Equiresidual algebraic geometry I: The affine theory

Abstract

In this first work dedicated to the generalisation of classic algebraic geometry to non algebraically closed fields and axiomatisable classes of fields, we develop the foundations for equiresidual algebraic geometry (EQAG), i.e. algebraic geometry over any commutative field $k$, algebraically closed or not. This is possible thanks to the existence in non algebraically closed fields of many normic forms, i.e. homogeneous polynomials with no non-trivial zero rational in $k$, and relies on an equiresidual generalisation of Hilbert's Nullstellensatz and the Jacobson radical in finitely presented $k$-algebras. Usual algebraic constructions are naturally worked out using a new type of $k$-algebras which correspond to localisations of $k$-algebras by means of polynomials over $k$ with no inner zero. The theory leads to a fruitful characterisation of the sections of sheaves of regular functions over affine algebraic sets, providing a dualisation of the (equiresidual) affine algebraic varieties over $k$ by an equiresidual analogue of reduced algebras of finite type. The true generalisation of reduced algebras are defined using the normic forms and encompass all rings of rational functions over affine subvarieties. These "special" algebras are interpreted as giving rise to a "special" radical, an equiresidual version of the classic radical of an ideal, of which we give a clean algebraic characterisation. Special extensions are the "right" field extensions of the ground field in EQAG and provide a connexion with model-theoretic algebraic geometry, as exemplified with real closed fields and p-adically closed fields. The special radical is well-behaved with respect to localisation, and this leads to an equiresidual analogue of the prime spectrum, which connects affine EQAG with scheme theory.