Differential Geometry of Curves - Reparametrization and Equivalence Relation

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 Cr curves and are central objects studied in the differential geometry of curves.

Two parametric curves of class Cr

and

are said to be equivalent if there exists a bijective Cr map

such that

and

γ2 is said to be a reparametrisation of γ1. This reparametrisation of γ1 defines the equivalence relation on the set of all parametric Cr curves. The equivalence class is called a Cr curve.

We can define an even finer equivalence relation of oriented Cr curves by requiring φ to be φ‘(t) > 0.

Equivalent Cr curves have the same image. And equivalent oriented Cr curves even traverse the image in the same direction.

Read more about this topic:  Differential Geometry Of Curves

Famous quotes containing the word relation:

    “Much poetry seems to be aware of its situation in time and of its relation to the metronome, the clock, and the calendar. ... The season or month is there to be felt; the day is there to be seized. Poems beginning “When” are much more numerous than those beginning “Where” of “If.” As the meter is running, the recurrent message tapped out by the passing of measured time is mortality.”
    —William Harmon (b. 1938)