Expecting the unexpected: Quantifying the persistence of unexpected hypersurfaces

Abstract

The independence of imposed vanishing conditions is a foundational issue for a wide range of research in algebraic geometry. In this spirit, if X⊂Pn is a reduced subscheme, we say that X admits an unexpected hypersurface of degree t and multiplicity m if requiring multiplicity m at a general point P fails to impose the expected number of conditions on the linear system of hypersurfaces of degree t containing X. Conditions which either guarantee the occurrence of unexpected hypersurfaces, or which ensure that they cannot occur, are not well understood. Research to date has made surprising connections to root systems, hyperplane arrangements, and generic splitting types of vector bundles, among other diverse topics. In this paper we introduce new methods for studying unexpectedness, such as the use of generic initial ideals and partial elimination ideals to clarify when it can and when it cannot occur. We also exhibit algebraic and geometric properties of X which in some cases guarantee and in other cases preclude X having certain kinds of unexpectedness. In addition, we formulate a new way of quantifying unexpectedness (our AV sequence), which allows us to detect the extent to which unexpectedness persists as t increases but t−m remains constant. Finally, we study to what extent we can detect unexpectedness from the Hilbert function of X.