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 integrals over 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 change of variables. More general forms of the formula for Lipschitz functions were first established by Herbert Federer, and for [[Bounded variation|'' BV '' functions]] by. A precise statement of the formula is as follows. Suppose that Ω is an open set in \R^n 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 taking on values in \R^k where k ≤ n. In this case, the following identity holds where Jku is the k-dimensional Jacobian of u whose determinant is given by

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