Certified Monodromy Groups for Families of Hypersurfaces

Abstract

The monodromy group of a family of algebraic varieties is the group of isometries of the homology of a generic fibre, induced by parallel transport along a loop in the base space. We present a method to compute the monodromy group of families of generically smooth projective hypersurfaces using certified numerical algebraic geometry. This leads us to develop efficient algorithms for computing the discriminants of these families, which is of independent interest. Our flagship example is the case of families of symmetric cubic surfaces. The monodromy group is the Galois group of the 27 lines. We find that, when imposing symmetry, this Galois group is often a small subgroup of the Weyl group $W(E_6)$. We also include computational results for symmetric quartic surfaces.

arXiv comment36 pages, 2 figures, 8 tables To appear in Foundations of Computational Mathematics