*Published Paper*

**Inserted:** 11 apr 2003

**Last Updated:** 28 apr 2004

**Journal:** Crelle's Journal

**Volume:** 564

**Pages:** 63-83

**Year:** 2003

**Notes:**

to appear

**Abstract:**

The characteristic set $C(S)$ of a codimension 1 submanifold $S$ in the Heisenberg group $\hn$ consists of those points where the tangent space of $S$ coincides with the space spanned by the left invariant horizontal vector fields of $\hn$. We prove that if $S$ is $C^1$ smooth then $C(S)$ has vanishing $2n+1$-dimensional Hausdorff measure with respect to the Heisenberg metric. If $S$ is $C^2$ smooth then $C(S)$ has Hausdorff dimension less or equal than $n$, both with respect to the Euclidean and Heisenberg metrics. On the other hand, $C(S)$ can have a positive $2n$-dimensional Hausdorff measure with respect to the Euclidean metric even for hypersurfaces of class $\cap_{0< \alpha < 1} C^{1,\alpha}$. This is shown by constructing a function of class $\cap_{0< \alpha < 1} C^{1,\alpha}$ with a prescribed gradient on a large measure set.

**Keywords:**
Hausdorff measure, Heisenberg group, Characteristic set

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