Question. Can a degree 4 polynomial $p:\mathbb R^2 \to \mathbb R$ have $N\geq 2$ local minima and no other critical points?
I got this question when trying to answer: How many strict local minima a quartic polynomial in two variables might have?
A closely related question asks: If a two variable smooth function has two global minima, will it necessarily have a third critical point? Among the answers @RiverLi gives an example of the function $$ f(x,y)=(x^2-1)^2+(x^2y-x-1)^2, \tag{*}\label{RL} $$ which is a polynomial, but of degree 6.
Notice that polynomials like \eqref{RL} can be written as $p(x,y)=u(x,y)^2 + v(x,y)^2$. Since we need $p$ to be degree 4 polynomial, $u$ and $v$ must be quadratic polynomials. We need there to be a set $P\subset \Bbb R^2, |P|=N$, of points such that:
- $(x,y)\in P \ \ \Longleftrightarrow \ \ u(x,y)=v(x,y)=0$;
- $(x,y)\in P \ \ \Longleftrightarrow \ \ u_x u + v_x v = u_y u + v_y v =0$ when evaluated at $(x,y)$.
For what values of $N\geq 2$ is there a set $P\subset \Bbb R^2, |P|=N$ and quadratic polynomials $u,v:\Bbb R^2 \to \Bbb R$ satisfying conditions 1 and 2?
My guess is that for $N=2$ at least one of the polynomials $u,v$ has to be of the third order as in \eqref{RL}. However, it is possible that there would be desirable $u,v$ for $N=3$ or $N=4$. Note that by Bézout's theorem condition 1 can not be satisfied at more than 4 points given that $u,v$ are quadratic.
Alternatively, is there another form than $p=u^2+v^2$ that could represent a quartic polynomial $p$ with $N\geq 2$ local minima and no other critical points?
Disclaimer. I asked about the special case of $N=2$ on https://math.codidact.com.