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I quote from wiki:

The coarea formula can be applied to Lipschitz functions $u$ defined in $Ω ⊂ \mathbb R^n$, taking on values in $\mathbb R^k$ where $k < n$. In this case, the following identity holds $$\int_\Omega g(x) |J_k u(x)|\, dx = \int_{\mathbb{R}^k} \left(\int_{u^{-1}(t)}g(x)\,dH_{n-k}(x)\right)\,dt$$

where $J_ku$ is the $k$-dimensional Jacobian of $u$.

I know that if $Du$ has full rank, $|J_k u(x)|=\sqrt{\det DuDu^T}$. But when $Du$ does not have full rank, how is $|J_k u(x)|$ defined?

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    then it is zero – 2017-02-13
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    So this means the coarea formula fails? I am doubtful about that – 2017-02-14
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    For convenience: Now reposted at http://mathoverflow.net/questions/262375/how-is-j-k-ux-in-the-corea-formula-defined-when-du-does-not-have-full – 2017-02-16

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