Ingeometry, aconvex curve is aplane curve that has asupporting line through each of its points. There are many other equivalent definitions of these curves, going back toArchimedes. Examples of convex curves include theconvex polygons, theboundaries ofconvex sets, and thegraphs ofconvex functions. Important subclasses of convex curves include the closed convex curves (the boundaries of bounded convex sets), thesmooth curves that are convex, and the strictly convex curves, which have the additional property that each supporting line passes through a unique point of the curve.

Bounded convex curves have a well-defined length, which can be obtained by approximating them with polygons, or from the average length of their projections onto a line. The maximum number of grid points that can belong to a single curve is controlled by its length. The points at which a convex curve has a uniquesupporting line aredense within the curve, and the distance of these lines from the origin defines a continuoussupport function. A smooth simple closed curve is convex if and only if itscurvature has a consistent sign, which happens if and only if itstotal curvature equals itstotal absolute curvature.

Archimedes, in hisOn the Sphere and Cylinder, defines convex arcs as the plane curves that lie on one side of the line through their two endpoints, and for which allchords touch the same side of the curve.^[1] This may have been the first formal definition of any notion of convexity, although convex polygons and convex polyhedra were already long known before Archimedes.^[2] For the next two millennia, there was little study of convexity:^[2] its in-depth investigation began again only in the 19th century,^[3] whenAugustin-Louis Cauchy and others began usingmathematical analysis instead ofalgebraic methods to putcalculus on a more rigorous footing.^[1][2]

Many other equivalent definitions for the convex curves are possible, as detailed below. Convex curves have also been defined by their supporting lines, by the sets they form boundaries of, and by their intersections with lines. In order to distinguish closed convex curves from curves that are not closed, the closed convex curves have sometimes also been calledconvex loops, and convex curves that are not closed have also been calledconvex arcs.^[4]

Aplane curve is the image of anycontinuous function from aninterval to theEuclidean plane. Intuitively, it is a set of points that could be traced out by a moving point. More specifically,smooth curves generally at least require that the function from the interval to the plane becontinuously differentiable, and in some contexts are defined to require higher derivatives. The function parameterizing a smooth curve is often assumed to beregular, meaning that its derivative stays away from zero; intuitively, the moving point never slows to a halt or reverses direction. Each interior point of a smooth curve has atangent line. If, in addition, the second derivative exists everywhere, then each of these points has a well-definedcurvature.^[5]

A plane curve isclosed if the two endpoints of the interval are mapped to the same point in the plane, and it issimple if no other two points coincide.^[5] Less commonly, a simple plane curve may be said to beopen if it is topologically equivalent to a line, neither having an endpoint nor forming any limiting point that does not belong to it, and dividing the plane into two unbounded regions.^[6] However, this terminology is ambiguous as other sources refer to a curve with two distinct endpoints as an open curve.^[7] Here, we use the topological-line meaning of an open curve.

Asupporting line is a line containing at least one point of the curve, for which the curve is contained in one of the twohalf-planes bounded by the line. A plane curve is calledconvex if it has a supporting line through each of its points.^[8][9] For example, thegraph of aconvex function has a supporting line below the graph through each of its points. More strongly, at the points where the function has a derivative, there is exactly one supporting line, thetangent line.^[10]

Supporting lines and tangent lines are not the same thing,^[11] but for convex curves, every tangent line is a supporting line.^[8] At a point of a curve where a tangent line exists, there can only be one supporting line, the tangent line.^[12] Therefore, a smooth curve is convex if it lies on one side of each of its tangent lines. This may be used as an equivalent definition of convexity for smooth curves, or more generally forpiecewise smooth curves.^[13][a]

A convex curve may be alternatively defined as a connected subset of theboundary of aconvex set in theEuclidean plane.^[8][9] Not every convex set has a connected boundary,^[b] but when it does, the whole boundary is an example of a convex curve. When abounded convex set in the plane is not a line segment, its boundary forms a simple closed convex curve.^[16] By theJordan curve theorem, a simple closed curve divides the plane into interior and exterior regions, and another equivalent definition of a closed convex curve is that it is a simple closed curve whose union with its interior is a convex set.^[9][17] Examples of open and unbounded convex curves include the graphs of convex functions. Again, these are boundaries of convex sets, theepigraphs of the same functions.^[18]

This definition is equivalent to the definition of convex curves from support lines. Every convex curve, defined as a curve with a support line through each point, is a subset of the boundary of its ownconvex hull. Every connected subset of the boundary of a convex set has a support line through each of its points.^[8][9][19]

For a convex curve, every line in the plane intersects the curve in one of four ways: its intersection can be the empty set, a single point, a pair of points, or an interval. In the cases where a closed curve intersects in a single point or an interval, the line is a supporting line. This can be used as an alternative definition of the convex curves: they are theJordan curves (connected simple curves) for which every intersection with a line has one of these four types. This definition can be used to generalize convex curves from theEuclidean plane to certain otherlinear spaces such as thereal projective plane. In these spaces, like in the Euclidean plane, any curve with only these restricted line intersections has a supporting line for each point.^[20]

Thestrictly convex curves again have many equivalent definitions. They are the convex curves that do not contain anyline segments.^[21] They are the curves for which every intersection of the curve with a line consists of at most two points.^[20] They are the curves that can be formed as a connected subset of the boundary of astrictly convex set.^[22] Here, a set is strictly convex if every point of its boundary is anextreme point of the set, the unique maximizer of some linear function.^[23] As the boundaries of strictly convex sets, these are the curves that lie inconvex position, meaning that none of their points can be aconvex combination of any other subset of its points.^[24]

Closed strictly convex curves can be defined as the simple closed curves that arelocally equivalent (under an appropriate coordinate transformation) to the graphs of strictly convex functions. This means that, at each point of the curve, there is aneighborhood of the points and a system ofCartesian coordinates within that neighborhood such that, within that neighborhood, the curve coincides with the graph of a strictly convex function.^[25][c]

Smooth closed convex curves with anaxis of symmetry, such as anellipse orMoss's egg, may sometimes be calledovals.^[28] However, the same word has also been used to describe the sets for which each point has a unique line disjoint from the rest of the set, especially in the context ofovals in finiteprojective geometry. In Euclidean geometry these are the smooth strictly convex closed curves, without any requirement of symmetry.^[20]

Every bounded convex curve is arectifiable curve, meaning that it has a well-defined finitearc length, and can be approximated in length by a sequence of inscribedpolygonal chains. For closed convex curves, the length may be given by a form of theCrofton formula as${\displaystyle \pi }$  times the average length of its projections onto lines.^[8] It is also possible to approximate the area of the convex hull of a convex curve by a sequence of inscribedconvex polygons. For any integer${\displaystyle n}$ , the most accurate approximating${\displaystyle n}$ -gon has the property that each vertex has a supporting line parallel to the line through its two neighboring vertices.^[29] As Archimedes already knew, if two convex curves have the same endpoint, and one of the two curves lies between the other and the line through their endpoints, then the inner curve is shorter than the outer one.^[2]

According toNewton's theorem about ovals, the area cut off from aninfinitely differentiable convex curve by a line cannot be an algebraic function of the coefficients of the line.^[30]

It is not possible for a strictly convex curve to pass through many points of theinteger lattice. If the curve has length${\displaystyle L}$ , then according to a theorem ofVojtěch Jarník, the number of lattice points that it can pass through is at most$${\displaystyle \frac{3}{\sqrt[3]{2\pi }}{L}^{2/3}+O(L^{1/3}).}$$ Because this estimate usesbig O notation, it is accurate only in the limiting case of large lengths. Neither the leading constant nor the exponent in the error term can be improved.^[31]

A convex curve can have at most acountable set ofsingular points, where it has more than one supporting line. All of the remaining points must be non-singular, and the unique supporting line at these points is necessarily a tangent line. This implies that the non-singular points form adense set in the curve.^[10][32] It is also possible to construct convex curves for which the singular points are dense.^[19]

A closed strictly convex closed curve has a continuoussupport function, mapping each direction of supporting lines to their signed distance from the origin. It is an example of ahedgehog, a type of curve determined as theenvelope of a system of lines with a continuous support function. The hedgehogs also include non-convex curves, such as theastroid, and even self-crossing curves, but the smooth strictly convex curves are the only hedgehogs that have no singular points.^[33]

It is impossible for a convex curve to have three parallel tangent lines. More strongly, a smooth closed curve is convex if and only if it does not have three parallel tangent lines. In one direction, the middle of any three parallel tangent lines would separate the points of tangency of the other two lines, so it could not be a line of support. There could be no other line of support through its point of tangency, so a curve tangent to these three lines could not be convex. In the other direction, a non-convex smooth closed curve has at least one point with no support line. The tangent line through that point, and the two tangent supporting lines parallel to it, form a set of three parallel tangent lines.^[13][d]

According to thefour-vertex theorem, every smooth closed curve has at least fourvertices, points that are local minima or local maxima ofcurvature.^[36] The original proof of the theorem, bySyamadas Mukhopadhyaya in 1909, considered only convexcurves;^[37] it was later extended to all smooth closedcurves.^[36]

Curvature can be used to characterize the smooth closed curves that areconvex.^[13] The curvature depends in a trivial way on the parameterization of the curve: if a regularly parameterization of a curve is reversed, the same set of points results, but its curvature isnegated.^[5] A smooth simple closed curve, with a regular parameterization, is convex if and only if its curvature has a consistent sign: always non-negative, or alwaysnon-positive.^[13][e] Every smooth simple closed curve with strictly positive (or strictly negative) curvature is strictly convex, but some strictly convex curves can have points with curvaturezero.^[39]

Thetotal absolute curvature of a smooth convex curve,$${\displaystyle \int |\kappa (s)|ds,}$$  is atmost${\displaystyle 2\pi }$ . It is exactly${\displaystyle 2\pi }$  for closed convex curves, equalling thetotal curvature of these curves, and of any simple closed curve. For convex curves, the equality of total absolute curvature and total curvature follows from the fact that the curvature has a consistent sign. For closed curves that are not convex, the total absolute curvature is always greaterthan${\displaystyle 2\pi }$ , and its excess can be used as a measure of how far from convex the curve is. More generally, byFenchel's theorem, the total absolute curvature of a closed smoothspace curve is atleast${\displaystyle 2\pi }$ , with equality only for convex planecurves.^[40][41]

By theAlexandrov theorem, a non-smooth convex curve has a second derivative, and therefore a well-defined curvature,almost everywhere. This means that the subset of points without a second derivative hasmeasure zero in the curve. However, in other senses, the set of points with a second derivative can be small. In particular, for the graphs of generic non-smooth convex functions, it is ameager set, that is, a countable union ofnowhere dense sets.^[42]

The boundary of anyconvex polygon forms a convex curve (one that is apiecewise linear curve and not strictly convex). A polygon that isinscribed in any strictly convex curve, with its vertices in order along the curve, must be a convex polygon.^[43]

Theinscribed square problem is the problem of proving that every simple closed curve in the plane contains the four corners of a square. Although still unsolved in general, its solved cases include the convex curves.^[44] In connection with this problem, related problems of finding inscribed quadrilaterals have been studied for convex curves. A scaled and rotated copy of anyrectangle ortrapezoid can be inscribed in any given closed convex curve. When the curve is smooth, a scaled and rotated copy of anycyclic quadrilateral can be inscribed in it. However, the assumption of smoothness is necessary for this result, because someright kites cannot be inscribed in some obtuseisosceles triangles.^[45][46]Regular polygons with more than four sides cannot be inscribed in all closed convex curves, because the curve formed by asemicircle and its diameter does not contain any of these polygons.^[47]

• Convex surface, the higher-dimensional generalization of convex curves
• List of convexity topics