In the mathematical field of geometric measure theory, the coarea formula expresses the integral of a function over an open set in Euclidean space in terms of the integral of the level sets of another function. A special case is Fubini's theorem, which says under suitable hypotheses that the integral of a function over the region enclosed by a rectangular box can be written as the iterated integral over the level sets of the coordinate functions. Another special case is integration in spherical coordinates, in which the integral of a function on Rn is related to the integral of the function over spherical shells: level sets of the radial function. The formula plays a decisive role in the modern study of isoperimetric problems.
For smooth functions the formula is a result in multivariate calculus which follows from a simple change of variables. More general forms of the formula for Lipschitz functions were first established by Herbert Federer Template:Harv, and for BV functions by Template:Harvtxt.
where Hn − 1 is the (n − 1)-dimensional Hausdorff measure. In particular, by taking g to be one, this implies
and conversely the latter equality implies the former by standard techniques in Lebesgue integration.
More generally, the coarea formula can be applied to Lipschitz functions u defined in Ω ⊂ Rn, taking on values in Rk where k < n. In this case, the following identity holds
where Jku is the k-dimensional Jacobian of u.
- Taking u(x) = |x − x0| gives the formula for integration in spherical coordinates of an integrable function ƒ:
- Combining the coarea formula with the isoperimetric inequality gives a proof of the Sobolev inequality for W1,1 with best constant:
- where ωn is the volume of the unit ball in Rn.