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.^{[clarification needed]} 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 ∞, I be a non-empty interval of real numbers and t in I. A vector-valued function

$\mathbf{\gamma}:I \to {\mathbb R}^n$

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 ≤ r.

If γ:(a,b) → Rⁿ 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

$\gamma:[a,b] \rightarrow \mathbb{R}^n$

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

$\lbrace \gamma'(t), \gamma''(t), ...,\gamma^{(m)}(t) \rbrace \mbox {, } m \leq k$

are linearly independent in Rⁿ.

In particular, a C¹-curve γ is regular if

$\gamma'(t) \neq 0$ for any $t \in I$.

Reparametrization and equivalence relation

See also: Position vector and Vector-valued function

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

$\mathbf{\gamma_1}:I_1 \to R^n$

and

$\mathbf{\gamma_2}:I_2 \to R^n$

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

$\phi :I_1 \to I_2$

such that

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

and

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

γ₂ is said to be a reparametrisation of γ₁. This reparametrisation of γ₁ defines the equivalence relation on the set of all parametric C^r curves. The equivalence class 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

See also: Curve#Lengths of curves

The length l of a curve γ : [a, b] → Rⁿ of class C¹ can be defined as

$l = \int_a^b \vert \mathbf{\gamma}'(t) \vert dt.$

The length of a curve is invariant under reparametrization and therefore a differential geometric property of the curve.

For each regular C^r-curve (r at least 1) γ: [a, b] → Rⁿ we can define a function

$s(t) = \int_{t_0}^t \vert \mathbf{\gamma}'(x) \vert dx.$

Writing

$\bar{\mathbf{\gamma}}(s) = \gamma(t(s))$

where t(s) is the inverse of s(t), we get a reparametrization $\bar{\gamma}$of γ which is called natural, arc-length or unit speed parametrization. The parameter s(t) is called the natural parameter of γ.

We prefer this parametrization because the natural parameter s(t) traverses the image of γ at unit speed so that

$\vert \bar{\mathbf{\gamma}}'(s(t)) \vert = 1 \qquad (t \in I).$

In practice it is often very difficult to calculate the natural parametrization of a curve, but it is useful for theoretical arguments.

For a given parametrized curve γ(t) the natural parametrization is unique up to shift of parameter.

The quantity

$E(\gamma) = \frac{1}{2}\int_a^b \vert \mathbf{\gamma}'(t) \vert^2 dt$

is sometimes called the energy or action of the curve; this name is justified because the geodesic equations are the Euler–Lagrange equations of motion for this action.

Frenet frame

See also: Frenet–Serret formulas

An illustration of the Frenet frame for a point on a space curve. T is the unit tangent, P the unit normal, and B the unit binormal.

A Frenet frame is a moving reference frame of n orthonormal vectors e_i(t) which are used to describe a curve locally at each point γ(t). It is the main tool in the differential geometric treatment of curves as it is far easier and more natural to describe local properties (e.g. curvature, torsion) in terms of a local reference system than using a global one like the Euclidean coordinates.

Given a Cⁿ⁺¹-curve γ in Rⁿ which is regular of order n the Frenet frame for the curve is the set of orthonormal vectors

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

called Frenet vectors. They are constructed from the derivatives of γ(t) using the Gram–Schmidt orthogonalization algorithm with

$\mathbf{e}_1(t) = \frac{\mathbf{\gamma}'(t)}{\| \mathbf{\gamma}'(t) \|}$

$\mathbf{e}_{j}(t) = \frac{\overline{\mathbf{e}_{j}}(t)}{\|\overline{\mathbf{e}_{j}}(t) \|} \mbox{, } \overline{\mathbf{e}_{j}}(t) = \mathbf{\gamma}^{(j)}(t) - \sum _{i=1}^{j-1} \langle \mathbf{\gamma}^{(j)}(t), \mathbf{e}_i(t) \rangle \, \mathbf{e}_i(t)$

The real-valued functions χ_i(t) are called generalized curvatures and are defined as

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

The Frenet frame and the generalized curvatures are invariant under reparametrization and are therefore differential geometric properties of the curve.

Special Frenet vectors and generalized curvatures

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 C¹ curve γ = γ(t), for every value t = t₀ of the parameter, the vector

$\gamma'(t_0) = \frac{d}{d\,t}\mathbf{\gamma}(t)$ at t = t₀

is the tangent vector at the point P = γ(t₀). Generally speaking, the tangent vector may be zero. The magnitude of the tangent vector,

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

is the speed at the time t₀.

The first Frenet vector e₁(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 e₂(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

The first generalized curvature χ₁(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 binormal vector is the third Frenet vector e₃(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

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

Torsion

Main article: Torsion of a curve

The second generalized curvature χ₂(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 functions

$\chi_i \in C^{n-i}([a,b]) \mbox{, } 1 \leq i \leq n$

with

$\chi_i(t) > 0 \mbox{, } 1 \leq i \leq n-1$

then there exists a unique (up to transformations using the Euclidean group) Cⁿ⁺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 t₀ in I, a starting point p₀ in Rⁿ and an initial positive orthonormal Frenet frame {e₁, ..., 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

$\begin{bmatrix} \mathbf{e}_1'(t)\\ \mathbf{e}_2'(t) \\ \end{bmatrix} = \begin{bmatrix} 0 & -\kappa(t) \\ \kappa(t) & 0 \\ \end{bmatrix} \begin{bmatrix} \mathbf{e}_1(t)\\ \mathbf{e}_2(t) \\ \end{bmatrix}$

3 dimensions

$\begin{bmatrix} \mathbf{e}_1'(t) \\ \mathbf{e}_2'(t) \\ \mathbf{e}_3'(t) \\ \end{bmatrix} = \begin{bmatrix} 0 & -\kappa(t) & 0 \\ \kappa(t) & 0 & -\tau(t) \\ 0 & \tau(t) & 0 \\ \end{bmatrix} \begin{bmatrix} \mathbf{e}_1(t) \\ \mathbf{e}_2(t) \\ \mathbf{e}_3(t) \\ \end{bmatrix}$

n dimensions (general formula)

$\begin{bmatrix} \mathbf{e}_1'(t)\\ \vdots \\ \mathbf{e}_n'(t) \\ \end{bmatrix} = \begin{bmatrix} 0 & -\chi_1(t) & & 0 \\ \chi_1(t) & \ddots & \ddots & \\ & \ddots & 0 & -\chi_{n-1}(t) \\ 0 & & \chi_{n-1}(t) & 0 \\ \end{bmatrix} \begin{bmatrix} \mathbf{e}_1(t) \\ \vdots \\ \mathbf{e}_n(t) \\ \end{bmatrix}$

See also

• List of curve topics
• Affine geometry of curves
• Arc
• Tangent, contact, subtangent
• Osculating circle
• Envelope (mathematics), roulette (curve)
• Four-vertex theorem
• Geodesic
• Isoperimetry
• Linking coefficient

Additional reading

• Erwin Kreyszig, Differential Geometry, Dover Publications, New York, 1991, ISBN 9780484667218. Chapter II is a classical treatment of Theory of Curves in 3-dimensions.