Vertex (curve)

Short description: Point of extreme curvature on a curve

An ellipse (red) and its evolute (blue). The dots are the vertices of the curve, each corresponding to a cusp on the evolute.

In the geometry of plane curves, a vertex is a point of where the first derivative of curvature is zero.[1] This is typically a local maximum or minimum of curvature,[2] and some authors define a vertex to be more specifically a local extremum of curvature.[3] However, other special cases may occur, for instance when the second derivative is also zero, or when the curvature is constant. For space curves, on the other hand, a vertex is a point where the torsion vanishes.

## Contents

* 1 Examples
* 2 Cusps and osculation
* 3 Other properties
* 4 Notes
* 5 References

## Examples

A hyperbola has two vertices, one on each branch; they are the closest of any two points lying on opposite branches of the hyperbola, and they lie on the principal axis. On a parabola, the sole vertex lies on the axis of symmetry and in a quadratic of the form:

ax2+bx+c

it can be found by completing the square or by differentiation.[2] On an ellipse, two of the four vertices lie on the major axis and two lie on the minor axis.[4]

For a circle, which has constant curvature, every point is a vertex.

## Cusps and osculation

Vertices are points where the curve has 4-point contact with the osculating circle at that point.[5][6] In contrast, generic points on a curve typically only have 3-point contact with their osculating circle. The evolute of a curve will generically have a cusp when the curve has a vertex;[6] other, more degenerate and non-stable singularities may occur at higher-order vertices, at which the osculating circle has contact of higher order than four.[5] Although a single generic curve will not have any higher-order vertices, they will generically occur within a one-parameter family of curves, at the curve in the family for which two ordinary vertices coalesce to form a higher vertex and then annihilate.

The symmetry set of a curve has endpoints at the cusps corresponding to the vertices, and the medial axis, a subset of the symmetry set, also has its endpoints in the cusps.

## Other properties

According to the classical four-vertex theorem, every simple closed planar smooth curve must have at least four vertices.[7] A more general fact is that every simple closed space curve which lies on the boundary of a convex body, or even bounds a locally convex disk, must have four vertices.[8] Every curve of constant width must have at least six vertices.[9]

If a planar curve is bilaterally symmetric, it will have a vertex at the point or points where the axis of symmetry crosses the curve. Thus, the notion of a vertex for a curve is closely related to that of an optical vertex, the point where an optical axis crosses a lens surface.

## Notes

1. ↑ (Agoston 2005), p. 570; (Gibson 2001), p. 126.
2. ↑ 2.0 2.1 (Gibson 2001), p. 127.
3. ↑ (Fuchs Tabachnikov), p. 141.
4. ↑ (Agoston 2005), p. 570; (Gibson 2001), p. 127.
5. ↑ 5.0 5.1 (Gibson 2001), p. 126.
6. ↑ 6.0 6.1 (Fuchs Tabachnikov), p. 142.
7. ↑ (Agoston 2005), Theorem 9.3.9, p. 570; (Gibson 2001), Section 9.3, "The Four Vertex Theorem", pp. 133–136; (Fuchs Tabachnikov), Theorem 10.3, p. 149.
8. ↑ (Sedykh 1994); (Ghomi 2015)
9. ↑ (Martinez-Maure 1996); (Craizer Teixeira)

## References

* Agoston, Max K. (2005), Computer Graphics and Geometric Modelling: Mathematics, Springer, ISBN 9781852338176 .
* Craizer, Marcos; Teixeira, Ralph; Balestro, Vitor (2018), "Closed cycloids in a normed plane", Monatshefte für Mathematik 185 (1): 43–60, doi:10.1007/s00605-017-1030-5 .
* Fuchs, D. B.; Tabachnikov, Serge (2007), Mathematical Omnibus: Thirty Lectures on Classic Mathematics, American Mathematical Society, ISBN 9780821843161
* Ghomi, Mohammad (2015), Boundary torsion and convex caps of locally convex surfaces, Bibcode: 2015arXiv150107626G
* Gibson, C. G. (2001), Elementary Geometry of Differentiable Curves: An Undergraduate Introduction, Cambridge University Press, ISBN 9780521011075 .
* Martinez-Maure, Yves (1996), "A note on the tennis ball theorem", American Mathematical Monthly 103 (4): 338–340, doi:10.2307/2975192 .
* Sedykh, V.D. (1994), "Four vertices of a convex space curve", Bull. London Math. Soc. 26 (2): 177–180, doi:10.1112/blms/26.2.177

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