Cusp (singularity)

An ordinary cusp on the curve x³–y²=0

In mathematics a cusp, sometimes called spinode in old texts, is a point on a curve where a moving point on the curve must start to move backward. A typical example is given in the figure. A cusp is thus a type of singular point of a curve.

For a plane curve defined by a differentiable parametric equation

\begin{align} x&=f(t)\\ y&=g(t), \end{align}

a cusp is a point where both derivatives of $f$ and $g$ are zero, and at least one of them changes of sign. Cusps are local singularities in the sense that they involve only one value of the parameter $t$ , contrarily to self-intersection points that involve several values.

For a curve defined by an implicit equation

F(x,y)=0,

cusps are points where the terms of lowest degree of the Taylor expansion of $F$ are a power of a linear polynomial; however not all singular points that have this property are cusps. In some contexts, and in the remainder of this article, one restricts the definition of a cusp to the case where the non-zero part of lowest degree of the Taylor expansion of $F$ has degree two.

A plane curve cusp may be put in one of the following forms by a diffeomorphism of the plane: x² − y^2k+1 = 0, where k ≥ 1 is an integer.

Classification in differential geometry

Consider a smooth real-valued function of two variables, say f(x, y) where x and y are real numbers. So f is a function from the plane to the line. The space of all such smooth functions is acted upon by the group of diffeomorphisms of the plane and the diffeomorphisms of the line, i.e. diffeomorphic changes of coordinate in both the source and the target. This action splits the whole function space up into equivalence classes, i.e. orbits of the group action.

One such family of equivalence classes is denoted by A_k^±, where k is a non-negative integer. This notation was introduced by V. I. Arnold. A function f is said to be of type A_k^± if it lies in the orbit of x² ± y^k+1, i.e. there exists a diffeomorphic change of coordinate in source and target which takes f into one of these forms. These simple forms x² ± y^k+1 are said to give normal forms for the type Ak^±-singularities. Notice that the A_2n⁺ are the same as the A_2n⁻ since the diffeomorphic change of coordinate (x,y) → (x, −y) in the source takes x² + y²ⁿ⁺¹ to x² − y²ⁿ⁺¹. So we can drop the ± from A_2n^± notation.

The cusps are then given by the zero-level-sets of the representatives of the A_2n equivalence classes, where n ≥ 1 is an integer.

Examples

An ordinary cusp is given by x² − y³ = 0, i.e. the zero-level-set of a type A₂-singularity. Let f(x, y) be a smooth function of x and y and assume, for simplicity, that f(0,0) = 0. Then a type A₂-singularity of f at (0,0) can be characterised by:

Having a degenerate quadratic part, i.e. the quadratic terms in the Taylor series of f form a perfect square, say L(x, y)², where L(x, y) is linear in x and y, and
L(x, y) does not divide the cubic terms in the Taylor series of f(x, y).

A rhamphoid cusp (coming from the Greek meaning beak-like) is given by x² – y⁵ = 0, i.e. the zero-level-set of a type A₄-singularity. These cusps are non-generic as caustics and wavefronts. The rhamphoid cusp and the ordinary cusp are non-diffeomorphic.

For a type A₄-singularity we need f to have a degenerate quadratic part (this gives type A_≥2), that L does divide the cubic terms (this gives type A_≥3), another divisibility condition (giving type A_≥4), and a final non-divisibility condition (giving type exactly A₄).

To see where these extra divisibility conditions come from, assume that f has a degenerate quadratic part L² and that L divides the cubic terms. It follows that the third order taylor series of f is given by L² ± LQ where Q is quadratic in x and y. We can complete the square to show that L² ± LQ = (L ± ½Q)² – ¼Q⁴. We can now make a diffeomorphic change of variable (in this case we simply substitute polynomials with linearly independent linear parts) so that (L ± ½Q)² − ¼Q⁴ → x₁² + P₁ where P₁ is quartic (order four) in x₁ and y₁. The divisibility condition for type A_≥4 is that x₁ divides P₁. If x₁ does not divide P₁ then we have type exactly A₃ (the zero-level-set here is a tacnode). If x₁ divides P₁ we complete the square on x₁² + P₁ and change coordinates so that we have x₂² + P₂ where P₂ is quintic (order five) in x₂ and y₂. If x₂ does not divide P₂ then we have exactly type A₄, i.e. the zero-level-set will be a rhamphoid cusp.

Applications

An ordinary cusp occurring as the caustic of light rays in the bottom of a teacup.

Cusps appear naturally when projecting into a plane a smooth curve in the three dimensional Euclidean space. In general, such a projection is a curve whose singularities are self-crossing points and ordinary cusps. Self-crossing points appear when two different points of the curves have the same projection. Ordinary cusps appear when the tangent to the curve is parallel to the direction of projection (that is when the tangent projects on a single point). More complicated singularities occur when several phenomena occurs simultaneously. For example, rhamphoid cusps occur for inflection points (and for undulation points) for which the tangent is parallel to the direction of projection.

In many cases, and typically in computer vision and computer graphics, the curve that is projected is the curve of the critical points of the restriction to a (smooth) spatial object of the projection. A cusp appears thus as a singularity of the contour of the image of the object (vision) or of its shadow (computer graphics).

Caustics and wave fronts are other examples of curves having cusps that are visible in the real world.

References

Bruce, J. W.; Giblin, Peter (1984). Curves and Singularities. Cambridge University Press. ISBN 0-521-42999-4.
Porteous, Ian (1994). Geometric Differentiation. Cambridge University Press. ISBN 0-521-39063-X.

External links

Physicists See The Cosmos In A Coffee Cup

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