For a Lipschitz map $f: \mathbb{R}^n \to \mathbb{R}^m$ with $n \le m$, and for any measurable set $A \subset \mathbb{R}^n$, $$ \int_A J_n f , d\mathcal{L}^n = \int_{\mathbb{R}^m} \mathcal{H}^0(A \cap f^{-1}{y}) , d\mathcal{H}^n(y). $$
In plain English: integrating the Jacobian over the domain equals integrating the number of preimages over the target, with respect to $n$-dimensional Hausdorff measure. federer geometric measure theory pdf
Last month, I finally decided to stop treating the PDF on my hard drive as a sacred artifact and actually opened it. Here is the view from the trenches. First, a note on the PDF. The original Springer “Grundlehren” edition runs 676 pages. The typesetting is pure late-60s elegance: no LaTeX, yet strangely beautiful. The PDFs floating around (legally purchased, of course) are usually clean scans, but they preserve the original’s dense theorems and famously terse proofs. For a Lipschitz map $f: \mathbb{R}^n \to \mathbb{R}^m$