Crofton formula

Statement

Suppose γ is a rectifiable plane curve. Given an oriented line l, let n γ (l) be the number of points at which γ and l intersect. We can parametrize the general line l by the direction φ in which it points and its signed distance p from the origin. The Crofton formula expresses the arc length of the curve γ in terms of an integral over the space of all oriented lines:

The differential form

is invariant under rigid motions, so it is a natural integration measure for speaking of an "average" number of intersections. The right-hand side in the Crofton formula is sometimes called the Favard length.

Proof sketch

Both sides of the Crofton formula are additive over concatenation of curves, so it suffices to prove the formula for a single line segment. Since the right-hand side does not depend on the positioning of the line segment, it must equal some function of the segment's length. Because, again, the formula is additive over concatenation of line segments, the integral must be a constant times the length of the line segment. It remains only to determine the factor of 1/4; this is easily done by computing both sides when γ is the unit circle.

Other forms

The space of oriented lines is a double cover of the space of unoriented lines. The Crofton formula is often stated in terms of the corresponding density in the latter space, in which the numerical factor is not 1/4 but 1/2. Since a convex curve intersects almost every line either twice or not at all, the unoriented Crofton formula for convex curves can be stated without numerical factors: the measure of the set of straight lines which intersect a convex curve is equal to its length.

The Crofton formula generalizes to any Riemannian surface; the integral is then performed with the natural measure on the space of geodesics.

Applications

Crofton's formula yields elegant proofs of the following results, among others: