“Curvature”的概念、定义、习题解题方法及翻译-数学辞典

单词

Curvature

释义

Curvature

In general, there are two important types of curvature: Extrinsic Curvature and Intrinsic Curvature. TheExtrinsic Curvature of curves in 2- and 3-space was the first type of curvature to be studied historically,culminating in the Frenet Formulas, which describe a Space Curve entirely in terms of its ``curvature,''Torsion, and the initial starting point and direction.

After the curvature of 2- and 3-D curves was studied, attention turned to the curvature of surfaces in 3-space. The maincurvatures which emerged from this scrutiny are the Mean Curvature, Gaussian Curvature, and theWeingarten Map. Mean Curvature was the most important for applications at the time and was the most studied,but Gauß was the first to recognize the importance of the Gaussian Curvature.

Because Gaussian Curvature is ``intrinsic,'' it is detectable to 2-dimensional ``inhabitants'' of the surface, whereasMean Curvature and the Weingarten Map are not detectable to someone who can't study the 3-dimensional spacesurrounding the surface on which he resides. The importance of Gaussian Curvature to an inhabitant is that it controlsthe surface Area of Spheres around the inhabitant.

Riemann and many others generalized the concept of curvature to Sectional Curvature, Scalar Curvature,the Riemann Tensor, Ricci Curvature, and a host of other Intrinsic andExtrinsic Curvatures. General curvatures no longer need to be numbers, and can take the form ofa Map, Group, Groupoid, tensor field, etc.

The simplest form of curvature and that usually first encountered in Calculus is an Extrinsic Curvature. In 2-D,let a Plane Curve be given by Cartesian parametric equations and . Then the curvature is defined by

(1)

where

is the Polar Angle and

is the Arc Length. As can readily be seen from the definition, curvaturetherefore has units of inverse distance. The

derivative in the above equation can be eliminated by using theidentity

(2)

(3)

and


			(4)

Combining (2) and (4) gives

(5)

For a 2-D curve written in the form , the equation of curvature becomes

(6)

If the 2-D curve is instead parameterized in Polar Coordinates, then

(7)

where

(Gray 1993). In Pedal Coordinates, the curvature is given by

(8)

The curvature for a 2-D curve given implicitly by

is given by

(9)

(Gray 1993).

Now consider a parameterized Space Curve in 3-D for which the Tangent Vector is defined as

(10)

Therefore,

(11)

(12)

where

is the Normal Vector. But


			(13)

(14)

(15)

The curvature of a 2-D curve is related to the Radius of Curvature of the curve's Osculating Circle. Consider a Circle specified parametrically by

(16)

(17)

which is tangent to the curve at a given point. The curvature is then

(18)

or one over the Radius of Curvature. The curvature of a Circle can also be repeated in vector notation. Forthe Circle with

, the Arc Length is


			(19)

and the equations of the Circle can be rewritten as

(20)

(21)

The Position Vector is then given by

(22)

and the Tangent Vector is

(23)

so the curvature is related to the Radius of Curvature


			(24)

as expected.

Four very important derivative relations in differential geometry related to the Frenet Formulas are

			(25)
			(26)
			(27)
			(28)

where T is the Tangent Vector, N is the Normal Vector, B is the Binormal Vector,and

is the Torsion (Coxeter 1969, p. 322).

The curvature at a point on a surface takes on a variety of values as the Plane through the normal varies. As varies, it achieves a minimum and a maximum (which are in perpendicular directions) known as the Principal Curvatures. As shown in Coxeter (1969, pp. 352-353),

(29)

(30)

where

is the Gaussian Curvature,

is the Mean Curvature, and det denotes the Determinant.

The curvature is sometimes called the First Curvature and the Torsion the Second Curvature. In addition, a Third Curvature (sometimes called TotalCurvature)

(31)

is also defined. A signed version of the curvature of a Circle appearing in the Descartes Circle Theorem for theradius of the fourth of four mutually tangent circles is called the Bend.

See also Bend (Curvature), Curvature Center, Curvature Scalar, Extrinsic Curvature, FirstCurvature, Four-Vertex Theorem, Gaussian Curvature, Intrinsic Curvature, Lancret Equation,Line of Curvature, Mean Curvature, Normal Curvature, Principal Curvatures, Radius ofCurvature, Ricci Curvature, Riemann Tensor, Second Curvature, Sectional Curvature, SoddyCircles, Third Curvature, Torsion (Differential Geometry), Weingarten Map

References

Coxeter, H. S. M. Introduction to Geometry, 2nd ed. New York: Wiley, 1969.

Fischer, G. (Ed.). Plates 79-85 in Mathematische Modelle/Mathematical Models, Bildband/Photograph Volume. Braunschweig, Germany: Vieweg, pp. 74-81, 1986.

Gray, A. ``Curvature of Curves in the Plane,'' ``Drawing Plane Curves with Assigned Curvature,'' and ``Drawing Space Curves with Assigned Curvature.'' §1.5, 6.4, and 7.8 in Modern Differential Geometry of Curves and Surfaces. Boca Raton, FL: CRC Press, pp. 11-13, 68-69, 113-118, and 145-147, 1993.

Kreyszig, E. ``Principal Normal, Curvature, Osculating Circle.'' §12 in Differential Geometry. New York: Dover, pp. 34-36, 1991.

Yates, R. C. ``Curvature.'' A Handbook on Curves and Their Properties. Ann Arbor, MI: J. W. Edwards, pp. 60-64, 1952.

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