The Stacks project

Exercise 111.31.4. Give an example of an affine variety $X \subset \mathbf{C}^ n$ which is a cone (see Exercise 111.31.3) and a regular function $f$ on $U = X \setminus \{ (0, \ldots , 0)\}$ which is not the restriction of a polynomial function on $\mathbf{C}^ n$.