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 (Federer 1959), and for BV functions by Fleming & Rishel (1960).
A precise statement of the formula is as follows. Suppose that Ω is an open set in Rn, and u is a real-valued Lipschitz function on Ω. Then, for an L1 function g,
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.