Let (d geqslant 2). A flat cone in (mathbb{R}^{d}) is a cone in (mathbb{R}^{d-1}). Adapt the argument

Question:

Let $d \geqslant 2$. A flat cone in $\mathbb{R}^{d}$ is a cone in $\mathbb{R}^{d-1}$. Adapt the argument of Example 8.18.e) to show the following useful regularity criterion for a $\mathrm{BM}^{d}$ : The boundary point $x_{0}$ is regular for $D^{c}$ if there is a truncated flat cone with vertex at $x_{0}$ and lying entirely in $D^{c}$.

Data From 8.18 Example

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