Permuting the roots of univariate polynomials whose coefficients depend on parameters

Abstract

We address two interrelated problems concerning permutation of roots of univariate polynomials whose coefficients depend on parameters. First, we compute the Galois group of polynomials $\phi (x)\in \mathbb{C}[t_1,\dots ,t_k][x]$ over $\mathbb{C}(t_1,\dots ,t_k)$. Provided that the corresponding multivariate polynomial $\phi (x,t_1,\dots ,t_k)$ is generic with respect to its support set $A\subset {\mathbb{Z}}^{k+1}$, we determine the latter Galois group for any $A$. Second, we determine the Galois group of systems of polynomial equations of the form $p(x,t)=q(t)=0$ where $p$ and $q$ have prescribed support sets $A_1\subset {\mathbb{Z}}^2$ and $A_2\subset \{0\}\times \mathbb{Z}$ respectively. For each problem, we determine the image of an appropriate braid monodromy map in order to compute the sought Galois group. As applications, we compute the Galois group of any rational function that is generic with respect to its support. We also provide general obstructions on the Galois group of enumerative problems on algebraic groups. Eventually, the techniques we develop allow us to compute the kernel of the braid monodromy map associated to $\phi$.