In this paper we study the tangent space to the Hilbert scheme $\mathrm{ Hilb}^d \mathbf{ P}^3$, motivated by Haiman's work on $\mathrm{ Hilb}^d \mathbf{ P}^2$ and by a long-standing conjecture of Brian\c{ c}on and Iarrobino on the most singular point in $\mathrm{ Hilb}^d \mathbf{ P}^n$. For points parametrizing monomial subschemes, we consider a decomposition of the tangent space into six distinguished subspaces, and show that a fat point exhibits an extremal behavior in this respect. This decomposition is also used to characterize smooth monomial points on the Hilbert scheme. We prove the first Brian\c{ c}on-Iarrobino conjecture up to a factor of 4/3, and improve the known asymptotic bound on the dimension of $\mathrm{ Hilb}^d \mathbf{ P}^3$. Furthermore, we construct infinitely many counterexamples to the second Brian\c{ c}on-Iarrobino conjecture, and we also settle a weaker conjecture of Sturmfels in the negative.
Categories: math.AG math.AC