Talk:Bony–Brezis theorem
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More details needed about "exterior normal vector"
[edit]The page currently defines an "exterior normal vector" like so:
- A vector is an exterior normal at a point of the closed set if there is a real-valued continuously differentiable function maximized locally at the point with that vector as its derivative at the point.
On the face of it this doesn't make much sense. If a function is locally maximised at a point then its derivative is the zero vector. Presumably something else is meant - it would be helpful to spell it out more formally in the 'Statement' section.
Additionally, I was able to find little if any additional information by googling "exterior normal vector". Is the concept also known by another name?