Let $f$ be a mapping in the Sobolev space $W^{1,n}(\Omega,\bold R^n)$. Then the change of variables, or area formula holds for $f$ provided removing from counting into the multiplicity function the set where $f$ is not approximately Hölder continuous. This exceptional set has Hausdorff dimension zero.
@article{118668, author = {Jan Mal\'y}, title = {The area formula for $W^{1,n}$-mappings}, journal = {Commentationes Mathematicae Universitatis Carolinae}, volume = {35}, year = {1994}, pages = {291-298}, zbl = {0812.30006}, mrnumber = {1286576}, language = {en}, url = {http://dml.mathdoc.fr/item/118668} }
Malý, Jan. The area formula for $W^{1,n}$-mappings. Commentationes Mathematicae Universitatis Carolinae, Tome 35 (1994) pp. 291-298. http://gdmltest.u-ga.fr/item/118668/
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