Talk:Bony–Brezis theorem

More details needed about "exterior normal vector"
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?

Nathaniel Virgo (talk) 02:32, 18 August 2023 (UTC)