Analysis of blow-ups for the double obstacle problem in dimension two

In this article we study a normalised double obstacle problem with polynomial obstacles $p^1\le p^2$ under the assumption that $p^1(x)=p^2(x)$ iff $x=0$. In dimension two we give a complete characterisation of blow-up solutions depending on the coefficients of the polynomials $p^1,p^2$. In particular, we see that there exists a new type of blow-ups, that we call double-cone solutions since the coincidence sets $\{u=p^1\}$ and $\{u=p^2\}$ are cones with a common vertex. We prove the uniqueness of blow-up limits, and analyse the regularity of the free boundary in dimension two. In particular we show that if the solution to the double obstacle problem has a double-cone blow-up limit at the origin, then locally the free boundary consists of four $C^{1,\gamma }$-curves, meeting at the origin. In the end we give an example of a three-dimensional double-cone solution.