I am looking for a differentiable function, thats its partial derivatives equals 0 along $$|y|=|x|$$ I have thought about $$u(x,y)=\frac{(x-y)^2(x+y)^2}{2}$$ but I am looking for another one
if anyone has a method/tips of "building" functions like that I'd be happy to learn...
Thank you!