Differential geometry of curves

This article considers only curves in Euclidean space. Most of the notions presented here have analogues for curves in Riemannian and pseudo-Riemannian manifolds. For a discussion of curves in an arbitrary topological space, see the main article on curves.

Differential geometry of curves is the branch of geometry that deals with smooth curves in the plane and in the Euclidean space by methods of differential and integral calculus.

Starting in antiquity, many concrete curves have been thoroughly investigated using the synthetic approach. Differential geometry takes another path: curves are represented in a parametrized form, and their geometric properties and various quantities associated with them, such as the curvature and the arc length, are expressed via derivatives and integrals using vector calculus. One of the most important tools used to analyze a curve is the Frenet frame, a moving frame that provides a coordinate system at each point of the curve that is "best adapted" to the curve near that point.

The theory of curves is much simpler and narrower in scope than the theory of surfaces and its higher-dimensional generalizations, because a regular curve in a Euclidean space has no intrinsic geometry. Any regular curve may be parametrized by the arc length (the natural parametrization) and from the point of view of a bug on the curve that does not know anything about the ambient space, all curves would appear the same. Different space curves are only distinguished by the way in which they bend and twist. Quantitatively, this is measured by the differential-geometric invariants called the curvature and the torsion of a curve. The fundamental theorem of curves asserts that the knowledge of these invariants completely determines the curve.

Definitions

Main article: Curve

Let $n$ be a natural number, $r$ a natural number or $\infty$ , $I$ be a non-empty interval of real numbers and $t$ in $I$ . A vector-valued function

\gamma \colon I\to {{\mathbb R}}^{n}

of class $C r$ (i.e. $γ$ is $r$ times continuously differentiable) is called a parametric curve of class $C r$ or a $C r$ parametrization of the curve $γ$ . $t$ is called the parameter of the curve $γ$ . $γ (I)$ is called the image of the curve. It is important to distinguish between a curve $γ$ and the image of a curve $γ (I)$ because a given image can be described by several different $C r$ curves.

One may think of the parameter $t$ as representing time and the curve $γ (t)$ as the trajectory of a moving particle in space.

If $I$ is a closed interval $[a, b]$ , we call $γ (a)$ the starting point and $γ (b)$ the endpoint of the curve $γ$ .

If $γ (a) = γ (b)$ , we say $γ$ is closed or a loop. Furthermore, we call $γ$ a closed $C r$ -curve if $γ (k) (a) = γ (k) (b)$ for all $k \leq r$ .

If $γ : (a, b) \to R n$ is injective, we call the curve simple.

If $γ$ is a parametric curve which can be locally described as a power series, we call the curve analytic or of class $C ω$ .

We write $- γ$ to say the curve is traversed in opposite direction.

A $C k$ -curve

\gamma \colon I\rightarrow {\mathbb {R}}^{n}

is called regular of order $m$ if for any $t$ in interval $I$

\{ \gamma'(t), \gamma''(t), \dotsc,\gamma^{(m)}(t) \}, \quad m \leq k

are linearly independent in $R n$ .

In particular, a $C 1$ -curve $γ$ is regular if $γ' (t) \neq 0$ for any $t \in I$ .

Reparametrization and equivalence relation

Given the image of a curve one can define several different parameterizations of the curve. Differential geometry aims to describe properties of curves invariant under certain reparametrizations. So we have to define a suitable equivalence relation on the set of all parametric curves. The differential geometric properties of a curve (length, Frenet frame and generalized curvature) are invariant under reparametrization and therefore properties of the equivalence class.The equivalence classes are called $C r$ curves and are central objects studied in the differential geometry of curves.

Two parametric curves of class $C r$

\mathbf{\gamma_1}\colon I_1 \to R^n

and

\mathbf{\gamma_2}\colon I_2 \to R^n

are said to be equivalent if there exists a bijective $C r$ map

\phi\colon I_1 \to I_2

such that

\phi'(t) \neq 0 \qquad (t \in I_1)

and

\mathbf{\gamma_2}(\phi(t)) = \mathbf{\gamma_1}(t) \qquad (t \in I_1)

$γ 2$ is said to be a reparametrization of $γ 1$ .

Reparametrization defines an equivalence relation on the set of all parametric $C r$ curves. An equivalence class of this relation is called a $C r$ curve.

We can define an even finer equivalence relation of oriented $C r$ curves by requiring $φ$ to be $φ' (t) > 0$ .

Equivalent $C r$ curves have the same image. And equivalent oriented $C r$ curves even traverse the image in the same direction.

Length and natural parametrization

Main article: Arc length

Frenet frame

Bertrand curve

A Bertrand curve is a Frenet curve in $\mathbb {R} ^{3}$ with the additional property that there is a second curve in $\mathbb {R} ^{3}$ such that the principal normal vectors to these two curves are identical at each corresponding point. In other words, if ${\vec {r}}_{1}(t)$ and ${\vec {r}}_{2}(t)$ are two curves in $\mathbb {R} ^{3}$ such that for any $t$ , ${\vec {N}}_{1}={\vec {N}}_{2}$ , then ${\vec {r}}_{1}$ and ${\vec {r}}_{2}$ are Bertrand curves. For this reason it is common to speak of a Bertrand pair of curves (like ${\vec {r}}_{1}$ and ${\vec {r}}_{2}$ in the previous example). According to problem 25 in Kühnel's "Differential Geometry Curves - Surfaces - Manifolds", it is also true that two Bertrand curves that do not lie in the same 2-dimensional plane are characterized by the existence of a linear relation $a\kappa +b\tau =1$ where $a,b$ are real constants and $a\neq 0$ .^[1] Furthermore, the product of torsions of Bertrand pairs of curves are constant.^[2]

Special Frenet vectors and generalized curvatures

Main article: Frenet–Serret formulas

The first three Frenet vectors and generalized curvatures can be visualized in three-dimensional space. They have additional names and more semantic information attached to them.

Tangent vector

If a curve $γ$ represents the path of a particle, then the instantaneous velocity of the particle at a given point $P$ is expressed by a vector, called the tangent vector to the curve at $P$ . Mathematically, given a parametrized $C 1$ curve $γ = γ (t)$ , for every value $t = t 0$ of the parameter, the vector

\gamma'(t_0) = \frac{d}{d\,t}\mathbf{\gamma}(t)

{t=t_0}

is the tangent vector at the point $P = γ (t 0)$ . Generally speaking, the tangent vector may be zero. The magnitude of the tangent vector,

\|\mathbf{\gamma}'(t_0)\|,

is the speed at the time $t 0$ .

The first Frenet vector $e 1 (t)$ is the unit tangent vector in the same direction, defined at each regular point of $γ$ :

\mathbf{e}_{1}(t) = \frac{ \mathbf{\gamma}'(t) }{ \| \mathbf{\gamma}'(t) \|}.

If $t = s$ is the natural parameter then the tangent vector has unit length, so that the formula simplifies:

\mathbf{e}_{1}(s) = \mathbf{\gamma}'(s).

The unit tangent vector determines the orientation of the curve, or the forward direction, corresponding to the increasing values of the parameter. The unit tangent vector taken as a curve traces the spherical image of the original curve.

Normal or curvature vector

The normal vector, sometimes called the curvature vector, indicates the deviance of the curve from being a straight line.

It is defined as

\overline{\mathbf{e}_2}(t) = \mathbf{\gamma}''(t) - \langle \mathbf{\gamma}''(t), \mathbf{e}_1(t) \rangle \, \mathbf{e}_1(t).

Its normalized form, the unit normal vector, is the second Frenet vector $e 2 (t)$ and defined as

\mathbf{e}_2(t) = \frac{\overline{\mathbf{e}_2}(t)} {\| \overline{\mathbf{e}_2}(t) \|}.

The tangent and the normal vector at point $t$ define the osculating plane at point $t$ .

Curvature

Main article: Curvature of space curves

The first generalized curvature $χ 1 (t)$ is called curvature and measures the deviance of $γ$ from being a straight line relative to the osculating plane. It is defined as

\kappa (t)=\chi _{1}(t)={\frac {\langle {\mathbf {e}}_{1}'(t),{\mathbf {e}}_{2}(t)\rangle }{\|{\mathbf {\gamma }}'(t)\|}}

and is called the curvature of $γ$ at point $t$ .

The reciprocal of the curvature

\frac{1}{\kappa(t)}

is called the radius of curvature.

A circle with radius $r$ has a constant curvature of

\kappa(t) = \frac{1}{r}

whereas a line has a curvature of 0.

Binormal vector

The unit binormal vector is the third Frenet vector $e 3 (t)$ . It is always orthogonal to the unit tangent and normal vectors at $t$ , and is defined as

\mathbf{e}_3(t) = \frac{\overline{\mathbf{e}_3}(t)} {\| \overline{\mathbf{e}_3}(t) \|} \mbox{, } \overline{\mathbf{e}_3}(t) = \mathbf{\gamma}'''(t) - \langle \mathbf{\gamma}'''(t), \mathbf{e}_1(t) \rangle \, \mathbf{e}_1(t) - \langle \mathbf{\gamma}'''(t), \mathbf{e}_2(t) \rangle \,\mathbf{e}_2(t)

In 3-dimensional space the equation simplifies to

\mathbf{e}_3(t) = \mathbf{e}_1(t) \times \mathbf{e}_2(t)

or to

\mathbf{e}_3(t) = -\mathbf{e}_1(t) \times \mathbf{e}_2(t)

That either sign may occur is illustrated by the examples of a right handed helix and a left handed helix.

Torsion

Main article: Torsion of a curve

The second generalized curvature $χ 2 (t)$ is called torsion and measures the deviance of $γ$ from being a plane curve. Or, in other words, if the torsion is zero, the curve lies completely in the same osculating plane (there is only one osculating plane for every point $t$ ). It is defined as

\tau(t) = \chi_2(t) = \frac{\langle \mathbf{e}_2'(t), \mathbf{e}_3(t) \rangle}{\| \mathbf{\gamma}'(t) \|}

and is called the torsion of $γ$ at point $t$ .

Main theorem of curve theory

Main article: Fundamental theorem of curves

Given $n - 1$ functions:

\chi _{i}\in C^{n-i}([a,b],\mathbb {R} ^{n}){\mbox{, }}\chi _{i}(t)>0{\mbox{, }}1\leq i\leq n-1

then there exists a unique (up to transformations using the Euclidean group) $C n + 1$ -curve $γ$ which is regular of order n and has the following properties

\|\gamma'(t)\| = 1 \mbox{ } (t \in [a,b])

\chi_i(t) = \frac{ \langle \mathbf{e}_i'(t), \mathbf{e}_{i+1}(t) \rangle}{\| \mathbf{\gamma}'(t) \|}

where the set

\mathbf{e}_1(t), \ldots, \mathbf{e}_n(t)

is the Frenet frame for the curve.

By additionally providing a start $t 0$ in $I$ , a starting point $p 0$ in $R n$ and an initial positive orthonormal Frenet frame ${e 1, \dots, e n - 1}$ with

\mathbf{\gamma}(t_0) = \mathbf{p}_0

\mathbf{e}_i(t_0) = \mathbf{e}_i \mbox{, } 1 \leq i \leq n-1

we can eliminate the Euclidean transformations and get unique curve $γ$ .

Frenet–Serret formulas

Main article: Frenet–Serret formulas

The Frenet–Serret formulas are a set of ordinary differential equations of first order. The solution is the set of Frenet vectors describing the curve specified by the generalized curvature functions $χ i$ .

2 dimensions

{\begin{bmatrix}\mathbf {e} _{1}'(t)\\\mathbf {e} _{2}'(t)\\\end{bmatrix}}=\left\Vert \gamma '\left(t\right)\right\Vert {\begin{bmatrix}0&\kappa (t)\\-\kappa (t)&0\\\end{bmatrix}}{\begin{bmatrix}\mathbf {e} _{1}(t)\\\mathbf {e} _{2}(t)\\\end{bmatrix}}

3 dimensions

{\begin{bmatrix}\mathbf {e} _{1}'(t)\\\mathbf {e} _{2}'(t)\\\mathbf {e} _{3}'(t)\\\end{bmatrix}}=\left\Vert \gamma '\left(t\right)\right\Vert {\begin{bmatrix}0&\kappa (t)&0\\-\kappa (t)&0&\tau (t)\\0&-\tau (t)&0\\\end{bmatrix}}{\begin{bmatrix}\mathbf {e} _{1}(t)\\\mathbf {e} _{2}(t)\\\mathbf {e} _{3}(t)\\\end{bmatrix}}

$n$ dimensions (general formula)

{\begin{bmatrix}\mathbf {e} _{1}'(t)\\\mathbf {e} _{2}'(t)\\\vdots \\\mathbf {e} _{n-1}'(t)\\\mathbf {e} _{n}'(t)\\\end{bmatrix}}=\left\Vert \gamma '\left(t\right)\right\Vert {\begin{bmatrix}0&\chi _{1}(t)&\cdots &0&0\\-\chi _{1}(t)&0&\cdots &0&0\\\vdots &\vdots &\ddots &\vdots &\vdots \\0&0&\cdots &0&\chi _{n-1}(t)\\0&0&\cdots &-\chi _{n-1}(t)&0\\\end{bmatrix}}{\begin{bmatrix}\mathbf {e} _{1}(t)\\\mathbf {e} _{2}(t)\\\vdots \\\mathbf {e} _{n-1}(t)\\\mathbf {e} _{n}(t)\\\end{bmatrix}}

References

↑ Page 53 of "Differential Geometry Curves - Surfaces - Manifolds" by Wolfgang Kühnel ISBN 978-0821839881
↑ http://mathworld.wolfram.com/BertrandCurves.html

Additional reading

Erwin Kreyszig, Differential Geometry, Dover Publications, New York, 1991, ISBN 0-486-66721-9. Chapter II is a classical treatment of Theory of Curves in 3-dimensions.

Differential transforms of plane curves

Unary operations	Evolute Involute Dual curve Inverse curve Parallel curve Isoptic

Unary operations defined by a point	Pedal & Contrapedal curves Negative pedal curve Pursuit curve Caustic

Unary operations defined by two points	Strophoid

Binary operations defined by a point	Roulette

Operations on a family of curves	Envelope

Various notions of curvature defined in differential geometry

Differential geometry of curves	Curvature Torsion of a curve Frenet–Serret formulas Radius of curvature (applications) Affine curvature Total curvature Total absolute curvature

Differential geometry of surfaces	Principal curvatures Gaussian curvature Mean curvature Darboux frame Gauss–Codazzi equations First fundamental form Second fundamental form Third fundamental form

Riemannian geometry	Curvature of Riemannian manifolds Riemann curvature tensor Ricci curvature Scalar curvature Sectional curvature

Curvature of connections	Curvature form Torsion tensor Cocurvature Holonomy

Tensors

Glossary of tensor theory

Scope

Mathematics	coordinate system multilinear algebra Euclidean geometry tensor algebra differential geometry exterior calculus tensor calculus

Physics Engineering	continuum mechanics electromagnetism transport phenomena general relativity computer vision

Notation

Tensor definitions

Operations

Related abstractions

Notable tensors

Mathematics	Kronecker delta Levi-Civita symbol metric tensor nonmetricity tensor Christoffel symbols Ricci curvature Riemann curvature tensor Weyl tensor torsion tensor

Physics	moment of inertia angular momentum tensor spin tensor Cauchy stress tensor stress–energy tensor EM tensor gluon field strength tensor Einstein tensor metric tensor (GR)

Mathematicians

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