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Karl Fabian
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$$f(x,y)~=~ \frac{-1}{1+ x^2 + y^2}-\cos (a\,x) \,\cos (b\,y)\,e^{-\frac{x^2 + y^2}{c^2}}$$ for $a,b,c \in \mathbb{R}^+$, where $c $ is sufficiently large.

Karl Fabian
  • 1.7k
  • 10
  • 15