the real part of an algebraic curve that can be a topological curve (this is not always the case, as the real part of an algebraic curve may be disconnected and contain isolated points). The whole curve, that is the set of its complex point is, from the topological point of view a surface. In particular, the nonsingular complex projective algebraic curves are called Riemann surfaces.

The points of a curve *C* with coordinates in a field *G* are said to be rational over *G* and can be denoted *C*(*G*). When *G* is the field of the rational numbers, one simply talks of *rational points*. For example, Fermat's Last Theorem may be restated as: *For**n* > 2, *every rational point of the Fermat curve of degree n has a zero coordinate*.

Algebraic curves can also be space curves, or curves in a space of higher dimension, say *n*. They are defined as algebraic varieties of dimension one. They may be obtained as the common solutions of at least *n*–1 polynomial equations in *n* variables. If *n*–1 polynomials are sufficient to define a curve in a space of dimension *n*, the curve is said to be a complete intersection. By eliminating variables (by any tool of elimination theory), an algebraic curve may be projected onto a plane algebraic curve, which however may introduce new singularities such as cusps or double points.

A plane curve may also be completed to a curve in the projective plane: if a curve is defined by a polynomial *f* of total degree *d*, then *w*^{d}*f*(*u*/*w*, *v*/*w*) simplifies to a homogeneous polynomial *g*(*u*, *v*, *w*) of degree *d*. The values of *u*, *v*, *w* such that *g*(*u*, *v*, *w*) = 0 are the homogeneous coordinates of the points of the completion of the curve in the projective plane and the points of the initial curve are those such that *w* is not zero. An example is the Fermat curve *u*^{n} + *v*^{n} = *w*^{n}, which has an affine form *x*^{n} + *y*^{n} = 1. A similar process of homogenization may be defined for curves in higher dimensional spaces.

Except for lines, the simplest examples of algebraic curves are the conics, which are nonsingular curves of degree two and genus zero. Elliptic curves, which are nonsingular curves of genus one, are studied in number theory, and have important applications to cryptography.

- ↑ In current mathematical usage, a line is straight. Previously lines could be either curved or straight.

In the mathematical field of algebraic topology, the **fundamental group** of a topological space is the group of the equivalence classes under homotopy of the loops contained in the space. It records information about the basic shape, or holes, of the topological space. The fundamental group is the first and simplest homotopy group. The fundamental group is a homotopy invariant—topological spaces that are homotopy equivalent have isomorphic fundamental groups.

In the part of mathematics referred to as topology, a **surface** is a two-dimensional manifold. Some surfaces arise as the boundaries of three-dimensional solids; for example, the sphere is the boundary of the solid ball. Other surfaces arise as graphs of functions of two variables; see the figure at right. However, surfaces can also be defined abstractly, without reference to any ambient space. For example, the Klein bottle is a surface that cannot be embedded in three-dimensional Euclidean space.

In mathematics, the **tangent space** of a manifold facilitates the generalization of vectors from affine spaces to general manifolds, since in the latter case one cannot simply subtract two points to obtain a vector that gives the displacement of the one point from the other.

In mathematics, **curvature** is any of several strongly related concepts in geometry. Intuitively, the curvature is the amount by which a curve deviates from being a straight line, or a surface deviates from being a plane.

In vector calculus and physics, a **vector field** is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with a given magnitude and direction, each attached to a point in the plane. Vector fields are often used to model, for example, the speed and direction of a moving fluid throughout space, or the strength and direction of some force, such as the magnetic or gravitational force, as it changes from one point to another point.

In geometry, a **geodesic** is commonly a curve representing in some sense the shortest path (arc) between two points in a surface, or more generally in a Riemannian manifold. The term also has meaning in any differentiable manifold with a connection. It is a generalization of the notion of a "straight line" to a more general setting.

In differential geometry, a **Riemannian manifold** or **Riemannian space**(*M*, *g*) is a real, smooth manifold *M* equipped with a positive-definite inner product *g*_{p} on the tangent space *T*_{p}*M* at each point *p*. A common convention is to take *g* to be smooth, which means that for any smooth coordinate chart (*U*, *x*) on *M*, the *n*^{2} functions

In mathematics, an **affine algebraic plane curve** is the zero set of a polynomial in two variables. A **projective algebraic plane curve** is the zero set in a projective plane of a homogeneous polynomial in three variables. An affine algebraic plane curve can be completed in a projective algebraic plane curve by homogenizing its defining polynomial. Conversely, a projective algebraic plane curve of homogeneous equation *h*(*x*, *y*, *t*) = 0 can be restricted to the affine algebraic plane curve of equation *h*(*x*, *y*, 1) = 0. These two operations are each inverse to the other; therefore, the phrase **algebraic plane curve** is often used without specifying explicitly whether it is the affine or the projective case that is considered.

In the mathematical disciplines of topology and geometry, an **orbifold** is a generalization of a manifold. Roughly speaking, an orbifold is a topological space which is locally a finite group quotient of an Euclidean space.

In mathematics, a **submersion** is a differentiable map between differentiable manifolds whose differential is everywhere surjective. This is a basic concept in differential topology. The notion of a submersion is dual to the notion of an immersion.

In mathematics, the **linking number** is a numerical invariant that describes the linking of two closed curves in three-dimensional space. Intuitively, the linking number represents the number of times that each curve winds around the other. The linking number is always an integer, but may be positive or negative depending on the orientation of the two curves.

This is a glossary of some terms used in Riemannian geometry and metric geometry — it doesn't cover the terminology of differential topology.

In geometry, a **hypersurface** is a generalization of the concepts of hyperplane, plane curve, and surface. A hypersurface is a manifold or an algebraic variety of dimension *n* − 1, which is embedded in an ambient space of dimension *n*, generally a Euclidean space, an affine space or a projective space. Hypersurfaces share, with surfaces in a three-dimensional space, the property of being defined by a single implicit equation, at least locally, and sometimes globally.

In mathematics, a **sub-Riemannian manifold** is a certain type of generalization of a Riemannian manifold. Roughly speaking, to measure distances in a sub-Riemannian manifold, you are allowed to go only along curves tangent to so-called *horizontal subspaces*.

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 neighborhood that is homeomorphic to the Euclidean space of dimension n.

In mathematics, a **differentiable manifold** is a type of manifold that is locally similar enough to a linear space to allow one to do calculus. Any manifold can be described by a collection of charts, also known as an atlas. One may then apply ideas from calculus while working within the individual charts, since each chart lies within a linear space to which the usual rules of calculus apply. If the charts are suitably compatible, then computations done in one chart are valid in any other differentiable chart.

In mathematics, a **surface** is a generalization of a plane, which is not necessarily flat – that is, the curvature is not necessarily zero. This is analogous to a curve generalizing a straight line. There are many more precise definitions, depending on the context and the mathematical tools that are used to analyze the surface.

In algebraic geometry, a branch of mathematics, a **Hilbert scheme** is a scheme that is the parameter space for the closed subschemes of some projective space, refining the Chow variety. The Hilbert scheme is a disjoint union of projective subschemes corresponding to Hilbert polynomials. The basic theory of Hilbert schemes was developed by Alexander Grothendieck (1961). Hironaka's example shows that non-projective varieties need not have Hilbert schemes.

In mathematics, the **differential geometry of surfaces** deals with the differential geometry of smooth surfaces with various additional structures, most often, a Riemannian metric. Surfaces have been extensively studied from various perspectives: *extrinsically*, relating to their embedding in Euclidean space and *intrinsically*, reflecting their properties determined solely by the distance within the surface as measured along curves on the surface. One of the fundamental concepts investigated is the Gaussian curvature, first studied in depth by Carl Friedrich Gauss, who showed that curvature was an intrinsic property of a surface, independent of its isometric embedding in Euclidean space.

In mathematics, a **configuration space** is a construction closely related to state spaces or phase spaces in physics. In physics, these are used to describe the state of a whole system as a single point in a high-dimensional space. In mathematics, they are used to describe assignments of a collection of points to positions in a topological space. More specifically, configuration spaces in mathematics are particular examples of configuration spaces in physics in the particular case of several non-colliding particles.

- ↑ In (rather old) French: "La ligne est la première espece de quantité, laquelle a tant seulement une dimension à sçavoir longitude, sans aucune latitude ni profondité, & n'est autre chose que le flux ou coulement du poinct, lequel […] laissera de son mouvement imaginaire quelque vestige en long, exempt de toute latitude." Pages 7 and 8 of
*Les quinze livres des éléments géométriques d'Euclide Megarien, traduits de Grec en François, & augmentez de plusieurs figures & demonstrations, avec la corrections des erreurs commises és autres traductions*, by Pierre Mardele, Lyon, MDCXLV (1645). - 1 2 Lockwood p. ix
- ↑ Heath p. 153
- ↑ Heath p. 160
- ↑ Lockwood p. 132
- ↑ Lockwood p. 129
- ↑ O'Connor, John J.; Robertson, Edmund F., "Spiral of Archimedes",
*MacTutor History of Mathematics archive*, University of St Andrews . - ↑ This term my be ambiguous, as a non-closed curve may be a closed set, as is a line in a plane
- ↑ "Jordan arc definition at Dictionary.com. Dictionary.com Unabridged. Random House, Inc". Dictionary.reference.com . Retrieved 2012-03-14.
- ↑ Osgood, William F. (January 1903). "A Jordan Curve of Positive Area".
*Transactions of the American Mathematical Society*. American Mathematical Society.**4**(1): 107–112. doi: 10.2307/1986455 . ISSN 0002-9947. JSTOR 1986455.

- A.S. Parkhomenko (2001) [1994], "Line (curve)",
*Encyclopedia of Mathematics*, EMS Press - B.I. Golubov (2001) [1994], "Rectifiable curve",
*Encyclopedia of Mathematics*, EMS Press - Euclid, commentary and trans. by T. L. Heath
*Elements*Vol. 1 (1908 Cambridge) Google Books - E. H. Lockwood
*A Book of Curves*(1961 Cambridge)

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- The Encyclopedia of Mathematics article on lines.
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