I'm looking for a simple two variable equation of a tilted sphere. I don't know what the precise terminology is, but it's similar to this.
Here is a picture:
The $f(x,y)=0$ at the dotted lines at the base of this shape. But this shape does not take on its maximum value when $x=0$ and $y=0$. Rather, it's sort of tilted to the side.
In particular, I need a continuously differentiable function $f : \mathbb{R}^2 \rightarrow \mathbb{R}$ such that $f(x,y)=0$ when $x^2+y^2=1$, but the point at which the tangent plane is parallel to the $xy$-plane is not at $(0,0).$ In other words, the top of the sphere is a little off center.
I'm having trouble constructing this. Whenever I introduce any "tilt/skew" to a single variable equation, I'm not sure how to make it satisfy the $f(x,y)=0$ when $x^2+y^2=1$ condition.