Lines and Curves

In Affine Space > s.a. affine structures.
$Line: Given an affine space, the line through two points P and Q is the set of all points S with SQ = t (PQ), for some t ∈ $$\mathbb R$$; Equivalently, with an abuse of notation, S = tP + (1–t) Q. * Line segment between two points: For points P and Q, the subset of the line through P and Q with 0 < t < 1. In Rn * Ray emanating from a point w: The straight line passing by w (the notion of straight line is given by the linear structure of $$\mathbb R$$n); Given w ∈ $$\mathbb R$$n, the ray emanating from w is Rw:= {x ∈ $$\mathbb R$$n | ∃ p ∈ $$\mathbb R$$n \ {0} such that x = w + tp for some t ∈ $$\mathbb R$$+} . @ References: Darst et al 09 [curious curves]. In Euclidean Geometry > s.a. differential and euclidean geometry.$ Line: The line whose distance from the origin is p and angle of the normal with the x axis φ is

x cosφ + y sinφp = 0 .

* Length of a curve: For a closed curve, $$L = \int_0^{2\pi}{\rm d}\phi\,p(\phi)$$, where p(ψ) is the support function for the compact set bounded by the curve with respect to an interior point O.
* Writhing number: The expression

$W[C] = {1\over4\pi} \oint_C {\rm d}x_\alpha \oint_C {\rm d}x_\beta\, \epsilon^{\alpha\beta\gamma} {(x-y)_\gamma\over|x-y|^3}\; .$

* Envelope of a family of curves: Given the family of curves F(x, y; λ) = 0 in the plane, the envelope is the curve every point of which is a point of contact with a curve in the family; Its equation can be obtained by eliminating λ from F = 0 and ∂F/∂λ = 0.
@ References: Toponogov & Rovenski 05; Balakrishnan & Satija mp/05 [linking number, twist and writhe].
$Curvature: The vector Ca:= ξmm ξa, where ξa is the unit tangent to the line; Ca is always perpendicular to ξa, and vanishes iff the line is a geodesic; In general relativity, one identifies Ca for world-lines with Aa, its acceleration. @ General references: Ehlers & Köhler JMP(77) [congruences of curves on manifolds]. @ In Riemannian manifolds, curvature: Castrillón et al DG&A(10) [total curvature]; Gutkin JGP(11) [in terms of invariants]. > Frenet curvature / Frenet-Serret formulas: see coordinates on a manifold; relativistic particles; MathWorld page; Wikipedia page. > More general settings: see manifolds [curves in supermanifolds]. Related Concepts > s.a. Congruence; Fiber; Field Line; Vorticity; Writhe.$ Line bundle: An $$\mathbb R$$-bundle; > s.a. fiber bundle; Quillen Determinant.